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 <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?
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. Kevin
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