[libvirt-users] Disk atomicity...

[David W King]

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It is okay to do concurrent mounts if _all_ of the mounts are RO, right?
 I frequently have many VM's mounting the same DVD iso (RO, of course).
 Should I be anticipating disaster?

David King
Goshen College ITS
davidwk at goshen.edu
574-535-7726


On 02/04/2011 05:14 AM, Daniel P. Berrange wrote:
> 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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