Re: [PATCH for-5.1] file-posix: Mitigate file fragmentation with extent size hints

[Kevin Wolf]

Am 13.07.2020 um 15:12 hat Kevin Wolf geschrieben:
> Am 13.07.2020 um 11:08 hat Max Reitz geschrieben:
> > On 10.07.20 18:12, Max Reitz wrote:
> > > On 07.07.20 18:17, Kevin Wolf wrote:
> > >> Am 07.07.2020 um 16:23 hat Kevin Wolf geschrieben:
> > >>> Espeically when O_DIRECT is used with image files so that the page cache
> > >>> indirection can't cause a merge of allocating requests, the file will
> > >>> fragment on the file system layer, with a potentially very small
> > >>> fragment size (this depends on the requests the guest sent).
> > >>>
> > >>> On Linux, fragmentation can be reduced by setting an extent size hint
> > >>> when creating the file (at least on XFS, it can't be set any more after
> > >>> the first extent has been allocated), basically giving raw files a
> > >>> "cluster size" for allocation.
> > >>>
> > >>> This adds an create option to set the extent size hint, and changes the
> > >>> default from not setting a hint to setting it to 1 MB. The main reason
> > >>> why qcow2 defaults to smaller cluster sizes is that COW becomes more
> > >>> expensive, which is not an issue with raw files, so we can choose a
> > >>> larger file. The tradeoff here is only potentially wasted disk space.
> > >>>
> > >>> For qcow2 (or other image formats) over file-posix, the advantage should
> > >>> even be greater because they grow sequentially without leaving holes, so
> > >>> there won't be wasted space. Setting even larger extent size hints for
> > >>> such images may make sense. This can be done with the new option, but
> > >>> let's keep the default conservative for now.
> > >>>
> > >>> The effect is very visible with a test that intentionally creates a
> > >>> badly fragmented file with qemu-img bench (the time difference while
> > >>> creating the file is already remarkable) and then looks at the number of
> > >>> extents and the take a simple "qemu-img map" takes.
> > >>>
> > >>> Without an extent size hint:
> > >>>
> > >>>     $ ./qemu-img create -f raw -o extent_size_hint=0 ~/tmp/test.raw 10G
> > >>>     Formatting '/home/kwolf/tmp/test.raw', fmt=raw size=10737418240 
> > >>> extent_size_hint=0
> > >>>     $ ./qemu-img bench -f raw -t none -n -w ~/tmp/test.raw -c 1000000 
> > >>> -S 8192 -o 0
> > >>>     Sending 1000000 write requests, 4096 bytes each, 64 in parallel 
> > >>> (starting at offset 0, step size 8192)
> > >>>     Run completed in 25.848 seconds.
> > >>>     $ ./qemu-img bench -f raw -t none -n -w ~/tmp/test.raw -c 1000000 
> > >>> -S 8192 -o 4096
> > >>>     Sending 1000000 write requests, 4096 bytes each, 64 in parallel 
> > >>> (starting at offset 4096, step size 8192)
> > >>>     Run completed in 19.616 seconds.
> > >>>     $ filefrag ~/tmp/test.raw
> > >>>     /home/kwolf/tmp/test.raw: 2000000 extents found
> > >>>     $ time ./qemu-img map ~/tmp/test.raw
> > >>>     Offset          Length          Mapped to       File
> > >>>     0               0x1e8480000     0               
> > >>> /home/kwolf/tmp/test.raw
> > >>>
> > >>>     real    0m1,279s
> > >>>     user    0m0,043s
> > >>>     sys     0m1,226s
> > >>>
> > >>> With the new default extent size hint of 1 MB:
> > >>>
> > >>>     $ ./qemu-img create -f raw -o extent_size_hint=1M ~/tmp/test.raw 10G
> > >>>     Formatting '/home/kwolf/tmp/test.raw', fmt=raw size=10737418240 
> > >>> extent_size_hint=1048576
> > >>>     $ ./qemu-img bench -f raw -t none -n -w ~/tmp/test.raw -c 1000000 
> > >>> -S 8192 -o 0
> > >>>     Sending 1000000 write requests, 4096 bytes each, 64 in parallel 
> > >>> (starting at offset 0, step size 8192)
> > >>>     Run completed in 11.833 seconds.
> > >>>     $ ./qemu-img bench -f raw -t none -n -w ~/tmp/test.raw -c 1000000 
> > >>> -S 8192 -o 4096
> > >>>     Sending 1000000 write requests, 4096 bytes each, 64 in parallel 
> > >>> (starting at offset 4096, step size 8192)
> > >>>     Run completed in 10.155 seconds.
> > >>>     $ filefrag ~/tmp/test.raw
> > >>>     /home/kwolf/tmp/test.raw: 178 extents found
> > >>>     $ time ./qemu-img map ~/tmp/test.raw
> > >>>     Offset          Length          Mapped to       File
> > >>>     0               0x1e8480000     0               
> > >>> /home/kwolf/tmp/test.raw
> > >>>
> > >>>     real    0m0,061s
> > >>>     user    0m0,040s
> > >>>     sys     0m0,014s
> > >>>
> > >>> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> > >>
> > >> I also need to squash in a few trivial qemu-iotests updates, for which I
> > >> won't send a v2:
> > > 
> > > The additional specifications in 243 make it print a warning on tmpfs
> > > (because the option doesn’t work there).  I suppose the same may be true
> > > on other filesystems as well.  Should it be filtered out?
> 
> I guess we just shouldn't print a warning if the requested hint is 0.
> 
> > This patch also breaks 059, 106, and 175.
> 
> Hm, I was sure I had tested raw... Anyway, 059 should filter out the
> actual size (how could this ever work?), and 175 is obvious, too - it
> tries to be clever, but not clever enough.
> 
> 106 is a bit mysterious because the error message implies that the
> images end up smaller than before, which shouldn't be the case. I'll
> have a look.

Ah, it misinterprets MiB as KiB, so the error says the image is smaller
than expected while it's actually larger. I'll just disable the extent
size hint for this one, too.

Kevin

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