* Peter Xu ([email protected]) wrote: > On Tue, Jan 20, 2026 at 07:04:09PM +0000, Dr. David Alan Gilbert wrote:
<snip> > > > (2) Failure happens _after_ applying the new checkpoint, but _before_ > > > the > > > whole checkpoint is applied. > > > > > > To be explicit, consider qemu_load_device_state() when the process > > > of > > > colo_incoming_process_checkpoint() failed. It means SVM applied > > > partial of PVM's checkpoint, I think it should mean PVM is > > > completely > > > corrupted. > > > > As long as the SVM has got the entire checkpoint, then it *can* apply it all > > and carry on from that point. > > Does it mean we assert() that qemu_load_device_state() will always success > for COLO syncs? Not sure; I'd expect if that load fails then the SVM fails; if that happens on a periodic checkpoint then the PVM should carry on. > Logically post_load() can invoke anything and I'm not sure if something can > start to fail, but I confess I don't know an existing device that can > trigger it. Like a postcopy, it shouldn't fail unless there's an underlying failure (e.g. storage died) > Lukas told me something was broken though with pc machine type, on > post_load() not re-entrant. I think it might be possible though when > post_load() is relevant to some device states (that guest driver can change > between two checkpoint loads), but that's still only theoretical. So maybe > we can indeed assert it here. I don't understand that non re-entrant bit? > > > > > Here either (1.b) or (2) seems fatal to me on the whole high level design. > > > Periodical syncs with x-checkpoint-delay can make this easier to happen, > > > so > > > larger windows of critical failures. That's also why I think it's > > > confusing COLO prefers more checkpoints - while it helps sync things up, > > > it > > > enlarges high risk window and overall overhead. > > > > No, there should be no point at which a failure leaves the SVM without a > > checkpoint > > that it can apply to take over. > > > > > > > > I have quite a few more performance and cleanup patches on my hands, > > > > > > for example to transfer dirty memory between checkpoints. > > > > > > > > > > > > > > > > > > > > IIUC, the critical path of COLO shouldn't be migration on its > > > > > > > own? It > > > > > > > should be when heartbeat gets lost; that normally should happen > > > > > > > when two > > > > > > > VMs are in sync. In this path, I don't see how multifd helps.. > > > > > > > because > > > > > > > there's no migration happening, only the src recording what has > > > > > > > changed. > > > > > > > Hence I think some number with description of the measurements > > > > > > > may help us > > > > > > > understand how important multifd is to COLO. > > > > > > > > > > > > > > Supporting multifd will cause new COLO functions to inject into > > > > > > > core > > > > > > > migration code paths (even if not much..). I want to make sure > > > > > > > such (new) > > > > > > > complexity is justified. I also want to avoid introducing a > > > > > > > feature only > > > > > > > because "we have XXX, then let's support XXX in COLO too, maybe > > > > > > > some day > > > > > > > it'll be useful". > > > > > > > > > > > > What COLO needs from migration at the low level: > > > > > > > > > > > > Primary/Outgoing side: > > > > > > > > > > > > Not much actually, we just need a way to incrementally send the > > > > > > dirtied memory and the full device state. > > > > > > Also, we ensure that migration never actually finishes since we will > > > > > > never do a switchover. For example we never set > > > > > > RAMState::last_stage with COLO. > > > > > > > > > > > > Secondary/Incoming side: > > > > > > > > > > > > colo cache: > > > > > > Since the secondary always needs to be ready to take over (even > > > > > > during > > > > > > checkpointing), we can not write the received ram pages directly to > > > > > > the guest ram to prevent having half of the old and half of the new > > > > > > contents. > > > > > > So we redirect the received ram pages to the colo cache. This is > > > > > > basically a mirror of the primary side ram. > > > > > > It also simplifies the primary side since from it's point of view > > > > > > it's > > > > > > just a normal migration target. So primary side doesn't have to care > > > > > > about dirtied pages on the secondary for example. > > > > > > > > > > > > Dirty Bitmap: > > > > > > With COLO we also need a dirty bitmap on the incoming side to track > > > > > > 1. pages dirtied by the secondary guest > > > > > > 2. pages dirtied by the primary guest (incoming ram pages) > > > > > > In the last step during the checkpointing, this bitmap is then used > > > > > > to overwrite the guest ram with the colo cache so the secondary > > > > > > guest > > > > > > is in sync with the primary guest. > > > > > > > > > > > > All this individually is very little code as you can see from my > > > > > > multifd patch. Just something to keep in mind I guess. > > > > > > > > > > > > > > > > > > At the high level we have the COLO framework outgoing and incoming > > > > > > threads which just tell the migration code to: > > > > > > Send all ram pages (qemu_savevm_live_state()) on the outgoing side > > > > > > paired with a qemu_loadvm_state_main on the incoming side. > > > > > > Send the device state (qemu_save_device_state()) paired with writing > > > > > > that stream to a buffer on the incoming side. > > > > > > And finally flusing the colo cache and loading the device state on > > > > > > the > > > > > > incoming side. > > > > > > > > > > > > And of course we coordinate with the colo block replication and > > > > > > colo-compare. > > > > > > > > > > Thank you. Maybe you should generalize some of the explanations and > > > > > put it > > > > > into docs/devel/migration/ somewhere. I think many of them are not > > > > > mentioned in the doc on how COLO works internally. > > > > > > > > > > Let me ask some more questions while I'm reading COLO today: > > > > > > > > > > - For each of the checkpoint (colo_do_checkpoint_transaction()), COLO > > > > > will > > > > > do the following: > > > > > > > > > > bql_lock() > > > > > vm_stop_force_state(RUN_STATE_COLO) # stop vm > > > > > bql_unlock() > > > > > > > > > > ... > > > > > > > > > > bql_lock() > > > > > qemu_save_device_state() # into a temp buffer fb > > > > > bql_unlock() > > > > > > > > > > ... > > > > > > > > > > qemu_savevm_state_complete_precopy() # send RAM, directly to > > > > > the wire > > > > > qemu_put_buffer(fb) # push temp buffer fb to > > > > > wire > > > > > > > > > > ... > > > > > > > > > > bql_lock() > > > > > vm_start() # start vm > > > > > bql_unlock() > > > > > > > > > > A few questions that I didn't ask previously: > > > > > > > > > > - If VM is stopped anyway, why putting the device states into a temp > > > > > buffer, instead of using what we already have for precopy phase, > > > > > or > > > > > just push everything directly to the wire? > > > > > > > > Actually we only do that to get the size of the device state and send > > > > the size out-of-band, since we can not use qemu_load_device_state() > > > > directly on the secondary side and look for the in-band EOF. > > > > > > I also don't understand why the size is needed.. > > > > > > Currently the streaming protocol for COLO is: > > > > > > - ... > > > - COLO_MESSAGE_VMSTATE_SEND > > > - RAM data > > > - EOF > > > - COLO_MESSAGE_VMSTATE_SIZE > > > - non-RAM data > > > - EOF > > > > > > My question is about, why can't we do this instead? > > > > > > - ... > > > - COLO_MESSAGE_VMSTATE_SEND > > > - RAM data > > [1] > > > > - non-RAM data > > > - EOF > > > > > > If the VM is stoppped during the whole process anyway.. > > > > > > Here RAM/non-RAM data all are vmstates, and logically can also be loaded > > > in > > > one shot of a vmstate load loop. > > > > You might be able to; in that case you would have to stream the > > entire thing into a buffer on the secondary rather than applying the > > RAM updates to the colo cache. > > I thought the colo cache is already such a buffering when receiving at [1] > above? Then we need to flush the colo cache (including scan the SVM bitmap > and only flush those pages in colo cache) like before. > > If something went wrong (e.g. channel broken during receiving non-ram > device states), SVM can directly drop all colo cache as the latest > checkpoint isn't complete. Oh, I think I've remembered why it's necessary to split it into RAM and non-RAM; you can't parse a non-RAM stream and know when you've got an EOF flag in the stream; especially for stuff that's open coded (like some of virtio); so there's no way to write a 'load until EOF' into a simple RAM buffer; you need to be given an explicit size to know how much to expect. You could do it for the RAM, but you'd need to write a protocol parser to follow the stream to watch for the EOF. It's actuallly harder with multifd; how would you make a temporary buffer with multiple streams like that? > > The thought of using userfaultfd-write had floated around at some time > > as ways to optimise this. > > It's an interesting idea. Yes it looks working, but as Lukas said, it looks > still unbounded. > > One idea to provide a strict bound: > > - admin sets a proper buffer to limit the extra pages to remember on SVM, > should be much smaller than total guest mem, but admin should make sure > in 99.99% cases it won't hit the limit with a proper x-checkpoint-delay, > > - if limit triggered, both VMs needs to pause (initiated by SVM), SVM > needs to explicitly request a checkpoint to src, > > - VMs can only start again after two VMs sync again Right, that should be doable with a userfault-write. Dave > Thanks, > > -- > Peter Xu > -- -----Open up your eyes, open up your mind, open up your code ------- / Dr. David Alan Gilbert | Running GNU/Linux | Happy \ \ dave @ treblig.org | | In Hex / \ _________________________|_____ http://www.treblig.org |_______/
