Re: [PATCH RFC 0/4] xfs: basic cow fork speculative preallocation

[Brian Foster <bfoster@redhat.com>]

On Tue, Nov 08, 2016 at 12:48:00PM -0800, Darrick J. Wong wrote:
> On Tue, Nov 08, 2016 at 03:27:32PM -0500, Brian Foster wrote:
> > Hi all,
> > 
> > This is an experiment based on an idea for COW fork speculative
> > preallocation. This is experimental, lightly/barely tested and sent in
> > RFC form to solicit thoughts, ideas or flames before I spend time taking
> > it further.
> > 
> > Patch 1 probably stands on its own. Patches 2 and 3 are some refactoring
> > and patch 4 implements the basic idea, which is described in the commit
> > log description. The testing I've done so far is basically similar to
> > how one would test the effects of traditional speculative preallocation:
> > write to multiple reflinked files in parallel and examine the resulting
> > fragmentation. Specifically, I wrote sequentially to 16 different
> > reflinked files of the same 8GB original (which has two data extents,
> > completely shared). Without preallocation, the test results in ~248
> > extents across the 16 files. With preallocation, the test results in 32
> > extents across the 16 files (i.e., 2 extents per file, same as the
> > source file).
> > 
> > An obvious tradeoff is the unnecessarily aggressive allocation that
> > might occur in the event of random writes to a large file (such as in
> > the cloned VM disk image use case), but my thinking is that the
> > cowblocks tagging and reclaim infrastructure should manage that
> > sufficiently (lack of testing notwithstanding). In any event, I'm
> > interested in any thoughts along the lines of whether this is useful at
> > all, alternative algorithm ideas, etc.
> 
> Was about to step out to lunch when this came in, but...
> 
> Is there an xfstest for this, so I can play too? :)
> 

Not yet.. I've only xfstests tested insofar as it hasn't blown anything
up yet. :) Otherwise, I've just run manual write tests to observe
whether it is doing what I expect it to in simple cases. It clearly
needs more work, as noted in the patch, but if this is something worth
pursuing further I can certainly come up with some tests as well.

FWIW, that COW fork fiemap hack I sent a bit ago came in handy for
playing with this as well. :)

> As far as random writes go, some of the reflink tests look at fragmentation
> behavior.  generic/301 generic/302 xfs/180 xfs/182 xfs/184 xfs/192 xfs/193
> xfs/198 xfs/200 xfs/204 xfs/208 xfs/208 xfs/211 xfs/215 xfs/218 xfs/219 xfs/221
> xfs/223 xfs/224 xfs/225 xfs/226 xfs/228 xfs/230 xfs/231 xfs/232 xfs/344 xfs/345
> xfs/346 xfs/347 are the ones that grep 'new extents:' picked up.
> 

Ok.

> Will look at the patches when I get back.
> 

Thanks!

Brian

> --D
> 
> > 
> > Brian
> > 
> > Brian Foster (4):
> >   xfs: clean up cow fork reservation and tag inodes correctly
> >   xfs: logically separate iomap range from allocation range
> >   xfs: reuse xfs_file_iomap_begin_delay() for cow fork delalloc
> >   xfs: implement basic COW fork speculative preallocation
> > 
> >  fs/xfs/xfs_iomap.c   | 132 +++++++++++++++++++++++++++++++++++++++++----------
> >  fs/xfs/xfs_reflink.c |  28 ++---------
> >  2 files changed, 111 insertions(+), 49 deletions(-)
> > 
> > -- 
> > 2.7.4
> > 
> > --
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