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 <kw...@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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