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? Max
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