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From: Daniel Stodden on 6 Aug 2010 17:30 On Fri, 2010-08-06 at 08:43 -0400, Christoph Hellwig wrote: > On Thu, Aug 05, 2010 at 02:07:42PM -0700, Daniel Stodden wrote: > > That one is read and well understood. > > Given that xen blkfront does not actually implement cache flushes > correctly that doesn't seem to be the case. Well, given the stuff below I might actually go and read it again, maybe I've been just too optimistic last time I did. :) > > I presently don't see a point in having the frontend perform its own > > pre or post flushes as long as there's a single queue in the block > > layer. But if the kernel drops the plain _TAG mode, there is no problem > > with that. Essentially the frontend may drain the queue as much as as it > > wants. It just won't buy you much if the backend I/O was actually > > buffered, other than adding latency to the transport. > > You do need the _FLUSH or _FUA modes (either with TAGs or DRAIN) to get > the block layer to send you pure cache flush requests (aka "empty > barriers") without this they don't work. They way the current barrier > code is implemented means you will always get manual cache flushes > before the actual barrier requests once you implement that. By using > the _FUA mode you can still do your own post flush. Okay. Well that's a frontend thing, let's absolutely fix according to kernel demand, and move on with whatever you guys are doing there. > I've been through doing all this, and given how hard it is to do a > semi-efficient drain in a backend driver, and given that non-Linux > guests don't even benefit from it just leaving the draining to the > guest is the easiest solution. Stop, now that's different thing, if we want to keep stuff simple (we really want) this asks for making draining the default mode for everyone? You basically want everybody to commit to a preflush, right? Only? Is that everything? Does the problem relate to merging barrier points, grouping frontends on shared storage, or am I missing something more general? Because otherwise it still sounds relatively straightforward. I wouldn't be against defaulting frontend barrier users to DRAIN if it's clearly beneficial for any kind of backend involved. For present blkback it's a no-brainer because we just map to the the blk queue, boring as we are. But even considering group synchronization, instead of just dumb serialization, the need for a guest queue drain doesn't look obvious. The backend would have to drain everybody else on its own terms anyway to find a good merge point. So I'm still wondering. Can you explain a little more what makes your backend depend on it? Otherwise one could always go and impose a couple extra flags on frontend authors, provided there's a benefit and it doesn't result in just mapping the entire QUEUE_ORDERED set into the control interface. :P But either way, that doesn't sound like a preferrable solution if we can spare it. > If you already have the draining around > and are confident that it gets all corner cases right you can of > course keep it and use the QUEUE_ORDERED_TAG_FLUSH/QUEUE_ORDERED_TAG_FUA > modes. But from dealing with data integrity issues in virtualized > environment I'm not confident that things will just work, both on the > backend side, especially if image formats are around, and also on the > guest side given that QUEUE_ORDERED_TAG* has zero real life testing. The image format stuff in Xen servers is not sitting immediately behind the frontend ring anymore. This one indeed orders with with a drain, but again we use the block layer to take care of that. Works, cheaply for now. > > Not sure if I understand your above comment regarding the flush and fua > > bits. Did you mean to indicate that _TAG on the frontend's request_queue > > is presently not coming up with the empty barrier request to make > > _explicit_ cache flushes happen? > > Yes. Well, I understand that _TAG is the only model in there which doesn't map easily to the concept of a full cache flush on the normal data path, after all it's the only one in there where the device wants to deal with it alone. Which happens to be exactly the reason why we wanted it in xen-blkfront. If it doesn't really work like that for a linux guest, tough luck. It certainly works for a backend sitting on the bio layer. To fully disqualify myself: What are the normal kernel entries going for a _full_ explicit cache flush? I'm only aware of BLKFLSBUF etc, and that even kicks down into the driver as far as I see, so I wonder who else would want empty barriers so badly under plain TAG ordering... > > That would be something which > > definitely needs a workaround in the frontend then. In that case, would > > PRE/POSTFLUSH help, to get a call into prepare_flush_fn, which might > > insert the tag itself then? It's sounds a bit over the top to combine > > this with a queue drain on the transport, but I'm rather after > > correctness. > > prepare_flush_fn is gone now. > > > I wonder if there's a userspace solution for that. Does e.g. fdatasync() > > deal with independent invocations other than serializing? > > fsync/fdatasync is serialized by i_mutex. That sounds like a fairly similar issue to me, despite starting out from a different layer. I understand that it's hardly up to a single disk queue to deal with that, just wondering about the overall impact. > > The blktap userspace component presently doesn't buffer, so a _DRAIN is > > sufficient. But if it did, then it'd be kinda cool if handled more > > carefully. If the kernel does it, all the better. > > Doesn't buffer as in using O_SYNC/O_DYSNC or O_DIRECT? Right. > You still need > to call fdatsync for the latter, to flush out transaction for block > allocations in sparse / fallocated images and Yup, this happens, most of our file-based backends run block preallocation synchronously before returning to aio. The rest I'm not sure (but don't care much about). > to flush the volatile > write cache of the host disks. I take it the RW_BARRIER at least in the fully allocated case is well taken care for? Just to make sure... Last time I checked I'm pretty sure I saw our aio writes completing with a proper hw barrier. Extra rules for sparse regions are already bad enough for a plain luserland interface, expecially since the lio documentation doesn't exactly seem to cry it out loud and far... :} Thanks, Daniel -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
From: Christoph Hellwig on 7 Aug 2010 13:30 On Fri, Aug 06, 2010 at 02:20:32PM -0700, Daniel Stodden wrote: > > I've been through doing all this, and given how hard it is to do a > > semi-efficient drain in a backend driver, and given that non-Linux > > guests don't even benefit from it just leaving the draining to the > > guest is the easiest solution. > > Stop, now that's different thing, if we want to keep stuff simple (we > really want) this asks for making draining the default mode for > everyone? > > You basically want everybody to commit to a preflush, right? Only? Is > that everything? Witht the barrier model we have in current kernels you basically need to a) do a drain (typically inside the guest) and you need to have a cache flush command if you have volatile write cache semantics. The cache flush command will be used for pre-flushes, standalone flushes and if you don't have a FUA bit in the protocol post-flushes. > So I'm still wondering. Can you explain a little more what makes your > backend depend on it? Which backend? Currently filesystems can in theory rely on the ordering semantics, although very few do. And we've not seen a working implementation except for draining for it - the _TAG versions exist, but they are basically untested, and no one has solved the issues of error handling for it yet. > Otherwise one could always go and impose a couple extra flags on > frontend authors, provided there's a benefit and it doesn't result in > just mapping the entire QUEUE_ORDERED set into the control interface. :P > But either way, that doesn't sound like a preferrable solution if we can > spare it. Basically the only think you need it a cache flush command right now, that solves everything the Linux kernel needs, as does windows or possibly other guests. The draining is something imposed on us by the current Linux barrier semantics, and I'm confident it will be a thing of the past by Linux 2.6.37. > Well, I understand that _TAG is the only model in there which doesn't > map easily to the concept of a full cache flush on the normal data path, > after all it's the only one in there where the device wants to deal with > it alone. Which happens to be exactly the reason why we wanted it in > xen-blkfront. If it doesn't really work like that for a linux guest, > tough luck. It certainly works for a backend sitting on the bio layer. I thikn you're another victim of the overloaded barrier concept. What the Linux barrier flags does is two only slightly related things: a) give the filesystem a way to flush volatile write caches and thus gurantee data integrity b) provide block level ordering losly modeled after the SCSI ordered tag model Practice has shown that we just need (a) in most cases, there's only two filesystems that can theoretically take advantage of (b), and even there I'm sure we could do better without the block level draining. The _TAG implementation of barriers is related to (b) only - the pure QUEUE_ORDERED_TAG is only safe if you do not have a volatile write cache - to do cache flushing you need the QUEUE_ORDERED_TAG_FLUSH or QUEUE_ORDERED_TAG_FUA modes. In theory we could also add another mode that not only integrates the post-flush in the _FUA mode but also a pre-flush into a single command, but so far there wasn't any demand, most likely because no on the wire storage protocol implements it. > To fully disqualify myself: What are the normal kernel entries going for > a _full_ explicit cache flush? The typical one is f(data)sync for the case where there have been no modifications of metadata, or when using an external log device. No metadata modifications are quite typical for databases or virtualization images, or other bulk storage that doesn't allocate space on the fly. > I'm only aware of BLKFLSBUF etc, and that > even kicks down into the driver as far as I see, so I wonder who else > would want empty barriers so badly under plain TAG ordering... It does issue normal write barriers when you have dirty metadata, else it sends empty barriers if supported. > > > The blktap userspace component presently doesn't buffer, so a _DRAIN is > > > sufficient. But if it did, then it'd be kinda cool if handled more > > > carefully. If the kernel does it, all the better. > > > > Doesn't buffer as in using O_SYNC/O_DYSNC or O_DIRECT? > > Right. Err, that was a question. For O_SYNC/O_DYSNC you don't need the explicit fsync. For O_DIRECT you do (or use O_SYNC/O_DYSNC in addition) > > to flush the volatile > > write cache of the host disks. > > I take it the RW_BARRIER at least in the fully allocated case is well > taken care for? Just to make sure... No, if you're using O_DIRECT you still need f(data)sync to flush out the host disk cache. > Last time I checked I'm pretty sure I saw our aio writes completing with > a proper hw barrier. Extra rules for sparse regions are already bad > enough for a plain luserland interface, expecially since the lio > documentation doesn't exactly seem to cry it out loud and far... :} All this will depends a lot on the filesystem. But if you're not doing any allocation and you're not using O_SYNC/O_DYSNC most filesystems will not send any barrier at all. The obvious exception is btrfs because it has to allocate new blocks anyway due to it's copy on write scheme. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
From: Daniel Stodden on 7 Aug 2010 19:20
On Sat, 2010-08-07 at 13:20 -0400, Christoph Hellwig wrote: > On Fri, Aug 06, 2010 at 02:20:32PM -0700, Daniel Stodden wrote: > > > I've been through doing all this, and given how hard it is to do a > > > semi-efficient drain in a backend driver, and given that non-Linux > > > guests don't even benefit from it just leaving the draining to the > > > guest is the easiest solution. > > > > Stop, now that's different thing, if we want to keep stuff simple (we > > really want) this asks for making draining the default mode for > > everyone? > > > > You basically want everybody to commit to a preflush, right? Only? Is > > that everything? > > Witht the barrier model we have in current kernels you basically need to > a) do a drain (typically inside the guest) and you need to have a cache > flush command if you have volatile write cache semantics. The cache > flush command will be used for pre-flushes, standalone flushes and > if you don't have a FUA bit in the protocol post-flushes. That's all true at the physical layer. I'm rather about the virtual one -- what consistutes the transport between the frontend and backend. So if the block queue above xen-blkfront wants to jump through a couple extra loops, such as declaring TAG_FUA mode, to realize proper out-of-band cache flushing, fine. Underneath a backend, whether that's blkback or qemu, that draining and flushing will happen on to the physical layer, too. Agreed. That still doesn't mean you have to impose a drain the transport in between. The block layer underneath the backend does all the draining necessary, with a request stream just submitted in-order and barrier bits set where the guest saw fit. Including an empty one for an explicit cache flush. Neither does a backend want to know how the physical layer will deal with it in detail, or can. Except for the NONE case, of course. And I still don't see where any backends can claim overall benefit from requiring the guest to drain. At that level, a "TAG" is the much simpler and efficient one. Even if it neither applies to a Linux guest, nor a caching disk. Especially the ones far below, underneath some image format. It maps well to the bio layer, it even maps well to a trivial datasync() implementation in userspace, and I don't see why it wouldn't map well to a non-trivial one either. These aren't just two shorted Linux block layers. So far I'd suggest we keep the ring model as TAG vs. NONE, fix xen-blkfront to keep the empty barrier stuff going, and keep additional details where they belong, which is on either end, respectively. On the Linux frontend side, does TAG_FUA sound about right to you? Because to me that appears to be the one with the least noise around the actual barrier request. According to barrier.txt, then I guess we will map the flush to an empty barrier on the ring and in turn drop a gratuitous empty barrier following that (?). I obviously didn't try that out yet. Please absolutely correct me so we maybe get it right this time. Also, is my understanding correct that on a Linux backend side, the empty barrier case at the bio layer isn't compromised? Provided all disk types declare established ordering modes. Which would be non-TAG for virtually anything I'm currently aware of. I hope that question is sufficiently silly. In the backend, we then keep mapping this to the normal data path above a gendisk. Pending some overdue optimizations for barriers on shared physical storage, I guess. Gulp. > > So I'm still wondering. Can you explain a little more what makes your > > backend depend on it? > > Which backend? My understanding so far was that you want to have a draining bit included as be the default mode on the frontend/backend link. Maybe I just got you wrong, in that case correct me and we get back to a proper frontend patch and drop half of this thread. If that's still the case, you need to enlighten me, I just don't seem to get it. We used to have one which actually was a DRAIN model, which was blktap v1. That's why the patch submitted has this DRAIN if no barrier mode was declared at all. We don't have a true application for that, it's mainly because nobody really wants to fix it. If we had to, we'd still rather fix it in the backend. The way I see it, that kind of thing is just not expensive enough to add any kind of complexity to the disk model than the idealized SCSI disk the Linux block layer has been modeled after, for ages. And still is. To me, you somewhat sound like you're working toward an entirely different queue model, but I might be mistaken. Well, that'd be *really* interesting, but not a particularly hot topic until you about to get there. I suspect we'd have throw the SCSI model over board entirely if we wanted to take advantage of it. At which point we'd be basically starting all over, that's hardly about a xen-blkfront bugfix then... :P > Currently filesystems can in theory rely on the ordering > semantics, although very few do. And we've not seen a working > implementation except for draining for it - the _TAG versions exist, > but they are basically untested, and no one has solved the issues of > error handling for it yet. > > > Otherwise one could always go and impose a couple extra flags on > > frontend authors, provided there's a benefit and it doesn't result in > > just mapping the entire QUEUE_ORDERED set into the control interface. :P > > But either way, that doesn't sound like a preferrable solution if we can > > spare it. > > Basically the only think you need it a cache flush command right now, > that solves everything the Linux kernel needs, as does windows or > possibly other guests. The draining is something imposed on us by > the current Linux barrier semantics, and I'm confident it will be a > thing of the past by Linux 2.6.37. > > > Well, I understand that _TAG is the only model in there which doesn't > > map easily to the concept of a full cache flush on the normal data path, > > after all it's the only one in there where the device wants to deal with > > it alone. Which happens to be exactly the reason why we wanted it in > > xen-blkfront. If it doesn't really work like that for a linux guest, > > tough luck. It certainly works for a backend sitting on the bio layer. > > I thikn you're another victim of the overloaded barrier concept. What > the Linux barrier flags does is two only slightly related things: > > a) give the filesystem a way to flush volatile write caches and thus > gurantee data integrity > b) provide block level ordering losly modeled after the SCSI ordered > tag model No victim, that's exactly the way I understand it. And without the overloading we wouldn't have this TAG discussion. We got ourselves b) as the virtual I/O model. We carried over a) for explicity cache flushes as well. Both because it's relatively painless, maps well to Linux dom0, and quite probably to any other backend design as well. All in turn just because it's so SCSIish. > Practice has shown that we just need (a) in most cases, there's only > two filesystems that can theoretically take advantage of (b), and even > there I'm sure we could do better without the block level draining. > > The _TAG implementation of barriers is related to (b) only - the pure > QUEUE_ORDERED_TAG is only safe if you do not have a volatile write > cache . That's not true. It just degrades the issue of a cache flush to a don't care to the driver on a sufficiently expensive disk. I don't mean to to imply it's not broken in blk-* if you state so, but that's the _model_ employed. I'm also not sure if even SCSI works that way in the explicit flush case, because I'm in that fortunate position of never having written a SCSI lld. It somewhat start to take it like it doesn't? Well, tough. I'm only after an idealizing TAG on the virtual layer. On the physical layer with a Linux dom0 it may have no practical application, but there it still works quite well for anyone, hence that that yet be fixed xen-blkfront patch. So to defend Jeremy and me, barrier.txt didn't exactly state it's plain broken, right? > - to do cache flushing you need the QUEUE_ORDERED_TAG_FLUSH or > QUEUE_ORDERED_TAG_FUA modes. In theory we could also add another > mode that not only integrates the post-flush in the _FUA mode but > also a pre-flush into a single command, but so far there wasn't > any demand, most likely because no on the wire storage protocol > implements it. > > > To fully disqualify myself: What are the normal kernel entries going for > > a _full_ explicit cache flush? > > The typical one is f(data)sync for the case where there have been no > modifications of metadata, or when using an external log device. > > No metadata modifications are quite typical for databases or > virtualization images, or other bulk storage that doesn't allocate space > on the fly. > > > I'm only aware of BLKFLSBUF etc, and that > > even kicks down into the driver as far as I see, so I wonder who else > > would want empty barriers so badly under plain TAG ordering... > > It does issue normal write barriers when you have dirty metadata, else > it sends empty barriers if supported. > > > > > The blktap userspace component presently doesn't buffer, so a _DRAIN is > > > > sufficient. But if it did, then it'd be kinda cool if handled more > > > > carefully. If the kernel does it, all the better. > > > > > > Doesn't buffer as in using O_SYNC/O_DYSNC or O_DIRECT? > > > > Right. > > Err, that was a question. For O_SYNC/O_DYSNC you don't need the > explicit fsync. For O_DIRECT you do (or use O_SYNC/O_DYSNC in addition) > > > > to flush the volatile > > > write cache of the host disks. > > > > I take it the RW_BARRIER at least in the fully allocated case is well > > taken care for? Just to make sure... > > No, if you're using O_DIRECT you still need f(data)sync to flush out > the host disk cache. > > > Last time I checked I'm pretty sure I saw our aio writes completing with > > a proper hw barrier. Extra rules for sparse regions are already bad > > enough for a plain luserland interface, expecially since the lio > > documentation doesn't exactly seem to cry it out loud and far... :} > > All this will depends a lot on the filesystem. But if you're not > doing any allocation and you're not using O_SYNC/O_DYSNC most > filesystems will not send any barrier at all. The obvious exception is > btrfs because it has to allocate new blocks anyway due to it's copy on > write scheme. Okay, that all sounds sufficiently terrible to just go on vacation. I'll be gone during the next week, can I return to bugging you about that stuff later on? Thanks a lot :) Daniel -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ |