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

[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.

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

[Paolo Bonzini]

On 03/11/2015 13:12, Richard W.M. Jones wrote:
> 
> 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.

Yes, this is correct.

Paolo