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

[Max Reitz]

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?

Max

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