[libvirt-users] Disk atomicity...

[Daniel P. Berrange]

On Fri, Feb 04, 2011 at 04:29:35AM -0500, Dale Amon wrote:
> I suspect the answer is no, as the very idea makes
> my head hurt, but a confirmation would be nice:
> 
> Am I correct in assuming that it is a very bad idea
> to mount a physical disk on both the physical machine
> and on a hosted VM at the same time? 
> 
> I can imagine getting away with it if one were RO, but
> not otherwise.

Even readonly is not safe. If the host OS has a RO mount
and has partial FS metadata cached, then the guest OS
writes to the FS, this might invalidate the host metadata
and potentially confuse the host OS enough that it would
crash if you're unlucky.

If you want safe concurrent mounts then you need a clustered
or network filesystem. You can also use libguestfs for readonly
access to live guest disks (still has the potential problem I
mention above, but libguestfs runs in a VM so it avoids any
risk to your host OS)

Daniel
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