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
>
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