Am 13.04.2017 um 15:21 hat Alberto Garcia geschrieben: > 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 ---------------------------------------------------------------- And sure enough, this is the output: ---------------------------------------------------------------- Formatting '/tmp/test.qcow2', fmt=qcow2 size=67108864 encryption=off cluster_size=65536 lazy_refcounts=off refcount_bits=16 even blocks odd blocks Offset Length Mapped to File 0 0x4000000 0x50000 /tmp/test.qcow2 Filesystem type is: 58465342 File size of /tmp/test.qcow2 is 67436544 (16464 blocks of 4096 bytes) ext: logical_offset: physical_offset: length: expected: flags: 0: 0.. 47: 142955.. 143002: 48: 1: 48.. 48: 143016.. 143016: 1: 143003: 2: 64.. 79: 142868.. 142883: 16: 143017: 3: 80.. 111: 155386.. 155417: 32: 142884: 4: 112.. 303: 227558.. 227749: 192: 155418: 5: 304.. 559: 228382.. 228637: 256: 227750: 6: 560.. 1071: 455069.. 455580: 512: 228638: 7: 1072.. 2095: 485544.. 486567: 1024: 455581: 8: 2096.. 4143: 497978.. 500025: 2048: 486568: 9: 4144.. 8239: 508509.. 512604: 4096: 500026: 10: 8240.. 16431: 563122.. 571313: 8192: 512605: 11: 16432.. 32815: 632969.. 649352: 16384: 571314: eof /tmp/test.qcow2: 12 extents found ---------------------------------------------------------------- That is, on the qcow2 level we have exactly 0% fragmentation, everything is completely contiguous in a single chunk. XFS as the container of the test image creates a few more extents, but as you can see, it uses fairly large extent sizes in the end (and it would use even larger ones if I wrote more than 64 MB). Kevin
