[Qemu-devel] Safety of killing qemu when it is doing an fstrim

[Richard W.M. Jones]

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.

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