[Linux-cluster] shared disk with virsh migration

[Sutton, Harry (HAS GSE)]

I'd have to do some research to verify, but I'm guessing that iSCSI (in 
option 3) would use the traditional SCSI reservation mechanism to 
prevent problems associated with multiple access.

     /Harry

On 09/16/2011 06:26 PM, Alan Wood wrote:
> Hi all,
>
> I'm trying to decide whether I really need a cluster implementation to do
> what I want to do and I figured I'd solicit opinions.
> Essentially I want to have two machines running as virtualization hosts
> with libvirt/kvm.  I have shared iSCSI storage available to both hosts and
> have to decide how to configure the storage for use with libvirt.  Right
> now I see three possibilities:
> 1.  Setting an iSCSI storage pool in libvirt
>   	Pros:   Migration seems painless, including live migration
>   	Cons:   Need to pre-allocate LUNs on iSCSI box.
>   		Does not seem to take advantage of iSCSI offloading or multipathing
> 2.  Setting up a two-node cluster and running CLVM
>   	Pros:   Very flexible storage management (is snapshotting supported yet in clvm?)
>   		Automatic failover
>   	Cons:	Cluster infrastructure adds complexity, more potential for bugs
>   		Possible split brain issues?
> 3.  A single iSCSI block device with partitions for each VM mounted on both hosts
>   	Pros:	Easy migration, setup
>   	Cons:	Two hosts accessing the same block device outside of a
>   		cluster seems like it might lead to disaster
>
> Right now I actually like option 3 but I'm wondering if I really am asking
> for trouble accessing a block device simultaneously on two hosts without a
> clustering infrastructure.  I did this a while back with a shared-SCSI box
> and it seemed to work.  I would never be accessing the same partition on
> both hosts and I understand that all partitioning has to be done while the
> other host is off, but is there something else I'm missing here?
>
> Also, are people out there running option 2?  Does it make sesne to set up
> a cluster as small as 2-nodes for HA virtualization or do I really need
> more nodes for it to be worthwhile?  I do have all the fencing
> infrastructure I might need (PDUs and Dracs).
>
> any help would be appreciated.  thanks
> -alan
>
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