Re: Thoughts on VM fence infrastructure
> On Oct 1, 2019, at 12:10 PM, Daniel P. Berrangé wrote: > > On Tue, Oct 01, 2019 at 10:46:24AM +, Felipe Franciosi wrote: >> Hi Daniel! >> >> >>> On Oct 1, 2019, at 11:31 AM, Daniel P. Berrangé wrote: >>> >>> On Tue, Oct 01, 2019 at 09:56:17AM +, Felipe Franciosi wrote: >> >> (Apologies for the mangled URL, nothing I can do about that.) :( >> >> There are several points which favour adding this to Qemu: >> - Not all environments use systemd. > > Sure, if you want to cope with that you can just use the HW watchdog > directly instead of via systemd. > >> - HW watchdogs always reboot the host, which is too drastic. >> - You may not want to protect all VMs in the same way. > > Same points repeated below, so I'll respond there > >>> IMHO doing this at the host OS level is going to be more reliable in >>> terms of detecting the problem in the first place, as well as more >>> reliable in taking the action - its very difficult for a hardware CPU >>> reset to fail to work. >> >> Absolutely, but it's a very drastic measure that: >> - May be unnecessary. > > Of course, the inability to predict future consequences is what > forces us into assuming the worst case & taking actions to > mitigate that. It will definitely result in unccessary killing > of hosts, but that is what gives you the safety guarantees you > can't otherwise achieve. The argument is that many configurations have controlled settings that do not require that drastic level of protection. And the feature we're discussing offer a softer way of dealing with these. > I gave the example elsewhere that even if you kill QEMU, the kernel > can have pending I/O associated with QEMU that can be sent if the > host later recovers. Even if an I/O is sent out of the host, there are no guarantees that it isn't queued somewhere and will reach its destination even after you pulled the power of a host. Such discussions were held separately a while back when we were talking about task cancellation. To that point, I've personally seen corruption with network storage which was debugged as: 1) write(lba=0, value='a') 2) host crashed (hard reset) 3) vm restarted elsewhere 4) write(lba=0, value='a') (resubmitted from "1") 5) write(lba=0, value='b') 6) I/O from step "1" reached controller 7) read(lba=0) == 'a' My argument is that you need to look into protection where protection is needed. Perhaps the example above could be avoided with a session distinction so that once I/O from step "4" was seen (coming from the new session), I/Os from older sessions should be rejected by the storage controller. In any case, all I'm saying is that there are different levels of protection. The feature we're discussing here offers one of them. > >> - Will fence everything even perhaps only some VMs need protection. > > I don't believe its viable to have offer real protection to only > a subset of VMs, principally because the kernel is doing I/O work > on behalf of the VM, so to protect just 1 VM you must fence the > kernel. You are assuming that all users of Qemu out there do I/O through the kernel. What if they don't? > >> What are your thoughts on this 3-level approach? >> 1) Qemu tries to log() + abort() (deadline) > > Just abort()'ing isn't going to be a viable strategy with QEMU's move > towards a multi-process architecture. This introduces the problem that > the "main" QEMU process has to enumerate all the helpers it is dealing > with and kill them all off in some way. This is non-trivial especially > if some of the helpers are running under different privilege levels. If this is to extend to a multi-process model, I don't think it should be one process killing others. It's called "self-fencing" because each process should be responsible for killing itself based on a heartbeat. (Or configuring the kernel to do it.) > > You could declare that multi-process QEMU is out of scope, but I think > QEMU self-fencing would need to offer compelling benefits over host OS > self-fencing to justify that exception. Personally I'm not seeing it. I would limit the feature for a monolithic model to begin with, but definitely keep an eye on ways to extend it to a multi-process model. The benefit is as described before, with the added arguments in this e-mail: - May not want to protect all VMs. - May not want to kill the entire host for a temporary network outage. - Killing Qemu is sufficient in various configurations. > >> 2) Kernel sends SIGKILL (harddeadline) > > This is slightly easier to deal with multiple processes in that it > isn't restricted by the privileges of the main QEMU vs helpers and > could take advantage of cgroups perhaps. Right, and it should be an option from the start. Thanks for weighting in with extra ideas around cgroup. F. > >> 3) HW watchdog kicks in (harderdeadline) > > > Regards, > Daniel > -- > |: > https://urldefense.proofpoint.com/v2/url?u=https-3A__berrange.com&d=DwIDaQ&c=s883GpUCOChKOHiocYtGcg&r=CCrJKVC5zGot8PrnI-
Re: Thoughts on VM fence infrastructure
On Tue, Oct 01, 2019 at 10:46:24AM +, Felipe Franciosi wrote: > Hi Daniel! > > > > On Oct 1, 2019, at 11:31 AM, Daniel P. Berrangé wrote: > > > > On Tue, Oct 01, 2019 at 09:56:17AM +, Felipe Franciosi wrote: > > (Apologies for the mangled URL, nothing I can do about that.) :( > > There are several points which favour adding this to Qemu: > - Not all environments use systemd. Sure, if you want to cope with that you can just use the HW watchdog directly instead of via systemd. > - HW watchdogs always reboot the host, which is too drastic. > - You may not want to protect all VMs in the same way. Same points repeated below, so I'll respond there > > IMHO doing this at the host OS level is going to be more reliable in > > terms of detecting the problem in the first place, as well as more > > reliable in taking the action - its very difficult for a hardware CPU > > reset to fail to work. > > Absolutely, but it's a very drastic measure that: > - May be unnecessary. Of course, the inability to predict future consequences is what forces us into assuming the worst case & taking actions to mitigate that. It will definitely result in unccessary killing of hosts, but that is what gives you the safety guarantees you can't otherwise achieve. I gave the example elsewhere that even if you kill QEMU, the kernel can have pending I/O associated with QEMU that can be sent if the host later recovers. > - Will fence everything even perhaps only some VMs need protection. I don't believe its viable to have offer real protection to only a subset of VMs, principally because the kernel is doing I/O work on behalf of the VM, so to protect just 1 VM you must fence the kernel. > What are your thoughts on this 3-level approach? > 1) Qemu tries to log() + abort() (deadline) Just abort()'ing isn't going to be a viable strategy with QEMU's move towards a multi-process architecture. This introduces the problem that the "main" QEMU process has to enumerate all the helpers it is dealing with and kill them all off in some way. This is non-trivial especially if some of the helpers are running under different privilege levels. You could declare that multi-process QEMU is out of scope, but I think QEMU self-fencing would need to offer compelling benefits over host OS self-fencing to justify that exception. Personally I'm not seeing it. > 2) Kernel sends SIGKILL (harddeadline) This is slightly easier to deal with multiple processes in that it isn't restricted by the privileges of the main QEMU vs helpers and could take advantage of cgroups perhaps. > 3) HW watchdog kicks in (harderdeadline) Regards, Daniel -- |: https://berrange.com -o-https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o-https://fstop138.berrange.com :| |: https://entangle-photo.org-o-https://www.instagram.com/dberrange :|
Re: Thoughts on VM fence infrastructure
On Mon, Sep 30, 2019 at 07:23:47PM +, Felipe Franciosi wrote: > > > > On Sep 30, 2019, at 6:59 PM, Dr. David Alan Gilbert > > wrote: > > > > * Felipe Franciosi (fel...@nutanix.com) wrote: > >> > >> > >>> On Sep 30, 2019, at 6:11 PM, Dr. David Alan Gilbert > >>> wrote: > >>> > >>> * Felipe Franciosi (fel...@nutanix.com) wrote: > > > > On Sep 30, 2019, at 5:03 PM, Dr. David Alan Gilbert > > wrote: > > > > * Felipe Franciosi (fel...@nutanix.com) wrote: > >> Hi David, > >> > >>> On Sep 30, 2019, at 3:29 PM, Dr. David Alan Gilbert > >>> wrote: > >>> > >>> * Felipe Franciosi (fel...@nutanix.com) wrote: > Heyall, > > We have a use case where a host should self-fence (and all VMs should > die) if it doesn't hear back from a heartbeat within a certain time > period. Lots of ideas were floated around where libvirt could take > care of killing VMs or a separate service could do it. The concern > with those is that various failures could lead to _those_ services > being unavailable and the fencing wouldn't be enforced as it should. > > Ultimately, it feels like Qemu should be responsible for this > heartbeat and exit (or execute a custom callback) on timeout. > >>> > >>> It doesn't feel doing it inside qemu would be any safer; something > >>> outside QEMU can forcibly emit a kill -9 and qemu *will* stop. > >> > >> The argument above is that we would have to rely on this external > >> service being functional. Consider the case where the host is > >> dysfunctional, with this service perhaps crashed and a corrupt > >> filesystem preventing it from restarting. The VMs would never die. > > > > Yeh that could fail. > > > >> It feels like a Qemu timer-driven heartbeat check and calls abort() / > >> exit() would be more reliable. Thoughts? > > > > OK, yes; perhaps using a timer_create and telling it to send a fatal > > signal is pretty solid; it would take the kernel to do that once it's > > set. > > I'm confused about why the kernel needs to be involved. If this is a > timer off the Qemu main loop, it can just check on the heartbeat > condition (which should be customisable) and call abort() if that's > not satisfied. If you agree on that I'd like to talk about how that > check could be made customisable. > >>> > >>> There are times when the main loop can get blocked even though the CPU > >>> threads can be running and can in some configurations perform IO > >>> even without the main loop (I think!). > >> > >> Ah, that's a very good point. Indeed, you can perform IO in those > >> cases specially when using vhost devices. > >> > >>> By setting a timer in the kernel that sends a signal to qemu, the kernel > >>> will send that signal however broken qemu is. > >> > >> Got you now. That's probably better. Do you reckon a signal is > >> preferable over SIGEV_THREAD? > > > > Not sure; probably the safest is getting the kernel to SIGKILL it - but > > that's a complete nightmare to debug - your process just goes *pop* > > with no apparent reason why. > > I've not used SIGEV_THREAD - it looks promising though. > > I'm worried that SIGEV_THREAD could be a bit heavyweight (if it fires > up a new thread each time). On the other hand, as you said, SIGKILL > makes it harder to debug. > > Also, asking the kernel to defer the SIGKILL (ie. updating the timer) > needs to come from Qemu itself (eg. a timer in the main loop, > something we already ruled unsuitable, or a qmp command which > constitutes an external dependency that we also ruled undesirable). > > What if, when self-fencing is enabled, Qemu kicks off a new thread > from the start which does nothing but periodically wake up, verify the > heartbeat condition and log()+abort() if required? (Then we wouldn't > need the kernel timer.) > > > > >> I'm still wondering how to make this customisable so that different > >> types of heartbeat could be implemented (preferably without creating > >> external dependencies per discussion above). Thoughts welcome. > > > > Yes, you need something to enable it, and some safe way to retrigger > > the timer. A qmp command marked as 'oob' might be the right way - > > another qm command can't block it. > > This qmp approach is slightly different than the external dependency > that itself kills Qemu; if it doesn't run, then Qemu dies because the > kernel timer is not updated. But this is also a heavyweight approach. > We are talking about a service that needs to frequently connect to all > running VMs on a host to reset the timer. > > But it does allow for the customisable heartbeat: the logic behind > what triggers the command is completely flexible. > > Thinking about this idea of a separate Qemu thread, one thing that > came to mind is this: > > qemu -fence heartb
Re: Thoughts on VM fence infrastructure
Hi Daniel! > On Oct 1, 2019, at 11:31 AM, Daniel P. Berrangé wrote: > > On Tue, Oct 01, 2019 at 09:56:17AM +, Felipe Franciosi wrote: >> >> >>> On Oct 1, 2019, at 9:23 AM, Dr. David Alan Gilbert >>> wrote: >>> >>> * Felipe Franciosi (fel...@nutanix.com) wrote: > On Sep 30, 2019, at 6:59 PM, Dr. David Alan Gilbert > wrote: > > * Felipe Franciosi (fel...@nutanix.com) wrote: >> >> >>> On Sep 30, 2019, at 6:11 PM, Dr. David Alan Gilbert >>> wrote: >>> >>> * Felipe Franciosi (fel...@nutanix.com) wrote: > On Sep 30, 2019, at 5:03 PM, Dr. David Alan Gilbert > wrote: > > * Felipe Franciosi (fel...@nutanix.com) wrote: >> Hi David, >> >>> On Sep 30, 2019, at 3:29 PM, Dr. David Alan Gilbert >>> wrote: >>> >>> * Felipe Franciosi (fel...@nutanix.com) wrote: Heyall, We have a use case where a host should self-fence (and all VMs should die) if it doesn't hear back from a heartbeat within a certain time period. Lots of ideas were floated around where libvirt could take care of killing VMs or a separate service could do it. The concern with those is that various failures could lead to _those_ services being unavailable and the fencing wouldn't be enforced as it should. Ultimately, it feels like Qemu should be responsible for this heartbeat and exit (or execute a custom callback) on timeout. >>> >>> It doesn't feel doing it inside qemu would be any safer; something >>> outside QEMU can forcibly emit a kill -9 and qemu *will* stop. >> >> The argument above is that we would have to rely on this external >> service being functional. Consider the case where the host is >> dysfunctional, with this service perhaps crashed and a corrupt >> filesystem preventing it from restarting. The VMs would never die. > > Yeh that could fail. > >> It feels like a Qemu timer-driven heartbeat check and calls abort() / >> exit() would be more reliable. Thoughts? > > OK, yes; perhaps using a timer_create and telling it to send a fatal > signal is pretty solid; it would take the kernel to do that once it's > set. I'm confused about why the kernel needs to be involved. If this is a timer off the Qemu main loop, it can just check on the heartbeat condition (which should be customisable) and call abort() if that's not satisfied. If you agree on that I'd like to talk about how that check could be made customisable. >>> >>> There are times when the main loop can get blocked even though the CPU >>> threads can be running and can in some configurations perform IO >>> even without the main loop (I think!). >> >> Ah, that's a very good point. Indeed, you can perform IO in those >> cases specially when using vhost devices. >> >>> By setting a timer in the kernel that sends a signal to qemu, the kernel >>> will send that signal however broken qemu is. >> >> Got you now. That's probably better. Do you reckon a signal is >> preferable over SIGEV_THREAD? > > Not sure; probably the safest is getting the kernel to SIGKILL it - but > that's a complete nightmare to debug - your process just goes *pop* > with no apparent reason why. > I've not used SIGEV_THREAD - it looks promising though. I'm worried that SIGEV_THREAD could be a bit heavyweight (if it fires up a new thread each time). On the other hand, as you said, SIGKILL makes it harder to debug. Also, asking the kernel to defer the SIGKILL (ie. updating the timer) needs to come from Qemu itself (eg. a timer in the main loop, something we already ruled unsuitable, or a qmp command which constitutes an external dependency that we also ruled undesirable). >>> >>> OK, there's two reasons I think this isn't that bad/is good: >>> a) It's an external dependency - but if it fails the result is the >>> system fails, rather than the system keeps on running; so I think >>> that's the balance you were after; it's the opposite from >>> the external watchdog. >> >> Right. I like where you are coming from. And I think a mix of these >> may be the best way forwards. I'll elaborate on it below. >> >>> >>> b) You need some external system anyway to tell QEMU when it's >>> OK - what's your definitino of a failed system? >> >> The feature is targeted at providing a self-fencing mechanism for >> Qemu. If a host is unreachable for whatever reason (eg. sshd down, ovs >> died, oomkiller took services out, physical network failure), it >> should guarant
Re: Thoughts on VM fence infrastructure
On Tue, Oct 01, 2019 at 09:56:17AM +, Felipe Franciosi wrote: > > > > On Oct 1, 2019, at 9:23 AM, Dr. David Alan Gilbert > > wrote: > > > > * Felipe Franciosi (fel...@nutanix.com) wrote: > >> > >> > >>> On Sep 30, 2019, at 6:59 PM, Dr. David Alan Gilbert > >>> wrote: > >>> > >>> * Felipe Franciosi (fel...@nutanix.com) wrote: > > > > On Sep 30, 2019, at 6:11 PM, Dr. David Alan Gilbert > > wrote: > > > > * Felipe Franciosi (fel...@nutanix.com) wrote: > >> > >> > >>> On Sep 30, 2019, at 5:03 PM, Dr. David Alan Gilbert > >>> wrote: > >>> > >>> * Felipe Franciosi (fel...@nutanix.com) wrote: > Hi David, > > > On Sep 30, 2019, at 3:29 PM, Dr. David Alan Gilbert > > wrote: > > > > * Felipe Franciosi (fel...@nutanix.com) wrote: > >> Heyall, > >> > >> We have a use case where a host should self-fence (and all VMs > >> should > >> die) if it doesn't hear back from a heartbeat within a certain time > >> period. Lots of ideas were floated around where libvirt could take > >> care of killing VMs or a separate service could do it. The concern > >> with those is that various failures could lead to _those_ services > >> being unavailable and the fencing wouldn't be enforced as it > >> should. > >> > >> Ultimately, it feels like Qemu should be responsible for this > >> heartbeat and exit (or execute a custom callback) on timeout. > > > > It doesn't feel doing it inside qemu would be any safer; something > > outside QEMU can forcibly emit a kill -9 and qemu *will* stop. > > The argument above is that we would have to rely on this external > service being functional. Consider the case where the host is > dysfunctional, with this service perhaps crashed and a corrupt > filesystem preventing it from restarting. The VMs would never die. > >>> > >>> Yeh that could fail. > >>> > It feels like a Qemu timer-driven heartbeat check and calls abort() / > exit() would be more reliable. Thoughts? > >>> > >>> OK, yes; perhaps using a timer_create and telling it to send a fatal > >>> signal is pretty solid; it would take the kernel to do that once it's > >>> set. > >> > >> I'm confused about why the kernel needs to be involved. If this is a > >> timer off the Qemu main loop, it can just check on the heartbeat > >> condition (which should be customisable) and call abort() if that's > >> not satisfied. If you agree on that I'd like to talk about how that > >> check could be made customisable. > > > > There are times when the main loop can get blocked even though the CPU > > threads can be running and can in some configurations perform IO > > even without the main loop (I think!). > > Ah, that's a very good point. Indeed, you can perform IO in those > cases specially when using vhost devices. > > > By setting a timer in the kernel that sends a signal to qemu, the kernel > > will send that signal however broken qemu is. > > Got you now. That's probably better. Do you reckon a signal is > preferable over SIGEV_THREAD? > >>> > >>> Not sure; probably the safest is getting the kernel to SIGKILL it - but > >>> that's a complete nightmare to debug - your process just goes *pop* > >>> with no apparent reason why. > >>> I've not used SIGEV_THREAD - it looks promising though. > >> > >> I'm worried that SIGEV_THREAD could be a bit heavyweight (if it fires > >> up a new thread each time). On the other hand, as you said, SIGKILL > >> makes it harder to debug. > >> > >> Also, asking the kernel to defer the SIGKILL (ie. updating the timer) > >> needs to come from Qemu itself (eg. a timer in the main loop, > >> something we already ruled unsuitable, or a qmp command which > >> constitutes an external dependency that we also ruled undesirable). > > > > OK, there's two reasons I think this isn't that bad/is good: > > a) It's an external dependency - but if it fails the result is the > > system fails, rather than the system keeps on running; so I think > > that's the balance you were after; it's the opposite from > > the external watchdog. > > Right. I like where you are coming from. And I think a mix of these > may be the best way forwards. I'll elaborate on it below. > > > > > b) You need some external system anyway to tell QEMU when it's > > OK - what's your definitino of a failed system? > > The feature is targeted at providing a self-fencing mechanism for > Qemu. If a host is unreachable for whatever reason (eg. sshd down, ovs > died, oomkiller took services out, physical network failure), it > should guarantee that VMs won't be running after a certain amount of > time. To your point, if
Re: Thoughts on VM fence infrastructure
* Felipe Franciosi (fel...@nutanix.com) wrote: > > > > On Oct 1, 2019, at 9:23 AM, Dr. David Alan Gilbert > > wrote: > > > > * Felipe Franciosi (fel...@nutanix.com) wrote: > >> > >> > >>> On Sep 30, 2019, at 6:59 PM, Dr. David Alan Gilbert > >>> wrote: > >>> > >>> * Felipe Franciosi (fel...@nutanix.com) wrote: > > > > On Sep 30, 2019, at 6:11 PM, Dr. David Alan Gilbert > > wrote: > > > > * Felipe Franciosi (fel...@nutanix.com) wrote: > >> > >> > >>> On Sep 30, 2019, at 5:03 PM, Dr. David Alan Gilbert > >>> wrote: > >>> > >>> * Felipe Franciosi (fel...@nutanix.com) wrote: > Hi David, > > > On Sep 30, 2019, at 3:29 PM, Dr. David Alan Gilbert > > wrote: > > > > * Felipe Franciosi (fel...@nutanix.com) wrote: > >> Heyall, > >> > >> We have a use case where a host should self-fence (and all VMs > >> should > >> die) if it doesn't hear back from a heartbeat within a certain time > >> period. Lots of ideas were floated around where libvirt could take > >> care of killing VMs or a separate service could do it. The concern > >> with those is that various failures could lead to _those_ services > >> being unavailable and the fencing wouldn't be enforced as it > >> should. > >> > >> Ultimately, it feels like Qemu should be responsible for this > >> heartbeat and exit (or execute a custom callback) on timeout. > > > > It doesn't feel doing it inside qemu would be any safer; something > > outside QEMU can forcibly emit a kill -9 and qemu *will* stop. > > The argument above is that we would have to rely on this external > service being functional. Consider the case where the host is > dysfunctional, with this service perhaps crashed and a corrupt > filesystem preventing it from restarting. The VMs would never die. > >>> > >>> Yeh that could fail. > >>> > It feels like a Qemu timer-driven heartbeat check and calls abort() / > exit() would be more reliable. Thoughts? > >>> > >>> OK, yes; perhaps using a timer_create and telling it to send a fatal > >>> signal is pretty solid; it would take the kernel to do that once it's > >>> set. > >> > >> I'm confused about why the kernel needs to be involved. If this is a > >> timer off the Qemu main loop, it can just check on the heartbeat > >> condition (which should be customisable) and call abort() if that's > >> not satisfied. If you agree on that I'd like to talk about how that > >> check could be made customisable. > > > > There are times when the main loop can get blocked even though the CPU > > threads can be running and can in some configurations perform IO > > even without the main loop (I think!). > > Ah, that's a very good point. Indeed, you can perform IO in those > cases specially when using vhost devices. > > > By setting a timer in the kernel that sends a signal to qemu, the kernel > > will send that signal however broken qemu is. > > Got you now. That's probably better. Do you reckon a signal is > preferable over SIGEV_THREAD? > >>> > >>> Not sure; probably the safest is getting the kernel to SIGKILL it - but > >>> that's a complete nightmare to debug - your process just goes *pop* > >>> with no apparent reason why. > >>> I've not used SIGEV_THREAD - it looks promising though. > >> > >> I'm worried that SIGEV_THREAD could be a bit heavyweight (if it fires > >> up a new thread each time). On the other hand, as you said, SIGKILL > >> makes it harder to debug. > >> > >> Also, asking the kernel to defer the SIGKILL (ie. updating the timer) > >> needs to come from Qemu itself (eg. a timer in the main loop, > >> something we already ruled unsuitable, or a qmp command which > >> constitutes an external dependency that we also ruled undesirable). > > > > OK, there's two reasons I think this isn't that bad/is good: > > a) It's an external dependency - but if it fails the result is the > > system fails, rather than the system keeps on running; so I think > > that's the balance you were after; it's the opposite from > > the external watchdog. > > Right. I like where you are coming from. And I think a mix of these > may be the best way forwards. I'll elaborate on it below. > > > > > b) You need some external system anyway to tell QEMU when it's > > OK - what's your definitino of a failed system? > > The feature is targeted at providing a self-fencing mechanism for > Qemu. If a host is unreachable for whatever reason (eg. sshd down, ovs > died, oomkiller took services out, physical network failure), it > should guarantee that VMs won't be running after a certain amount of > time. To your point, if this external softw
Re: Thoughts on VM fence infrastructure
> On Oct 1, 2019, at 9:23 AM, Dr. David Alan Gilbert > wrote: > > * Felipe Franciosi (fel...@nutanix.com) wrote: >> >> >>> On Sep 30, 2019, at 6:59 PM, Dr. David Alan Gilbert >>> wrote: >>> >>> * Felipe Franciosi (fel...@nutanix.com) wrote: > On Sep 30, 2019, at 6:11 PM, Dr. David Alan Gilbert > wrote: > > * Felipe Franciosi (fel...@nutanix.com) wrote: >> >> >>> On Sep 30, 2019, at 5:03 PM, Dr. David Alan Gilbert >>> wrote: >>> >>> * Felipe Franciosi (fel...@nutanix.com) wrote: Hi David, > On Sep 30, 2019, at 3:29 PM, Dr. David Alan Gilbert > wrote: > > * Felipe Franciosi (fel...@nutanix.com) wrote: >> Heyall, >> >> We have a use case where a host should self-fence (and all VMs should >> die) if it doesn't hear back from a heartbeat within a certain time >> period. Lots of ideas were floated around where libvirt could take >> care of killing VMs or a separate service could do it. The concern >> with those is that various failures could lead to _those_ services >> being unavailable and the fencing wouldn't be enforced as it should. >> >> Ultimately, it feels like Qemu should be responsible for this >> heartbeat and exit (or execute a custom callback) on timeout. > > It doesn't feel doing it inside qemu would be any safer; something > outside QEMU can forcibly emit a kill -9 and qemu *will* stop. The argument above is that we would have to rely on this external service being functional. Consider the case where the host is dysfunctional, with this service perhaps crashed and a corrupt filesystem preventing it from restarting. The VMs would never die. >>> >>> Yeh that could fail. >>> It feels like a Qemu timer-driven heartbeat check and calls abort() / exit() would be more reliable. Thoughts? >>> >>> OK, yes; perhaps using a timer_create and telling it to send a fatal >>> signal is pretty solid; it would take the kernel to do that once it's >>> set. >> >> I'm confused about why the kernel needs to be involved. If this is a >> timer off the Qemu main loop, it can just check on the heartbeat >> condition (which should be customisable) and call abort() if that's >> not satisfied. If you agree on that I'd like to talk about how that >> check could be made customisable. > > There are times when the main loop can get blocked even though the CPU > threads can be running and can in some configurations perform IO > even without the main loop (I think!). Ah, that's a very good point. Indeed, you can perform IO in those cases specially when using vhost devices. > By setting a timer in the kernel that sends a signal to qemu, the kernel > will send that signal however broken qemu is. Got you now. That's probably better. Do you reckon a signal is preferable over SIGEV_THREAD? >>> >>> Not sure; probably the safest is getting the kernel to SIGKILL it - but >>> that's a complete nightmare to debug - your process just goes *pop* >>> with no apparent reason why. >>> I've not used SIGEV_THREAD - it looks promising though. >> >> I'm worried that SIGEV_THREAD could be a bit heavyweight (if it fires >> up a new thread each time). On the other hand, as you said, SIGKILL >> makes it harder to debug. >> >> Also, asking the kernel to defer the SIGKILL (ie. updating the timer) >> needs to come from Qemu itself (eg. a timer in the main loop, >> something we already ruled unsuitable, or a qmp command which >> constitutes an external dependency that we also ruled undesirable). > > OK, there's two reasons I think this isn't that bad/is good: > a) It's an external dependency - but if it fails the result is the > system fails, rather than the system keeps on running; so I think > that's the balance you were after; it's the opposite from > the external watchdog. Right. I like where you are coming from. And I think a mix of these may be the best way forwards. I'll elaborate on it below. > > b) You need some external system anyway to tell QEMU when it's > OK - what's your definitino of a failed system? The feature is targeted at providing a self-fencing mechanism for Qemu. If a host is unreachable for whatever reason (eg. sshd down, ovs died, oomkiller took services out, physical network failure), it should guarantee that VMs won't be running after a certain amount of time. To your point, if this external software doesn't come in and touch the file, that's because it can't reach the host or it wants the host to self-fence. The qualifying Qemus should therefore be considered dead after a "deadline" period (since the last time the control file was touched). > >> What if, when self-fencing is enabled
Re: Thoughts on VM fence infrastructure
* Felipe Franciosi (fel...@nutanix.com) wrote: > > > > On Sep 30, 2019, at 6:59 PM, Dr. David Alan Gilbert > > wrote: > > > > * Felipe Franciosi (fel...@nutanix.com) wrote: > >> > >> > >>> On Sep 30, 2019, at 6:11 PM, Dr. David Alan Gilbert > >>> wrote: > >>> > >>> * Felipe Franciosi (fel...@nutanix.com) wrote: > > > > On Sep 30, 2019, at 5:03 PM, Dr. David Alan Gilbert > > wrote: > > > > * Felipe Franciosi (fel...@nutanix.com) wrote: > >> Hi David, > >> > >>> On Sep 30, 2019, at 3:29 PM, Dr. David Alan Gilbert > >>> wrote: > >>> > >>> * Felipe Franciosi (fel...@nutanix.com) wrote: > Heyall, > > We have a use case where a host should self-fence (and all VMs should > die) if it doesn't hear back from a heartbeat within a certain time > period. Lots of ideas were floated around where libvirt could take > care of killing VMs or a separate service could do it. The concern > with those is that various failures could lead to _those_ services > being unavailable and the fencing wouldn't be enforced as it should. > > Ultimately, it feels like Qemu should be responsible for this > heartbeat and exit (or execute a custom callback) on timeout. > >>> > >>> It doesn't feel doing it inside qemu would be any safer; something > >>> outside QEMU can forcibly emit a kill -9 and qemu *will* stop. > >> > >> The argument above is that we would have to rely on this external > >> service being functional. Consider the case where the host is > >> dysfunctional, with this service perhaps crashed and a corrupt > >> filesystem preventing it from restarting. The VMs would never die. > > > > Yeh that could fail. > > > >> It feels like a Qemu timer-driven heartbeat check and calls abort() / > >> exit() would be more reliable. Thoughts? > > > > OK, yes; perhaps using a timer_create and telling it to send a fatal > > signal is pretty solid; it would take the kernel to do that once it's > > set. > > I'm confused about why the kernel needs to be involved. If this is a > timer off the Qemu main loop, it can just check on the heartbeat > condition (which should be customisable) and call abort() if that's > not satisfied. If you agree on that I'd like to talk about how that > check could be made customisable. > >>> > >>> There are times when the main loop can get blocked even though the CPU > >>> threads can be running and can in some configurations perform IO > >>> even without the main loop (I think!). > >> > >> Ah, that's a very good point. Indeed, you can perform IO in those > >> cases specially when using vhost devices. > >> > >>> By setting a timer in the kernel that sends a signal to qemu, the kernel > >>> will send that signal however broken qemu is. > >> > >> Got you now. That's probably better. Do you reckon a signal is > >> preferable over SIGEV_THREAD? > > > > Not sure; probably the safest is getting the kernel to SIGKILL it - but > > that's a complete nightmare to debug - your process just goes *pop* > > with no apparent reason why. > > I've not used SIGEV_THREAD - it looks promising though. > > I'm worried that SIGEV_THREAD could be a bit heavyweight (if it fires > up a new thread each time). On the other hand, as you said, SIGKILL > makes it harder to debug. > > Also, asking the kernel to defer the SIGKILL (ie. updating the timer) > needs to come from Qemu itself (eg. a timer in the main loop, > something we already ruled unsuitable, or a qmp command which > constitutes an external dependency that we also ruled undesirable). OK, there's two reasons I think this isn't that bad/is good: a) It's an external dependency - but if it fails the result is the system fails, rather than the system keeps on running; so I think that's the balance you were after; it's the opposite from the external watchdog. b) You need some external system anyway to tell QEMU when it's OK - what's your definitino of a failed system? > What if, when self-fencing is enabled, Qemu kicks off a new thread > from the start which does nothing but periodically wake up, verify the > heartbeat condition and log()+abort() if required? (Then we wouldn't > need the kernel timer.) I'd make that thread bump the kernel timer along. > > > >> I'm still wondering how to make this customisable so that different > >> types of heartbeat could be implemented (preferably without creating > >> external dependencies per discussion above). Thoughts welcome. > > > > Yes, you need something to enable it, and some safe way to retrigger > > the timer. A qmp command marked as 'oob' might be the right way - > > another qm command can't block it. > > This qmp approach is slightly different than the external dependency > that itself kills Qemu; if it doesn't run, then Qemu die
Re: Thoughts on VM fence infrastructure
> On Sep 30, 2019, at 8:45 PM, Rafael David Tinoco > wrote: > > There are times when the main loop can get blocked even though the CPU threads can be running and can in some configurations perform IO even without the main loop (I think!). >>> Ah, that's a very good point. Indeed, you can perform IO in those >>> cases specially when using vhost devices. >>> By setting a timer in the kernel that sends a signal to qemu, the kernel will send that signal however broken qemu is. >>> Got you now. That's probably better. Do you reckon a signal is >>> preferable over SIGEV_THREAD? >> Not sure; probably the safest is getting the kernel to SIGKILL it - but >> that's a complete nightmare to debug - your process just goes *pop* >> with no apparent reason why. >> I've not used SIGEV_THREAD - it looks promising though. > > Sorry to "enter" the discussion, but, in "real" HW, its not by accident > that watchdog devices timeout generates a NMI to CPUs, causing the > kernel to handle the interrupt - and panic (or to take other action set > by specific watchdog drivers that re-implements the default ones). Not sure what you mean by "sorry"... thanks for joining. :) > Can't you simple "inject" a NMI in all guest vCPUs BEFORE you take any > action in QEMU itself? Just like the virtual watchdog device would do, > from inside the guest (/dev/watchdog), but capable of being updated by > outside, in this case of yours (if I understood correctly). It's unclear to me how this relates to this use case, perhaps that's not clear. The idea is that on various cloud deployments, a host could be temporarily unavailable. Imagine that a network cable snapped. A management layer could then restart the unreachable VMs elsewhere (as part of High Availability offerings), but it needs to ensure that disconnected host is not just going to come back from the dead with older incarnations of the VMs running. (Imagine that someone replaced the broken network cable.) That would result in lots of issues from colliding IP addresses to different writers on shared storage leading to data corruption. The ask is for a mechanism to fence the host, essentially causing all (or selected) VMs on that host to die. There are several mechanisms for that, mostly requiring some sort of HW support (eg. STONITH). Those are often focused on cases where the host requires manual intervention to recover or at least a reset. I'm looking to implement a mechanism for self-fencing, which doesn't require external hardware and cover most failure scenarios (from partially/totally broken hosts to simply a temporary network failure). In several cases rebooting the host is unnecessary; just ensuring the VMs are down is enough. That's almost always true on temporary network unavailability (eg. split network). > Possibly you would have to have a dedicated loop for this "watchdog > device" (AIO threads ?) not to compete with existing coroutines/BH Tasks > and their jittering on your "realtime watchdog needs". Only when this feature is needed (which isn't the case for most people), there would be an extra thread (according to the latest proposal) which is mostly idle. It would wake up every few seconds and stat() a file, which is a very lightweight operation. That would not measurably impact/jitter other work. > Regarding remaining existing I/OS for the guest's devices in question > (vhost/vhost-user etc), would be just like a real host where the "bus" > received commands, but sender died right after... I hope the above clarifies the idea. Cheers, F.
Re: Thoughts on VM fence infrastructure
>>> There are times when the main loop can get blocked even though the CPU >>> threads can be running and can in some configurations perform IO >>> even without the main loop (I think!). >> Ah, that's a very good point. Indeed, you can perform IO in those >> cases specially when using vhost devices. >> >>> By setting a timer in the kernel that sends a signal to qemu, the kernel >>> will send that signal however broken qemu is. >> Got you now. That's probably better. Do you reckon a signal is >> preferable over SIGEV_THREAD? > Not sure; probably the safest is getting the kernel to SIGKILL it - but > that's a complete nightmare to debug - your process just goes *pop* > with no apparent reason why. > I've not used SIGEV_THREAD - it looks promising though. Sorry to "enter" the discussion, but, in "real" HW, its not by accident that watchdog devices timeout generates a NMI to CPUs, causing the kernel to handle the interrupt - and panic (or to take other action set by specific watchdog drivers that re-implements the default ones). Can't you simple "inject" a NMI in all guest vCPUs BEFORE you take any action in QEMU itself? Just like the virtual watchdog device would do, from inside the guest (/dev/watchdog), but capable of being updated by outside, in this case of yours (if I understood correctly). Possibly you would have to have a dedicated loop for this "watchdog device" (AIO threads ?) not to compete with existing coroutines/BH Tasks and their jittering on your "realtime watchdog needs". Regarding remaining existing I/OS for the guest's devices in question (vhost/vhost-user etc), would be just like a real host where the "bus" received commands, but sender died right after...
Re: Thoughts on VM fence infrastructure
> On Sep 30, 2019, at 6:59 PM, Dr. David Alan Gilbert > wrote: > > * Felipe Franciosi (fel...@nutanix.com) wrote: >> >> >>> On Sep 30, 2019, at 6:11 PM, Dr. David Alan Gilbert >>> wrote: >>> >>> * Felipe Franciosi (fel...@nutanix.com) wrote: > On Sep 30, 2019, at 5:03 PM, Dr. David Alan Gilbert > wrote: > > * Felipe Franciosi (fel...@nutanix.com) wrote: >> Hi David, >> >>> On Sep 30, 2019, at 3:29 PM, Dr. David Alan Gilbert >>> wrote: >>> >>> * Felipe Franciosi (fel...@nutanix.com) wrote: Heyall, We have a use case where a host should self-fence (and all VMs should die) if it doesn't hear back from a heartbeat within a certain time period. Lots of ideas were floated around where libvirt could take care of killing VMs or a separate service could do it. The concern with those is that various failures could lead to _those_ services being unavailable and the fencing wouldn't be enforced as it should. Ultimately, it feels like Qemu should be responsible for this heartbeat and exit (or execute a custom callback) on timeout. >>> >>> It doesn't feel doing it inside qemu would be any safer; something >>> outside QEMU can forcibly emit a kill -9 and qemu *will* stop. >> >> The argument above is that we would have to rely on this external >> service being functional. Consider the case where the host is >> dysfunctional, with this service perhaps crashed and a corrupt >> filesystem preventing it from restarting. The VMs would never die. > > Yeh that could fail. > >> It feels like a Qemu timer-driven heartbeat check and calls abort() / >> exit() would be more reliable. Thoughts? > > OK, yes; perhaps using a timer_create and telling it to send a fatal > signal is pretty solid; it would take the kernel to do that once it's > set. I'm confused about why the kernel needs to be involved. If this is a timer off the Qemu main loop, it can just check on the heartbeat condition (which should be customisable) and call abort() if that's not satisfied. If you agree on that I'd like to talk about how that check could be made customisable. >>> >>> There are times when the main loop can get blocked even though the CPU >>> threads can be running and can in some configurations perform IO >>> even without the main loop (I think!). >> >> Ah, that's a very good point. Indeed, you can perform IO in those >> cases specially when using vhost devices. >> >>> By setting a timer in the kernel that sends a signal to qemu, the kernel >>> will send that signal however broken qemu is. >> >> Got you now. That's probably better. Do you reckon a signal is >> preferable over SIGEV_THREAD? > > Not sure; probably the safest is getting the kernel to SIGKILL it - but > that's a complete nightmare to debug - your process just goes *pop* > with no apparent reason why. > I've not used SIGEV_THREAD - it looks promising though. I'm worried that SIGEV_THREAD could be a bit heavyweight (if it fires up a new thread each time). On the other hand, as you said, SIGKILL makes it harder to debug. Also, asking the kernel to defer the SIGKILL (ie. updating the timer) needs to come from Qemu itself (eg. a timer in the main loop, something we already ruled unsuitable, or a qmp command which constitutes an external dependency that we also ruled undesirable). What if, when self-fencing is enabled, Qemu kicks off a new thread from the start which does nothing but periodically wake up, verify the heartbeat condition and log()+abort() if required? (Then we wouldn't need the kernel timer.) > >> I'm still wondering how to make this customisable so that different >> types of heartbeat could be implemented (preferably without creating >> external dependencies per discussion above). Thoughts welcome. > > Yes, you need something to enable it, and some safe way to retrigger > the timer. A qmp command marked as 'oob' might be the right way - > another qm command can't block it. This qmp approach is slightly different than the external dependency that itself kills Qemu; if it doesn't run, then Qemu dies because the kernel timer is not updated. But this is also a heavyweight approach. We are talking about a service that needs to frequently connect to all running VMs on a host to reset the timer. But it does allow for the customisable heartbeat: the logic behind what triggers the command is completely flexible. Thinking about this idea of a separate Qemu thread, one thing that came to mind is this: qemu -fence heartbeat=/path/to/file,deadline=60[,recheck=5] Qemu could fire up a thread that stat()s (every seconds or on a default interval) and log()+abort() the whole process if the last modification time of the file is older than . If goes away (ie. stat() gives ENOENT), then it either fences immedia
Re: Thoughts on VM fence infrastructure
* Felipe Franciosi (fel...@nutanix.com) wrote: > > > > On Sep 30, 2019, at 6:11 PM, Dr. David Alan Gilbert > > wrote: > > > > * Felipe Franciosi (fel...@nutanix.com) wrote: > >> > >> > >>> On Sep 30, 2019, at 5:03 PM, Dr. David Alan Gilbert > >>> wrote: > >>> > >>> * Felipe Franciosi (fel...@nutanix.com) wrote: > Hi David, > > > On Sep 30, 2019, at 3:29 PM, Dr. David Alan Gilbert > > wrote: > > > > * Felipe Franciosi (fel...@nutanix.com) wrote: > >> Heyall, > >> > >> We have a use case where a host should self-fence (and all VMs should > >> die) if it doesn't hear back from a heartbeat within a certain time > >> period. Lots of ideas were floated around where libvirt could take > >> care of killing VMs or a separate service could do it. The concern > >> with those is that various failures could lead to _those_ services > >> being unavailable and the fencing wouldn't be enforced as it should. > >> > >> Ultimately, it feels like Qemu should be responsible for this > >> heartbeat and exit (or execute a custom callback) on timeout. > > > > It doesn't feel doing it inside qemu would be any safer; something > > outside QEMU can forcibly emit a kill -9 and qemu *will* stop. > > The argument above is that we would have to rely on this external > service being functional. Consider the case where the host is > dysfunctional, with this service perhaps crashed and a corrupt > filesystem preventing it from restarting. The VMs would never die. > >>> > >>> Yeh that could fail. > >>> > It feels like a Qemu timer-driven heartbeat check and calls abort() / > exit() would be more reliable. Thoughts? > >>> > >>> OK, yes; perhaps using a timer_create and telling it to send a fatal > >>> signal is pretty solid; it would take the kernel to do that once it's > >>> set. > >> > >> I'm confused about why the kernel needs to be involved. If this is a > >> timer off the Qemu main loop, it can just check on the heartbeat > >> condition (which should be customisable) and call abort() if that's > >> not satisfied. If you agree on that I'd like to talk about how that > >> check could be made customisable. > > > > There are times when the main loop can get blocked even though the CPU > > threads can be running and can in some configurations perform IO > > even without the main loop (I think!). > > Ah, that's a very good point. Indeed, you can perform IO in those > cases specially when using vhost devices. > > > By setting a timer in the kernel that sends a signal to qemu, the kernel > > will send that signal however broken qemu is. > > Got you now. That's probably better. Do you reckon a signal is > preferable over SIGEV_THREAD? Not sure; probably the safest is getting the kernel to SIGKILL it - but that's a complete nightmare to debug - your process just goes *pop* with no apparent reason why. I've not used SIGEV_THREAD - it looks promising though. > I'm still wondering how to make this customisable so that different > types of heartbeat could be implemented (preferably without creating > external dependencies per discussion above). Thoughts welcome. Yes, you need something to enable it, and some safe way to retrigger the timer. A qmp command marked as 'oob' might be the right way - another qm command can't block it. Dave > F. > > > > >> > >>> > >>> IMHO the safer way is to kick the host off the network by reprogramming > >>> switches; so even if the qemu is actually alive it can't get anywhere. > >>> > >>> Dave > >> > >> Naturally some off-host STONITH is preferable, but that's not always > >> available. A self-fencing mechanism right at the heart of the emulator > >> can do the job without external hardware dependencies. > > > > Dave > > > >> Cheers, > >> Felipe > >> > >>> > >>> > Felipe > > > > >> Does something already exist for this purpose which could be used? > >> Would a generic Qemu-fencing infrastructure be something of interest? > > Dave > > > > > >> Cheers, > >> F. > >> > > -- > > Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK > > >>> -- > >>> Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK > >> > > -- > > Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK > -- Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
Re: Thoughts on VM fence infrastructure
> On Sep 30, 2019, at 6:11 PM, Dr. David Alan Gilbert > wrote: > > * Felipe Franciosi (fel...@nutanix.com) wrote: >> >> >>> On Sep 30, 2019, at 5:03 PM, Dr. David Alan Gilbert >>> wrote: >>> >>> * Felipe Franciosi (fel...@nutanix.com) wrote: Hi David, > On Sep 30, 2019, at 3:29 PM, Dr. David Alan Gilbert > wrote: > > * Felipe Franciosi (fel...@nutanix.com) wrote: >> Heyall, >> >> We have a use case where a host should self-fence (and all VMs should >> die) if it doesn't hear back from a heartbeat within a certain time >> period. Lots of ideas were floated around where libvirt could take >> care of killing VMs or a separate service could do it. The concern >> with those is that various failures could lead to _those_ services >> being unavailable and the fencing wouldn't be enforced as it should. >> >> Ultimately, it feels like Qemu should be responsible for this >> heartbeat and exit (or execute a custom callback) on timeout. > > It doesn't feel doing it inside qemu would be any safer; something > outside QEMU can forcibly emit a kill -9 and qemu *will* stop. The argument above is that we would have to rely on this external service being functional. Consider the case where the host is dysfunctional, with this service perhaps crashed and a corrupt filesystem preventing it from restarting. The VMs would never die. >>> >>> Yeh that could fail. >>> It feels like a Qemu timer-driven heartbeat check and calls abort() / exit() would be more reliable. Thoughts? >>> >>> OK, yes; perhaps using a timer_create and telling it to send a fatal >>> signal is pretty solid; it would take the kernel to do that once it's >>> set. >> >> I'm confused about why the kernel needs to be involved. If this is a >> timer off the Qemu main loop, it can just check on the heartbeat >> condition (which should be customisable) and call abort() if that's >> not satisfied. If you agree on that I'd like to talk about how that >> check could be made customisable. > > There are times when the main loop can get blocked even though the CPU > threads can be running and can in some configurations perform IO > even without the main loop (I think!). Ah, that's a very good point. Indeed, you can perform IO in those cases specially when using vhost devices. > By setting a timer in the kernel that sends a signal to qemu, the kernel > will send that signal however broken qemu is. Got you now. That's probably better. Do you reckon a signal is preferable over SIGEV_THREAD? I'm still wondering how to make this customisable so that different types of heartbeat could be implemented (preferably without creating external dependencies per discussion above). Thoughts welcome. F. > >> >>> >>> IMHO the safer way is to kick the host off the network by reprogramming >>> switches; so even if the qemu is actually alive it can't get anywhere. >>> >>> Dave >> >> Naturally some off-host STONITH is preferable, but that's not always >> available. A self-fencing mechanism right at the heart of the emulator >> can do the job without external hardware dependencies. > > Dave > >> Cheers, >> Felipe >> >>> >>> Felipe > >> Does something already exist for this purpose which could be used? >> Would a generic Qemu-fencing infrastructure be something of interest? > Dave > > >> Cheers, >> F. >> > -- > Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK >>> -- >>> Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK >> > -- > Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
Re: Thoughts on VM fence infrastructure
* Felipe Franciosi (fel...@nutanix.com) wrote: > > > > On Sep 30, 2019, at 5:03 PM, Dr. David Alan Gilbert > > wrote: > > > > * Felipe Franciosi (fel...@nutanix.com) wrote: > >> Hi David, > >> > >>> On Sep 30, 2019, at 3:29 PM, Dr. David Alan Gilbert > >>> wrote: > >>> > >>> * Felipe Franciosi (fel...@nutanix.com) wrote: > Heyall, > > We have a use case where a host should self-fence (and all VMs should > die) if it doesn't hear back from a heartbeat within a certain time > period. Lots of ideas were floated around where libvirt could take > care of killing VMs or a separate service could do it. The concern > with those is that various failures could lead to _those_ services > being unavailable and the fencing wouldn't be enforced as it should. > > Ultimately, it feels like Qemu should be responsible for this > heartbeat and exit (or execute a custom callback) on timeout. > >>> > >>> It doesn't feel doing it inside qemu would be any safer; something > >>> outside QEMU can forcibly emit a kill -9 and qemu *will* stop. > >> > >> The argument above is that we would have to rely on this external > >> service being functional. Consider the case where the host is > >> dysfunctional, with this service perhaps crashed and a corrupt > >> filesystem preventing it from restarting. The VMs would never die. > > > > Yeh that could fail. > > > >> It feels like a Qemu timer-driven heartbeat check and calls abort() / > >> exit() would be more reliable. Thoughts? > > > > OK, yes; perhaps using a timer_create and telling it to send a fatal > > signal is pretty solid; it would take the kernel to do that once it's > > set. > > I'm confused about why the kernel needs to be involved. If this is a > timer off the Qemu main loop, it can just check on the heartbeat > condition (which should be customisable) and call abort() if that's > not satisfied. If you agree on that I'd like to talk about how that > check could be made customisable. There are times when the main loop can get blocked even though the CPU threads can be running and can in some configurations perform IO even without the main loop (I think!). By setting a timer in the kernel that sends a signal to qemu, the kernel will send that signal however broken qemu is. > > > > > IMHO the safer way is to kick the host off the network by reprogramming > > switches; so even if the qemu is actually alive it can't get anywhere. > > > > Dave > > Naturally some off-host STONITH is preferable, but that's not always > available. A self-fencing mechanism right at the heart of the emulator > can do the job without external hardware dependencies. Dave > Cheers, > Felipe > > > > > > >> Felipe > >> > >>> > Does something already exist for this purpose which could be used? > Would a generic Qemu-fencing infrastructure be something of interest? > >>> Dave > >>> > >>> > Cheers, > F. > > >>> -- > >>> Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK > >> > > -- > > Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK > -- Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
Re: Thoughts on VM fence infrastructure
> On Sep 30, 2019, at 5:03 PM, Dr. David Alan Gilbert > wrote: > > * Felipe Franciosi (fel...@nutanix.com) wrote: >> Hi David, >> >>> On Sep 30, 2019, at 3:29 PM, Dr. David Alan Gilbert >>> wrote: >>> >>> * Felipe Franciosi (fel...@nutanix.com) wrote: Heyall, We have a use case where a host should self-fence (and all VMs should die) if it doesn't hear back from a heartbeat within a certain time period. Lots of ideas were floated around where libvirt could take care of killing VMs or a separate service could do it. The concern with those is that various failures could lead to _those_ services being unavailable and the fencing wouldn't be enforced as it should. Ultimately, it feels like Qemu should be responsible for this heartbeat and exit (or execute a custom callback) on timeout. >>> >>> It doesn't feel doing it inside qemu would be any safer; something >>> outside QEMU can forcibly emit a kill -9 and qemu *will* stop. >> >> The argument above is that we would have to rely on this external >> service being functional. Consider the case where the host is >> dysfunctional, with this service perhaps crashed and a corrupt >> filesystem preventing it from restarting. The VMs would never die. > > Yeh that could fail. > >> It feels like a Qemu timer-driven heartbeat check and calls abort() / >> exit() would be more reliable. Thoughts? > > OK, yes; perhaps using a timer_create and telling it to send a fatal > signal is pretty solid; it would take the kernel to do that once it's > set. I'm confused about why the kernel needs to be involved. If this is a timer off the Qemu main loop, it can just check on the heartbeat condition (which should be customisable) and call abort() if that's not satisfied. If you agree on that I'd like to talk about how that check could be made customisable. > > IMHO the safer way is to kick the host off the network by reprogramming > switches; so even if the qemu is actually alive it can't get anywhere. > > Dave Naturally some off-host STONITH is preferable, but that's not always available. A self-fencing mechanism right at the heart of the emulator can do the job without external hardware dependencies. Cheers, Felipe > > >> Felipe >> >>> Does something already exist for this purpose which could be used? Would a generic Qemu-fencing infrastructure be something of interest? >>> Dave >>> >>> Cheers, F. >>> -- >>> Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK >> > -- > Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
Re: Thoughts on VM fence infrastructure
* Felipe Franciosi (fel...@nutanix.com) wrote: > Hi David, > > > On Sep 30, 2019, at 3:29 PM, Dr. David Alan Gilbert > > wrote: > > > > * Felipe Franciosi (fel...@nutanix.com) wrote: > >> Heyall, > >> > >> We have a use case where a host should self-fence (and all VMs should > >> die) if it doesn't hear back from a heartbeat within a certain time > >> period. Lots of ideas were floated around where libvirt could take > >> care of killing VMs or a separate service could do it. The concern > >> with those is that various failures could lead to _those_ services > >> being unavailable and the fencing wouldn't be enforced as it should. > >> > >> Ultimately, it feels like Qemu should be responsible for this > >> heartbeat and exit (or execute a custom callback) on timeout. > > > > It doesn't feel doing it inside qemu would be any safer; something > > outside QEMU can forcibly emit a kill -9 and qemu *will* stop. > > The argument above is that we would have to rely on this external > service being functional. Consider the case where the host is > dysfunctional, with this service perhaps crashed and a corrupt > filesystem preventing it from restarting. The VMs would never die. Yeh that could fail. > It feels like a Qemu timer-driven heartbeat check and calls abort() / > exit() would be more reliable. Thoughts? OK, yes; perhaps using a timer_create and telling it to send a fatal signal is pretty solid; it would take the kernel to do that once it's set. IMHO the safer way is to kick the host off the network by reprogramming switches; so even if the qemu is actually alive it can't get anywhere. Dave > Felipe > > > > >> Does something already exist for this purpose which could be used? > >> Would a generic Qemu-fencing infrastructure be something of interest? > > Dave > > > > > >> Cheers, > >> F. > >> > > -- > > Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK > -- Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
Re: Thoughts on VM fence infrastructure
Hi David, > On Sep 30, 2019, at 3:29 PM, Dr. David Alan Gilbert > wrote: > > * Felipe Franciosi (fel...@nutanix.com) wrote: >> Heyall, >> >> We have a use case where a host should self-fence (and all VMs should >> die) if it doesn't hear back from a heartbeat within a certain time >> period. Lots of ideas were floated around where libvirt could take >> care of killing VMs or a separate service could do it. The concern >> with those is that various failures could lead to _those_ services >> being unavailable and the fencing wouldn't be enforced as it should. >> >> Ultimately, it feels like Qemu should be responsible for this >> heartbeat and exit (or execute a custom callback) on timeout. > > It doesn't feel doing it inside qemu would be any safer; something > outside QEMU can forcibly emit a kill -9 and qemu *will* stop. The argument above is that we would have to rely on this external service being functional. Consider the case where the host is dysfunctional, with this service perhaps crashed and a corrupt filesystem preventing it from restarting. The VMs would never die. It feels like a Qemu timer-driven heartbeat check and calls abort() / exit() would be more reliable. Thoughts? Felipe > >> Does something already exist for this purpose which could be used? >> Would a generic Qemu-fencing infrastructure be something of interest? > Dave > > >> Cheers, >> F. >> > -- > Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
Re: Thoughts on VM fence infrastructure
* Felipe Franciosi (fel...@nutanix.com) wrote: > Heyall, > > We have a use case where a host should self-fence (and all VMs should > die) if it doesn't hear back from a heartbeat within a certain time > period. Lots of ideas were floated around where libvirt could take > care of killing VMs or a separate service could do it. The concern > with those is that various failures could lead to _those_ services > being unavailable and the fencing wouldn't be enforced as it should. > > Ultimately, it feels like Qemu should be responsible for this > heartbeat and exit (or execute a custom callback) on timeout. It doesn't feel doing it inside qemu would be any safer; something outside QEMU can forcibly emit a kill -9 and qemu *will* stop. > Does something already exist for this purpose which could be used? > Would a generic Qemu-fencing infrastructure be something of interest? Dave > Cheers, > F. > -- Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
Thoughts on VM fence infrastructure
Heyall, We have a use case where a host should self-fence (and all VMs should die) if it doesn't hear back from a heartbeat within a certain time period. Lots of ideas were floated around where libvirt could take care of killing VMs or a separate service could do it. The concern with those is that various failures could lead to _those_ services being unavailable and the fencing wouldn't be enforced as it should. Ultimately, it feels like Qemu should be responsible for this heartbeat and exit (or execute a custom callback) on timeout. Does something already exist for this purpose which could be used? Would a generic Qemu-fencing infrastructure be something of interest? Cheers, F.
