On Thu 13 Apr 2017 03:51:55 PM CEST, Kevin Wolf wrote: >> This invariant is already broken by the very design of the qcow2 >> format, subclusters don't really add anything new there. For any >> given cluster size you can write 4k in every odd cluster, then do the >> same in every even cluster, and you'll get an equally fragmented >> image. > > Because this scenario has appeared repeatedly in this thread: Can we > please use a more realistic one that shows an actual problem? Because > with 8k or more for the cluster size you don't get any qcow2 > fragmentation with 4k even/odd writes (which is a pathological case > anyway), and the file systems are clever enough to cope with it, too. > > Just to confirm this experimentally, I ran this short script: > > ---------------------------------------------------------------- > #!/bin/bash > ./qemu-img create -f qcow2 /tmp/test.qcow2 64M > > echo even blocks > for i in $(seq 0 32767); do echo "write $((i * 8))k 4k"; done | ./qemu-io > /tmp/test.qcow2 > /dev/null > echo odd blocks > for i in $(seq 0 32767); do echo "write $((i * 8 + 4))k 4k"; done | ./qemu-io > /tmp/test.qcow2 > /dev/null > > ./qemu-img map /tmp/test.qcow2 > filefrag -v /tmp/test.qcow2 > ----------------------------------------------------------------
But that's because while you're writing on every other 4k block the cluster size is 64k, so you're effectively allocating clusters in sequential order. That's why you get this: > Offset Length Mapped to File > 0 0x4000000 0x50000 /tmp/test.qcow2 You would need to either have 4k clusters, or space writes even more. Here's a simpler example, mkfs.ext4 on an empty drive gets you something like this: ---------------------------------------------------------------- Offset Length Mapped to File [...] 0x1040000 0x10000 0x150000 test.qcow2 0x1050000 0x50000 0x100000 test.qcow2 0x10a0000 0x10000 0x190000 test.qcow2 0x10b0000 0x30000 0x160000 test.qcow2 0x10e0000 0x10000 0x200000 test.qcow2 0x10f0000 0x30000 0x1a0000 test.qcow2 0x1120000 0x10000 0x210000 test.qcow2 0x1130000 0x30000 0x1d0000 test.qcow2 0x1160000 0x10000 0x280000 test.qcow2 0x1170000 0x30000 0x220000 test.qcow2 [...] ---------------------------------------------------------------- Now, I haven't measured the effect of this on I/O performance, but Denis's point seems in principle valid to me. Berto