On Fri 07 Apr 2017 02:20:21 PM CEST, Stefan Hajnoczi wrote: >> Here are the results when writing to an empty 40GB qcow2 image with no >> backing file. The numbers are of course different but as you can see >> the patterns are similar: >> >> |-----------------+----------------+-----------------+-------------------| >> | cluster size | subclusters=on | subclusters=off | Max L2 cache size | >> |-----------------+----------------+-----------------+-------------------| >> | 2 MB (256 KB) | 1200 IOPS | 255 IOPS | 160 KB | >> | 512 KB (64 KB) | 3000 IOPS | 700 IOPS | 640 KB | >> | 64 KB (8 KB) | 7200 IOPS | 3300 IOPS | 5 MB | >> | 32 KB (4 KB) | 12300 IOPS | 4200 IOPS | 10 MB | >> | 4 KB (512 B) | 100 IOPS | 100 IOPS | 80 MB | >> |-----------------+----------------+-----------------+-------------------| > > I don't understand why subclusters=on performs so much better when > there's no backing file. Is qcow2 zeroing out the 64 KB cluster with > subclusters=off?
It is, see https://lists.nongnu.org/archive/html/qemu-devel/2015-07/msg05877.html Berto
