[libvirt] virsh blockresize syntax is inconsistent with vol-resize and somewhat dangerous

Cole Robinson crobinso at redhat.com
Mon Nov 18 20:16:38 UTC 2019

On 10/25/19 4:28 AM, Patrik Martinsson wrote:
> Hi Tim,
> I recently stumbled on the same thing, accidentally shrinking a blockdevice.
> I have written a patch for virsh that will force the user to append a
> '--force' flag if shrinking is desired.
> The behavior is somewhat still inconsistent with the vol-resize
> command, however a bigger rewrite is needed to make both commands
> operate exactly the same, which I don't know if actually needed.
> Previous discussion can be found below,
> - https://www.redhat.com/archives/libvir-list/2019-October/msg00258.html
> - https://www.redhat.com/archives/libvir-list/2019-October/msg01437.html
> Best regards,
> Patrik
> On Thu, Oct 24, 2019 at 6:04 PM Tim Small <tim at seoss.co.uk> wrote:
>> Hello,
>> virsh has two commands which can be used to resize block devices -
>> "blockresize" for volumes in use by and active guest, and "vol-resize"
>> for volumes which are not in use.
>> The vol-resize syntax allows to specify the size as a delta (increase or
>> decrease vs. the current size), and also refuses to shrink a volume
>> unless the "--shrink" argument is also passed.
>> Most other tools which can be used for block device resizing (outside of
>> libvirt) also have similar "--shrink" argument requirements when
>> reducing the size of an existing block device.  e.g. ceph requires
>> "--allow-shrink" when using the "rbd resize" command.
>> The lack of such a safety device makes "blockresize" a foot-gun (which I
>> recently found to great effect when I typoed the domain name to another
>> valid domain).
>> It seems I am not alone in making this error e.g.
>> https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=902171
>> One possible solution would be to make a new command e.g. "domblkresize"
>> or perhaps "live-resize", which implement the "--shrink" and "--delta"
>> behaviour to make it consistent with "vol-resize" syntax, and mark the
>> "blockresize" command as deprecated in the documentation and help (so
>> that existing automation which depends on the current behaviour doesn't
>> break).
>> Any thoughts?  Should I open this as an RFE?

Considering there's been multiple people hitting it, I think it's
something we should fix in libvirt. Just need buy in from other devs. To

'virsh blockresize' will online resize an image path for a running VM.
It does this with the qemu block_resize monitor command via the
virDomainBlockResize API. The API doesn't provide any protection against
shrinking the disk image though, which I presume is both the less common
intention of the operation, and much less often safe to do for a running
VM. And a user typo can mean data loss

virsh vol-resize, which is storage API virStorageVolResize, is for
offline image resizing, mostly using qemu-img. It has had a SHRINK API
flag from the outset, rejecting requests to reduce the image size unless
the flag is passed. Seems like a safe pattern to follow.

Can we change existing blockresize behavior? I think it's reasonable;
we've added flags to other APIs that are required to restore old
behavior, UNDEFINE_NVRAM for one example.

danpb, pkrempa, eblake, thoughts?


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