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Re: [Linux-cluster] shared disk with virsh migration


I don't use KVM and libvirt but my experiment concerne clustering storage :
1. Don't know
2. Snapshotting is supported in clvm (since 5.7 I think)
  Complexity... yes
  Bugs... yes
  Split brain... yes 
  2 nodes is sufficient for HA, juste think what happens if 1 node shuts down and your VMs are very loded (needs 3rd nodes ?)
3. No experiment too but it sounds like it's not the right usage

Best regards

-----Message d'origine-----
De : linux-cluster-bounces redhat com [mailto:linux-cluster-bounces redhat com] De la part de Alan Wood
Envoyé : samedi 17 septembre 2011 00:26
À : linux-cluster redhat com
Objet : [Linux-cluster] shared disk with virsh migration

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

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