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

[Max Reitz]

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?

This patch also breaks 059, 106, and 175.

Max

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