I wrote a tool called virt-sparsify which runs fstrim on disks via
qemu. My colleague asked me a good question: Is this safe if qemu is
killed (^C)? Could it corrupt the guest?
Using 'virt-sparsify --inplace disk.img' is essentially equivalent to
doing:
qemu-kvm \
-kernel <recent-kernel> \
-drive file=disk.img,discard=unmap,[virtio-scsi] \
-drive file=appliance
And in the appliance doing:
foreach fs in filesystems:
mount -o discard fs /sysroot
fstrim /sysroot
umount /sysroot
sync
poweroff
I think the answer is "safe", as long as the Linux kernel and qemu are
written carefully, but it would be good to get an expert opinion.
It looks like fstrim just sends discard requests. And mount/umount
should be safe by the usual rules of journalling.
Rich.
--
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
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