Re: [Qemu-devel] [PATCH 0/2] qemu-img: Let "info" warn and go ahead without -U

[Daniel P. Berrange]

On Wed, Jan 10, 2018 at 03:07:12PM +0100, Kevin Wolf wrote:
> Am 10.01.2018 um 13:49 hat Daniel P. Berrange geschrieben:
> > On Mon, Jan 08, 2018 at 03:41:36PM +0100, Kevin Wolf wrote:
> > > Am 05.01.2018 um 07:55 hat Fam Zheng geschrieben:
> > > > Management and users are accustomed to "qemu-img info" to query status 
> > > > of
> > > > images even when they are used by guests. Since image locking was 
> > > > added, the -U
> > > > (--force-share) option is needed for that to work. The reason has been 
> > > > that due
> > > > to possible race with image header update, the output can be misleading.
> > > > 
> > > > But what are likely to happen after we emit the error are that, for 
> > > > interactive
> > > > users, '-U' will be used and the command retried; for management (nova, 
> > > > RHV,
> > > > etc.), the operation is broken with no knob to workaround this.
> > > > 
> > > > This series changes that error to a warning so that it doesn't get in 
> > > > the way.
> > > 
> > > Are management tools actually doing this? There is no good reason to
> > > call 'qemu-img info' for an image that is in use by a VM.
> > 
> > OpenStack will frequently call 'qemu-img info' for disks that are in use by
> > VMs. It is looking at the sizes to understand the relation between the 
> > current
> > size used by qcow2 vs the possible future usage. In this context, it does 
> > not
> > matter if the data is slightly outdated, as it will catch up next time it 
> > reads
> > it a few mins later.
> > 
> > It has been patched to just retry with -U to avoid this error on new
> > QEMU.
> 
> The proper, though somewhat more intrusive fix would be to use QMP
> commands for images of running VMs. You already need to do the same for
> anything modifying the image (to avoid corruption), so I think doing the
> same with 'query-block' instead of 'qemu-img info' for guaranteed
> consistent results on running VMs only makes sense.

The problem with 'query-block' is that you assume the code that is
processing this set of disk images knows which image is used where.

This code in question merely sees a directory full of images, and does
not directly know whether any of them are in use or not. So trying to
use query-block would make it significantly more complex for little
obvious benefit to OpenStack.

Regards,
Daniel
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