Re: [Qemu-devel] bdrv_flush for qemu block drivers nbd, rbd and sheepdog
At Fri, 22 Oct 2010 10:47:44 +0200, Kevin Wolf wrote: > > Am 22.10.2010 07:43, schrieb MORITA Kazutaka: > > At Thu, 21 Oct 2010 16:07:28 +0200, > > Kevin Wolf wrote: > >> > >> Hi all, > >> > >> I'm currently looking into adding a return value to qemu's bdrv_flush > >> function and I noticed that your block drivers (nbd, rbd and sheepdog) > >> don't implement bdrv_flush at all. bdrv_flush is going to return > >> -ENOTSUP for any block driver not implementing this, effectively > >> breaking these three drivers for anything but cache=unsafe. > >> > >> Is there a specific reason why your drivers don't implement this? I > >> think I remember that one of the drivers always provides > >> cache=writethough semantics. It would be okay to silently "upgrade" to > >> cache=writethrough, so in this case I'd just need to add an empty > >> bdrv_flush implementation. > >> > >> Otherwise, we really cannot allow any option except cache=unsafe because > >> that's the semantics provided by the driver. > >> > >> In any case, I think it would be a good idea to implement a real > >> bdrv_flush function to allow the write-back cache modes cache=off and > >> cache=writeback in order to improve performance over writethrough. > >> > >> Is this possible with your protocols, or can the protocol be changed to > >> consider this? Any hints on how to proceed? > >> > > > > It is a bit difficult to implement an effective bdrv_flush in the > > sheepdog block driver. Sheepdog virtual disks are splited and > > distributed to all cluster servers, so the block driver needs to send > > flush requests to all of them. I'm not sure this could improve > > performance more than writethrough semantics. > > It could probably be optimized so that you only send flush requests to > servers that have actually received write requests since the last flush. > > But yes, that's probably a valid point. I guess there's only one way to > find out how it performs: Trying it out. Agreed, I'll try it out. > > > So I think it is better to support only writethrough semantics > > currently (I'll modify sheepdog server codes to open stored objects > > with O_SYNC or O_DIRECT) and leave write-back semantics as a future > > work. > > I agree, that makes sense. > > Note that O_DIRECT does not provide write-through semantics. It bypasses > the page cache, but it doesn't flush other caches like a volatile disk > write cache. If you want to use it, you still need explicit flushes or > O_DIRECT | O_SYNC. Thanks for your comment. I've modified server codes to use O_SYNC, so now sheepdog gives cache=writethrough semantics always. Kazutaka
Re: [Qemu-devel] bdrv_flush for qemu block drivers nbd, rbd and sheepdog
Am 22.10.2010 15:45, schrieb Anthony Liguori: >>> On a physical system, if you don't have a battery backed disk and you >>> enable the WC on your disk, then even with cache=writethrough we're unsafe. >>> >> I don't think that's right. O_SYNC should guarantee that the volatile >> disk cache is flushed. >> > > If your filesystem does the right thing which an awful lot of them don't > today. The list of really relevant filesystems is rather short, though. >>> Likewise, if you mount your filesystem with barrier=0, QEMU is unsafe. >>> >> Yeah, if you do something equivalent to cache=unsafe on a lower layer, >> then qemu can't do much about it. Maybe you can apply the same argument >> to NBD, even though it's unsafe by default. >> >> >>> QEMU can't guarantee safety. The underlying storage needs to be >>> configured correctly. As long as we're not introducing caching within >>> QEMU, I don't think we should assume we're unsafe. >>> >>> Do we have any place where we can add docs on a per-block format basis? >>> It would be good to at least mention for each block device how the >>> backing storage needed to be configured for safety. >>> >> docs/block-protocols.txt? >> > > Maybe docs/block/.txt? Would be a good home for the qed spec too. I think spec and documentation for users should be kept separate. I thought that's the reason why docs/specs/ exists. And if you exclude specs, I'm not sure if there's a lot left to say for each format. Having ten files under docs/block/ which consist of two lines each would be ridiculous. If contrary to my expectations we actually do have content for it, docs/block/.txt works for me as well. Kevin
Re: [Qemu-devel] bdrv_flush for qemu block drivers nbd, rbd and sheepdog
On 10/22/2010 08:57 AM, Kevin Wolf wrote: Am 22.10.2010 15:45, schrieb Anthony Liguori: On a physical system, if you don't have a battery backed disk and you enable the WC on your disk, then even with cache=writethrough we're unsafe. I don't think that's right. O_SYNC should guarantee that the volatile disk cache is flushed. If your filesystem does the right thing which an awful lot of them don't today. The list of really relevant filesystems is rather short, though. Likewise, if you mount your filesystem with barrier=0, QEMU is unsafe. Yeah, if you do something equivalent to cache=unsafe on a lower layer, then qemu can't do much about it. Maybe you can apply the same argument to NBD, even though it's unsafe by default. QEMU can't guarantee safety. The underlying storage needs to be configured correctly. As long as we're not introducing caching within QEMU, I don't think we should assume we're unsafe. Do we have any place where we can add docs on a per-block format basis? It would be good to at least mention for each block device how the backing storage needed to be configured for safety. docs/block-protocols.txt? Maybe docs/block/.txt? Would be a good home for the qed spec too. I think spec and documentation for users should be kept separate. I thought that's the reason why docs/specs/ exists. And if you exclude specs, I'm not sure if there's a lot left to say for each format. Having ten files under docs/block/ which consist of two lines each would be ridiculous. If contrary to my expectations we actually do have content for it, docs/block/.txt works for me as well. Okay, sounds reasonable. Regards, Anthony Liguori Kevin
Re: [Qemu-devel] bdrv_flush for qemu block drivers nbd, rbd and sheepdog
On 10/22/2010 08:35 AM, Kevin Wolf wrote: Am 22.10.2010 14:58, schrieb Anthony Liguori: On 10/22/2010 03:29 AM, Kevin Wolf wrote: I agree. Of course, as Laurent said a while ago, there is no specification for NBD, so it's hard to say what the intended semantics is. However, I did have a look at the nbdserver code and it looks as if it implements something similar to writethrough (namely fsync after each write) only if configured this way on the server side. qemu-nbd defaults to writethrough, but can be configured to use cache=none. So with either server qemu as a client can't tell whether the data is safe on disk or not. In my book this is a strong argument for refusing to open nbd connections with anything but cache=unsafe. On a physical system, if you don't have a battery backed disk and you enable the WC on your disk, then even with cache=writethrough we're unsafe. I don't think that's right. O_SYNC should guarantee that the volatile disk cache is flushed. If your filesystem does the right thing which an awful lot of them don't today. Likewise, if you mount your filesystem with barrier=0, QEMU is unsafe. Yeah, if you do something equivalent to cache=unsafe on a lower layer, then qemu can't do much about it. Maybe you can apply the same argument to NBD, even though it's unsafe by default. QEMU can't guarantee safety. The underlying storage needs to be configured correctly. As long as we're not introducing caching within QEMU, I don't think we should assume we're unsafe. Do we have any place where we can add docs on a per-block format basis? It would be good to at least mention for each block device how the backing storage needed to be configured for safety. docs/block-protocols.txt? Maybe docs/block/.txt? Would be a good home for the qed spec too. Regards, Anthony Liguori Kevin
Re: [Qemu-devel] bdrv_flush for qemu block drivers nbd, rbd and sheepdog
Am 22.10.2010 14:58, schrieb Anthony Liguori: > On 10/22/2010 03:29 AM, Kevin Wolf wrote: >>> I agree. >>> >> Of course, as Laurent said a while ago, there is no specification for >> NBD, so it's hard to say what the intended semantics is. >> >> However, I did have a look at the nbdserver code and it looks as if it >> implements something similar to writethrough (namely fsync after each >> write) only if configured this way on the server side. qemu-nbd defaults >> to writethrough, but can be configured to use cache=none. So with either >> server qemu as a client can't tell whether the data is safe on disk or not. >> >> In my book this is a strong argument for refusing to open nbd >> connections with anything but cache=unsafe. >> > > On a physical system, if you don't have a battery backed disk and you > enable the WC on your disk, then even with cache=writethrough we're unsafe. I don't think that's right. O_SYNC should guarantee that the volatile disk cache is flushed. > Likewise, if you mount your filesystem with barrier=0, QEMU is unsafe. Yeah, if you do something equivalent to cache=unsafe on a lower layer, then qemu can't do much about it. Maybe you can apply the same argument to NBD, even though it's unsafe by default. > QEMU can't guarantee safety. The underlying storage needs to be > configured correctly. As long as we're not introducing caching within > QEMU, I don't think we should assume we're unsafe. > > Do we have any place where we can add docs on a per-block format basis? > It would be good to at least mention for each block device how the > backing storage needed to be configured for safety. docs/block-protocols.txt? Kevin
Re: [Qemu-devel] bdrv_flush for qemu block drivers nbd, rbd and sheepdog
On 10/22/2010 03:29 AM, Kevin Wolf wrote: I agree. Of course, as Laurent said a while ago, there is no specification for NBD, so it's hard to say what the intended semantics is. However, I did have a look at the nbdserver code and it looks as if it implements something similar to writethrough (namely fsync after each write) only if configured this way on the server side. qemu-nbd defaults to writethrough, but can be configured to use cache=none. So with either server qemu as a client can't tell whether the data is safe on disk or not. In my book this is a strong argument for refusing to open nbd connections with anything but cache=unsafe. On a physical system, if you don't have a battery backed disk and you enable the WC on your disk, then even with cache=writethrough we're unsafe. Likewise, if you mount your filesystem with barrier=0, QEMU is unsafe. QEMU can't guarantee safety. The underlying storage needs to be configured correctly. As long as we're not introducing caching within QEMU, I don't think we should assume we're unsafe. Do we have any place where we can add docs on a per-block format basis? It would be good to at least mention for each block device how the backing storage needed to be configured for safety. Regards, Anthony Liguori Kevin
Re: [Qemu-devel] bdrv_flush for qemu block drivers nbd, rbd and sheepdog
Am 22.10.2010 07:43, schrieb MORITA Kazutaka: > At Thu, 21 Oct 2010 16:07:28 +0200, > Kevin Wolf wrote: >> >> Hi all, >> >> I'm currently looking into adding a return value to qemu's bdrv_flush >> function and I noticed that your block drivers (nbd, rbd and sheepdog) >> don't implement bdrv_flush at all. bdrv_flush is going to return >> -ENOTSUP for any block driver not implementing this, effectively >> breaking these three drivers for anything but cache=unsafe. >> >> Is there a specific reason why your drivers don't implement this? I >> think I remember that one of the drivers always provides >> cache=writethough semantics. It would be okay to silently "upgrade" to >> cache=writethrough, so in this case I'd just need to add an empty >> bdrv_flush implementation. >> >> Otherwise, we really cannot allow any option except cache=unsafe because >> that's the semantics provided by the driver. >> >> In any case, I think it would be a good idea to implement a real >> bdrv_flush function to allow the write-back cache modes cache=off and >> cache=writeback in order to improve performance over writethrough. >> >> Is this possible with your protocols, or can the protocol be changed to >> consider this? Any hints on how to proceed? >> > > It is a bit difficult to implement an effective bdrv_flush in the > sheepdog block driver. Sheepdog virtual disks are splited and > distributed to all cluster servers, so the block driver needs to send > flush requests to all of them. I'm not sure this could improve > performance more than writethrough semantics. It could probably be optimized so that you only send flush requests to servers that have actually received write requests since the last flush. But yes, that's probably a valid point. I guess there's only one way to find out how it performs: Trying it out. > So I think it is better to support only writethrough semantics > currently (I'll modify sheepdog server codes to open stored objects > with O_SYNC or O_DIRECT) and leave write-back semantics as a future > work. I agree, that makes sense. Note that O_DIRECT does not provide write-through semantics. It bypasses the page cache, but it doesn't flush other caches like a volatile disk write cache. If you want to use it, you still need explicit flushes or O_DIRECT | O_SYNC. Kevin
Re: [Qemu-devel] bdrv_flush for qemu block drivers nbd, rbd and sheepdog
Am 21.10.2010 21:32, schrieb Laurent Vivier: > Le jeudi 21 octobre 2010 à 10:07 -0500, Anthony Liguori a écrit : >> On 10/21/2010 09:07 AM, Kevin Wolf wrote: >>> Hi all, >>> >>> I'm currently looking into adding a return value to qemu's bdrv_flush >>> function and I noticed that your block drivers (nbd, rbd and sheepdog) >>> don't implement bdrv_flush at all. bdrv_flush is going to return >>> -ENOTSUP for any block driver not implementing this, effectively >>> breaking these three drivers for anything but cache=unsafe. >>> >>> Is there a specific reason why your drivers don't implement this? >> >> NBD doesn't have a notion of flush. Only read/write and the block-nbd >> implementation doesn't do write-caching so flush would be a nop. >> >> I'm not sure what the right semantics would be for QEMU. My guess is a >> nop flush. > > I agree. Of course, as Laurent said a while ago, there is no specification for NBD, so it's hard to say what the intended semantics is. However, I did have a look at the nbdserver code and it looks as if it implements something similar to writethrough (namely fsync after each write) only if configured this way on the server side. qemu-nbd defaults to writethrough, but can be configured to use cache=none. So with either server qemu as a client can't tell whether the data is safe on disk or not. In my book this is a strong argument for refusing to open nbd connections with anything but cache=unsafe. Kevin
Re: [Qemu-devel] bdrv_flush for qemu block drivers nbd, rbd and sheepdog
At Thu, 21 Oct 2010 16:07:28 +0200, Kevin Wolf wrote: > > Hi all, > > I'm currently looking into adding a return value to qemu's bdrv_flush > function and I noticed that your block drivers (nbd, rbd and sheepdog) > don't implement bdrv_flush at all. bdrv_flush is going to return > -ENOTSUP for any block driver not implementing this, effectively > breaking these three drivers for anything but cache=unsafe. > > Is there a specific reason why your drivers don't implement this? I > think I remember that one of the drivers always provides > cache=writethough semantics. It would be okay to silently "upgrade" to > cache=writethrough, so in this case I'd just need to add an empty > bdrv_flush implementation. > > Otherwise, we really cannot allow any option except cache=unsafe because > that's the semantics provided by the driver. > > In any case, I think it would be a good idea to implement a real > bdrv_flush function to allow the write-back cache modes cache=off and > cache=writeback in order to improve performance over writethrough. > > Is this possible with your protocols, or can the protocol be changed to > consider this? Any hints on how to proceed? > It is a bit difficult to implement an effective bdrv_flush in the sheepdog block driver. Sheepdog virtual disks are splited and distributed to all cluster servers, so the block driver needs to send flush requests to all of them. I'm not sure this could improve performance more than writethrough semantics. So I think it is better to support only writethrough semantics currently (I'll modify sheepdog server codes to open stored objects with O_SYNC or O_DIRECT) and leave write-back semantics as a future work. Thanks, Kazutaka
Re: [Qemu-devel] bdrv_flush for qemu block drivers nbd, rbd and sheepdog
Le jeudi 21 octobre 2010 à 10:07 -0500, Anthony Liguori a écrit : > On 10/21/2010 09:07 AM, Kevin Wolf wrote: > > Hi all, > > > > I'm currently looking into adding a return value to qemu's bdrv_flush > > function and I noticed that your block drivers (nbd, rbd and sheepdog) > > don't implement bdrv_flush at all. bdrv_flush is going to return > > -ENOTSUP for any block driver not implementing this, effectively > > breaking these three drivers for anything but cache=unsafe. > > > > Is there a specific reason why your drivers don't implement this? > > NBD doesn't have a notion of flush. Only read/write and the block-nbd > implementation doesn't do write-caching so flush would be a nop. > > I'm not sure what the right semantics would be for QEMU. My guess is a > nop flush. I agree. Regards, Laurent > Regards, > > Anthony Liguori > > > I > > think I remember that one of the drivers always provides > > cache=writethough semantics. It would be okay to silently "upgrade" to > > cache=writethrough, so in this case I'd just need to add an empty > > bdrv_flush implementation. > > > > Otherwise, we really cannot allow any option except cache=unsafe because > > that's the semantics provided by the driver. > > > > In any case, I think it would be a good idea to implement a real > > bdrv_flush function to allow the write-back cache modes cache=off and > > cache=writeback in order to improve performance over writethrough. > > > > Is this possible with your protocols, or can the protocol be changed to > > consider this? Any hints on how to proceed? > > > > Kevin > > > > > -- - laur...@vivier.eu -- "Tout ce qui est impossible reste à accomplir"Jules Verne "Things are only impossible until they're not" Jean-Luc Picard
Re: [Qemu-devel] bdrv_flush for qemu block drivers nbd, rbd and sheepdog
On 10/21/2010 09:07 AM, Kevin Wolf wrote: Hi all, I'm currently looking into adding a return value to qemu's bdrv_flush function and I noticed that your block drivers (nbd, rbd and sheepdog) don't implement bdrv_flush at all. bdrv_flush is going to return -ENOTSUP for any block driver not implementing this, effectively breaking these three drivers for anything but cache=unsafe. Is there a specific reason why your drivers don't implement this? NBD doesn't have a notion of flush. Only read/write and the block-nbd implementation doesn't do write-caching so flush would be a nop. I'm not sure what the right semantics would be for QEMU. My guess is a nop flush. Regards, Anthony Liguori I think I remember that one of the drivers always provides cache=writethough semantics. It would be okay to silently "upgrade" to cache=writethrough, so in this case I'd just need to add an empty bdrv_flush implementation. Otherwise, we really cannot allow any option except cache=unsafe because that's the semantics provided by the driver. In any case, I think it would be a good idea to implement a real bdrv_flush function to allow the write-back cache modes cache=off and cache=writeback in order to improve performance over writethrough. Is this possible with your protocols, or can the protocol be changed to consider this? Any hints on how to proceed? Kevin
