Re: [libvirt-users] FYI: Notes on setting up KVM guests using iSCSI
On Wed, May 05, 2010 at 08:09:50AM -0400, Tom Georgoulias wrote:
On 05/05/2010 07:23 AM, Daniel P. Berrange wrote:
>DV suggested that we document some libvirt setups using shared storage. I'm
>not a fan of NFS, so I wrote some blog posts on how to use iSCSI in the
>context of libvirt + KVM.
Thank you for creating and sharing these.
>
http://berrange.com/posts/2010/05/05/provisioning-kvm-virtual-machines-on...
I have a question about this example, specifically the shareable
permission option and tag: (perm=sh, <shareable/>)
What does that do, exactly? Is it simply the required r/w option to
prevent corruption of VMs when using shared storage, or is it needed
only if the disk will be actively by more than one VM? (That 2nd idea
doesn't seem quite right to me, I would expect to have to use some kind
of locking technology to prevent it two or more VMs trying to write to
the same disk at the same time).
The <shareable/> flag tells libvirt that the disk will be *concurrently*
accessed by multiple VMs. This means that the hypervisor will not try to
take an exclusive lock on the disk, that all I/O caching is disabled,
and any SELinux labelling allows use by all VMs.
NB: if you are marking a disk as <sharable/> you need to know what you
are doing in the guest OS. You would typically use a cluster filesystem
on the disk like GFS or OCFS. Attempts to use a normal FS like ext3
from multiple VMs at once is not healthy for your data :-)
Daniel
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