On Sat, Dec 19, 2015 at 12:56:23AM -0800, Darrick J. Wong wrote: > Hi all, > ... > Fixed since RFCv3: > > * The reflink and dedupe ioctls are being hoisted to the VFS, as > provided in the first few patches. Patch 81 connects to this > functionality. > > * Copy on write has been rewritten for v4. We now use the existing > delayed allocation mechanism to coalesce writes together, deferring > allocation until writeout time. This enables CoW to make better > block placement decisions and significantly reduces overhead. > CoW is still pretty slow, but not as slow as before. > > * Direct IO CoW has been implemented using the same mechanism as > above, but modified to perform the allocation and remapping right > then and there. Throughput is much higher than pushing data > through the page cache CoW. (It's the same mechanism, but we're > playing with chunks bigger than a single memory page.) > > * CoW ENOSPC works correctly now, except in the pathological case > that the AG fills up and the rmap btree cannot expand. That will > be addressed for v5. > > * fallocate will now unshare blocks to prevent future ENOSPC, as > you'd expect. > > * refcount btree blocks are preallocated at mount time to prevent > ENOSPC while trying to expand the tree. This also has the effect > of grouping the btree blocks together, which can speed up CoW > remapping. > Can you elaborate on how these blocks are preallocated? E.g., is the tree "preconstructed" in some sense? However that is done, is this the anticipated solution or a temporary workaround..? Also, shouldn't the enospc condition be handled by the agfl? I take it there is something going on here that renders that solution flawed, so I'm just curious what it is. (Sorry if this is all explained elsewhere, but I haven't yet had a chance to take a close enough look at this feature..). Brian > Issues: > > * The extent swapping ioctl still allocates a bigger fixed-size > transaction. That's most likely a stupid thing to do, so getting a > better grip on how the journalling code works and auditing all the > new transaction users will have to happen. Right now it mostly > gets lucky. > > * EFI tracking for the allocated-but-not-yet-mapped blocks is > nonexistant. A crash will leak them. > > * ENOSPC while expanding the rmap btree can crash the FS. For now we > work around this problem by making the AGFL as big as possible, > failing CoW attempts with ENOSPC if there aren't enough AGFL blocks > available, and hoping that doesn't actually happen. > > If you're going to start using this mess, you probably ought to just > pull from my github trees for kernel[1], xfsprogs[2], and xfstests[3]. > There are also updates for xfs-docs[4] and man-pages[5]. > > The patches have been xfstested with x64, i386, and ppc64; while in > general the tests run to completion, there are still periodic bugs > that will be addressed by the next RFC. There's a persistent crash on > arm64 and ppc64el that I haven't been able to triage. > > This is an extraordinary way to eat your data. Enjoy! > Comments and questions are, as always, welcome. > > --D > > [1] https://github.com/djwong/linux/tree/for-dave > [2] https://github.com/djwong/xfsprogs/tree/for-dave > [3] https://github.com/djwong/xfstests/tree/for-dave > [4] https://github.com/djwong/xfs-documentation/tree/for-dave > [5] https://github.com/djwong/man-pages/commits/for-mtk > > _______________________________________________ > xfs mailing list > xfs@xxxxxxxxxxx > http://oss.sgi.com/mailman/listinfo/xfs _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs
