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-iscsi-the-hard-way-part-2-of-2/ > > 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 -- |: Red Hat, Engineering, London -o- http://people.redhat.com/berrange/ :| |: http://libvirt.org -o- http://virt-manager.org -o- http://deltacloud.org :| |: http://autobuild.org -o- http://search.cpan.org/~danberr/ :| |: GnuPG: 7D3B9505 -o- F3C9 553F A1DA 4AC2 5648 23C1 B3DF F742 7D3B 9505 :|
