Avi Kivity wrote:
That setting has some unfortunate side effects. Partitioning tools will
locate the first partition at the second cylinder, which is at the 63rd
sector. This means that if the guest uses a 4K block filesystem on the
first partition (an incredibly common occurance), then every single
access will not be 4K aligned with respect to the virtual block device.
This will cause fragmentation and read/modify/write cycles with:
- qcow2 (which uses aligned 4K blocks)
- any host filesystem which uses 4K blocks
- any host disk which uses 4K blocks (not yet common)
I can think of a few workarounds, all bad:
- add a partitioning tool (or option to qemu-img) to format the disk,
placing the first partition on the fourth cylinder, aligning it. tell
the users not to wipe the disks out but instead install to one of the
existing parititions
- add a tool to optimize an existing disk by extending it and moving the
partitions around so they are aligned. may break boot loaders.
- make qcow4 use 512 byte sectors. will increase overhead and doesn't
solve problems on the host filesystem and disk.
- have qcow51 detect misaligned accesses and adjust itself somehow.
doesn't help raw and other formats. likely very difficult.
Does anybody know if scsi will have the same problems? Can anyone
suggest other workarounds?
This is not just limited to KVM. The right thing is for partitioning
tools to realize that this isn't 1981 and actually align partitions to
block boundaries. Windows Vista aligns partitions to megabyte
boundaries; I personally think that is somewhat excessive, but aligning
partitions to 128K (or possibly 64K) would be better.
This idiotic default damages virtualizers, flash drives, hardware RAID,
and pretty soon spinning media as well.
-hpa
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