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[Qemu-devel] Re: [et-mgmt-tools] Image Corruption Possible with qemu and

From: Daniel P. Berrange
Subject: [Qemu-devel] Re: [et-mgmt-tools] Image Corruption Possible with qemu and qemu-kvm
Date: Mon, 27 Aug 2007 21:27:48 +0100
User-agent: Mutt/1.4.1i

On Mon, Aug 27, 2007 at 10:19:42PM +0200, Sven Oehme wrote:
> i thought it might be good to post this on qemu-devel as well, as i see 
> this as a general qemu / kvm / xen /whatevercomesnext issue .
> are there any plans to implement a default in qemu to prevent accessing 
> the same image multiple times ? 

Each QEMU instance is unware of each other so can't directly check this. One
possiblity would be for QEMU to use fcntl() to take a lock on a disk image
when opening it. With raw disk images at least, it is, however, safe to let
multiple QEMU instances use it at once *provided* you have a clustered
filesystem installed, rather than regular FAT or ext2/3, so one wouldn't want
to exclude this use case.

> i know that in xen they deal with this problem somehow xen specific.
> wouldn't it be good to prevent multiple access to the same image by 
> default and add a switch to overwrite it ? 

In the scenario you mention, libvirt should probably do a sanity check for
this before letting you start the guest. libvirt already supports the idea
of 'shared' disk images where two or more guests can be optionally configured 
to have write access - basically assumes the admin requesting sharing knows
what they're doing.

> i corrupted a couple of virtual Linux images last week as i accidentally 
> started them two times .. 
> i opened a bug against qemu (not sure if this was a good idea) -->  
> https://bugzilla.redhat.com/show_bug.cgi?id=253533 
> are there any plans to ensure that this can't happen in future releases of 
> virt-manager with lock files in the image directory or similar ? 

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