On Fri, Aug 27, 2021 at 12:55:39PM +1000, Chris Dunlop wrote: > On Fri, Aug 27, 2021 at 06:56:35AM +1000, Chris Dunlop wrote: > > On Thu, Aug 26, 2021 at 10:05:00AM -0500, Eric Sandeen wrote: > > > On 8/25/21 9:06 PM, Chris Dunlop wrote: > > > > > > > > fallocate -l 1GB image.img > > > > mkfs.xfs -f image.img > > > > mkdir mnt > > > > mount -o loop ./image.img mnt > > > > fallocate -o 0 -l 700mb mnt/image.img > > > > fallocate -o 0 -l 700mb mnt/image.img > > > > > > > > Why does the second fallocate fail with ENOSPC, and is that considered an XFS bug? > > > > > > Interesting. Off the top of my head, I assume that xfs is not looking at > > > current file space usage when deciding how much is needed to satisfy the > > > fallocate request. While filesystems can return ENOSPC at any time for > > > any reason, this does seem a bit suboptimal. > > > > Yes, I would have thought the second fallocate should be a noop. > > On further reflection, "filesystems can return ENOSPC at any time" is > certainly something apps need to be prepared for (and in this case, it's > doing the right thing, by logging the error and aborting), but it's not > really a "not a bug" excuse for the filesystem in all circumstances (or this > one?), is it? E.g. a write(fd, buf, 1) returning ENOSPC on an fresh > filesystem would be considered a bug, no? Sure, but the fallocate case here is different. You're asking to preallocate up to 700MB of space on a filesystem that only has 300MB of space free. Up front, without knowing anything about the layout of the file we might need to allocate 700MB of space into, there's a very good chance that we'll get ENOSPC partially through the operation. The real problem with preallocation failing part way through due to overcommit of space is that we can't go back an undo the allocation(s) made by fallocate because when we get ENOSPC we have lost all the state of the previous allocations made. If fallocate is filling holes between unwritten extents already in the file, then we have no way of knowing where the holes we filled were and hence cannot reliably free the space we've allocated before ENOSPC was hit. Hence if we allow the fallocate to go ahead and preallocate space until we hit ENOSPC, we still end up returning to userspace with ENOSPC, but we've also consumed all the remaining space in the filesystem. So there's a very good argument for simply rejecting any attempt to preallocate space that has the possibility of over-committing space and hence hitting ENOSPC part way through. Given that we spend a lot of effort in XFS to avoid over-committing resources so that ENOSPC is reliable and not prone to deadlocks, the choice to make fallocate avoid a potential over-commit is at least internally consistent with the XFS ENOSPC architecture. IOWs, either behaviour could be considered a "bug" because it is sub-optimal behaviour, but at some point you've got to choose what is the least worst behaviour and run with it. > ...or maybe your "suboptimal" was entirely tongue in cheek? > > > > > Background: I'm chasing a mysterious ENOSPC error on an XFS > > > > filesystem with way more space than the app should be asking > > > > for. There are no quotas on the fs. Unfortunately it's a third > > > > party app and I can't tell what sequence is producing the error, > > > > but this fallocate issue is a possibility. More likely speculative preallocation is causing this than fallocate. However, we've had a background worker that cleans up speculative prealloc before reporting ENOSPC for a while now - what kernel version are seeing this on? Also, it might not even be data allocation that is the issue - if the filesystem is full and free space is fragmented, you could be getting ENOSPC because inodes cannot be allocated. In which case, the output of xfs-info would be useful so we can see if sparse inode clusters are enabled or not.... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx
