On Thu, May 25 2023 at 7:39P -0400, Dave Chinner <david@xxxxxxxxxxxxx> wrote: > On Wed, May 24, 2023 at 04:02:49PM -0400, Mike Snitzer wrote: > > On Tue, May 23 2023 at 8:40P -0400, > > Dave Chinner <david@xxxxxxxxxxxxx> wrote: > > > > > On Tue, May 23, 2023 at 11:26:18AM -0400, Mike Snitzer wrote: > > > > On Tue, May 23 2023 at 10:05P -0400, Brian Foster <bfoster@xxxxxxxxxx> wrote: > > > > > On Mon, May 22, 2023 at 02:27:57PM -0400, Mike Snitzer wrote: > > > > > ... since I also happen to think there is a potentially interesting > > > > > development path to make this sort of reserve pool configurable in terms > > > > > of size and active/inactive state, which would allow the fs to use an > > > > > emergency pool scheme for managing metadata provisioning and not have to > > > > > track and provision individual metadata buffers at all (dealing with > > > > > user data is much easier to provision explicitly). So the space > > > > > inefficiency thing is potentially just a tradeoff for simplicity, and > > > > > filesystems that want more granularity for better behavior could achieve > > > > > that with more work. Filesystems that don't would be free to rely on the > > > > > simple/basic mechanism provided by dm-thin and still have basic -ENOSPC > > > > > protection with very minimal changes. > > > > > > > > > > That's getting too far into the weeds on the future bits, though. This > > > > > is essentially 99% a dm-thin approach, so I'm mainly curious if there's > > > > > sufficient interest in this sort of "reserve mode" approach to try and > > > > > clean it up further and have dm guys look at it, or if you guys see any > > > > > obvious issues in what it does that makes it potentially problematic, or > > > > > if you would just prefer to go down the path described above... > > > > > > > > The model that Dave detailed, which builds on REQ_PROVISION and is > > > > sticky (by provisioning same blocks for snapshot) seems more useful to > > > > me because it is quite precise. That said, it doesn't account for > > > > hard requirements that _all_ blocks will always succeed. > > > > > > Hmmm. Maybe I'm misunderstanding the "reserve pool" context here, > > > but I don't think we'd ever need a hard guarantee from the block > > > device that every write bio issued from the filesystem will succeed > > > without ENOSPC. > > > > > > If the block device can provide a guarantee that a provisioned LBA > > > range is always writable, then everything else is a filesystem level > > > optimisation problem and we don't have to involve the block device > > > in any way. All we need is a flag we can ready out of the bdev at > > > mount time to determine if the filesystem should be operating with > > > LBA provisioning enabled... > > > > > > e.g. If we need to "pre-provision" a chunk of the LBA space for > > > filesystem metadata, we can do that ahead of time and track the > > > pre-provisioned range(s) in the filesystem itself. > > > > > > In XFS, That could be as simple as having small chunks of each AG > > > reserved to metadata (e.g. start with the first 100MB) and limiting > > > all metadata allocation free space searches to that specific block > > > range. When we run low on that space, we pre-provision another 100MB > > > chunk and then allocate all metadata out of that new range. If we > > > start getting ENOSPC to pre-provisioning, then we reduce the size of > > > the regions and log low space warnings to userspace. If we can't > > > pre-provision any space at all and we've completely run out, we > > > simply declare ENOSPC for all incoming operations that require > > > metadata allocation until pre-provisioning succeeds again. > > > > This is basically saying the same thing but: > > > > It could be that the LBA space is fragmented and so falling back to > > the smallest region size (that matches the thinp block size) would be > > the last resort? Then if/when thinp cannot even service allocating a > > new free thin block, dm-thinp will transition to out-of-data-space > > mode. > > Yes, something of that sort, though we'd probably give up if we > can't get at least megabyte scale reservations - a single > modification in XFS can modify many structures and require > allocation of a lot of new metadata, so the fileystem cut-off would > for metadata provisioning failure would be much larger than the > dm-thinp region size.... > > > > This is built entirely on the premise that once proactive backing > > > device provisioning fails, the backing device is at ENOSPC and we > > > have to wait for that situation to go away before allowing new data > > > to be ingested. Hence the block device really doesn't need to know > > > anything about what the filesystem is doing and vice versa - The > > > block dev just says "yes" or "no" and the filesystem handles > > > everything else. > > > > Yes. > > > > > It's worth noting that XFS already has a coarse-grained > > > implementation of preferred regions for metadata storage. It will > > > currently not use those metadata-preferred regions for user data > > > unless all the remaining user data space is full. Hence I'm pretty > > > sure that a pre-provisioning enhancment like this can be done > > > entirely in-memory without requiring any new on-disk state to be > > > added. > > > > > > Sure, if we crash and remount, then we might chose a different LBA > > > region for pre-provisioning. But that's not really a huge deal as we > > > could also run an internal background post-mount fstrim operation to > > > remove any unused pre-provisioning that was left over from when the > > > system went down. > > > > This would be the FITRIM with extension you mention below? Which is a > > filesystem interface detail? > > No. We might reuse some of the internal infrastructure we use to > implement FITRIM, but that's about it. It's just something kinda > like FITRIM but with different constraints determined by the > filesystem rather than the user... > > As it is, I'm not sure we'd even need it - a preiodic userspace > FITRIM would acheive the same result, so leaked provisioned spaces > would get cleaned up eventually without the filesystem having to do > anything specific... > > > So dm-thinp would _not_ need to have new > > state that tracks "provisioned but unused" block? > > No idea - that's your domain. :) > > dm-snapshot, for certain, will need to track provisioned regions > because it has to guarantee that overwrites to provisioned space in > the origin device will always succeed. Hence it needs to know how > much space breaking sharing in provisioned regions after a snapshot > has been taken with be required... dm-thinp offers its own much more scalable snapshot support (doesn't use old dm-snapshot N-way copyout target). dm-snapshot isn't going to be modified to support this level of hardening (dm-snapshot is basically in "maintenance only" now). But I understand your meaning: what you said is 100% applicable to dm-thinp's snapshot implementation and needs to be accounted for in thinp's metadata (inherent 'provisioned' flag). > > Nor would the block > > layer need an extra discard flag for a new class of "provisioned" > > blocks. > > Right, I don't see that the discard operations need to care whether > the underlying storage is provisioned. dm-thinp and dm-snapshot can > treat REQ_OP_DISCARD as "this range is not longer in use" and do > whatever they want with them. > > > If XFS tracked this "provisioned but unused" state, dm-thinp could > > just discard the block like its told. Would be nice to avoid dm-thinp > > needing to track "provisioned but unused". > > > > That said, dm-thinp does still need to know if a block was provisioned > > (given our previous designed discussion, to allow proper guarantees > > from this interface at snapshot time) so that XFS and other > > filesystems don't need to re-provision areas they already > > pre-provisioned. > > Right. > > I've simply assumed that dm-thinp would need to track entire > provisioned regions - used or unused - so it knows which writes to > empty or shared regions have a reservation to allow allocation to > succeed when the backing pool is otherwise empty..... > > > However, it may be that if thinp did track "provisioned but unused" > > it'd be useful to allow snapshots to share provisioned blocks that > > were never used. Meaning, we could then avoid "breaking sharing" at > > snapshot-time for "provisioned but unused" blocks. But allowing this > > "optimization" undercuts the gaurantee that XFS needs for thinp > > storage that allows snapshots... SO, I think I answered my own > > question: thinp doesnt need to track "provisioned but unused" blocks > > but we must always ensure snapshots inherit provisoned blocks ;) > > Sounds like a potential optimisation, but I haven't thought through > a potential snapshot device implementation that far to comment > sanely. I stopped once I got to the point where accounting tricks > count be used to guarantee space is available for breaking sharing > of used provisioned space after a snapshot was taken.... > > > > Further, managing shared pool exhaustion doesn't require a > > > reservation pool in the backing device and for the filesystems to > > > request space from it. Filesystems already have their own reserve > > > pools via pre-provisioning. If we want the filesystems to be able to > > > release that space back to the shared pool (e.g. because the shared > > > backing pool is critically short on space) then all we need is an > > > extension to FITRIM to tell the filesystem to also release internal > > > pre-provisioned reserves. > > > > So by default FITRIM will _not_ discard provisioned blocks. Only if > > a flag is used will it result in discarding provisioned blocks. > > No. FITRIM results in discard of any unused free space in the > filesystem that matches the criteria set by the user. We don't care > if free space was once provisioned used space - we'll issue a > discard for the range regardless. The "special" FITRIM extension I > mentioned is to get filesystem metadata provisioning released; > that's completely separate to user data provisioning through > fallocate() which FITRIM will always discard if it has been freed... > > IOWs, normal behaviour will be that a FITRIM ends up discarding a > mix of unprovisioned and provisioned space. Nobody will be able to > predict what mix the device is going to get at any point in time. > Also, if we turn on online discard, the block device is going to get > a constant stream of discard operations that will also be a mix of > provisioned and unprovisioned space that is not longer in use by the > filesystem. > > I suspect that you need to stop trying to double guess what > operations the filesystem will use provisioning for, what it will > send discards for and when it will send discards for them.. Just > assume the device will receive a constant stream of both > REQ_PROVISION and REQ_OP_DISCARD (for both provisioned and > unprovisioned regions) operations whenver the filesystem is active > on a thinp device..... Yeah, I was getting tripped up in the weeds a bit. It's pretty straight-forward (and like I said at the start of our subthread here: this follow-on work, to inherit provisioned flag, can build on this REQ_PROVISION patchset). All said, I've now gotten this sub-thread on Joe Thornber's radar and we've started discussing. We'll be discussing with more focus tomorrow. Mike
