On Wed, Mar 30, 2016 at 11:27:55AM -0700, Darrick J. Wong wrote: > Hi all, > > Christoph and I have been working on adding reflink and CoW support to > XFS recently. Since the purpose of (mode 0) fallocate is to make sure > that future file writes cannot ENOSPC, I extended the XFS fallocate > handler to unshare any shared blocks via the copy on write mechanism I > built for it. However, Christoph shared the following concerns with > me about that interpretation: > > > I know that I suggested unsharing blocks on fallocate, but it turns out > > this is causing problems. Applications expect falloc to be a fast > > metadata operation, and copying a potentially large number of blocks > > is against that expextation. This is especially bad for the NFS > > server, which should not be blocked for a long time in a synchronous > > operation. > > > > I think we'll have to remove the unshare and just fail the fallocate > > for a reflinked region for now. I still think it makes sense to expose > > an unshare operation, and we probably should make that another > > fallocate mode. I'm expecting fallocate to be fast, too. Well, btrfs fallocate doesn't allocate space if it's a shared one because it thinks the space is already allocated. So a later overwrite over this shared extent may hit enospc errors. > > With that in mind, how do you all think we ought to resolve this? > Should we add a new fallocate mode flag that means "unshare the shared > blocks"? Obviously, this unshare flag cannot be used in conjunction > with hole punching, zero range, insert range, or collapse range. This > breaks the expectation that writing to a file after fallocate won't > ENOSPC. > > Or is it ok that fallocate could block, potentially for a long time as > we stream cows through the page cache (or however unshare works > internally)? Those same programs might not be expecting fallocate to > take a long time. > > Can we do better than either solution? It occurs to me that XFS does > unshare by reading the file data into the pagecache, marking the pages > dirty, and flushing the dirty pages; performance could be improved by > skipping the flush at the end. We won't ENOSPC, because the XFS > delalloc system is careful enough to check that there are enough free > blocks to handle both the allocation and the metadata updates. The > only gap in this scheme that I can see is if we fallocate, crash, and > upon restart the program then tries to write without retrying the > fallocate. Can we trade some performance for the added requirement > that we must fallocate -> write -> fsync, and retry the trio if we > crash before the fsync returns? I think that's already an implicit > requirement, so we might be ok here. > > Opinions? I rather like the last option, though I've only just > thought of it and have not had time to examine it thoroughly, and it's > specific to XFS. :) I'd vote for another mode for 'unshare the shared blocks'. Thanks, -liubo _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs
