[Linux-cluster] Could some one explain why SCSI_Fence Agent can not be used in 2 nodes cluster?

[Ryan O'Hara]

On Sun, Jun 14, 2009 at 11:21:08AM +1000, yu song wrote:
> Hi,
> 
> I am planning to build a 2 nodes cluster on rhcl 5.3, and looking for what
> fencing method I could use.
> 
> On the storage side, it is EMC clarion and supports scsi 3 reservation.
> 
> So I'm thinking to use fence_scsi agent to do the disk fencing. however,
> according the redhat website,  it states that fence_scsi does not support
> two nodes cluster.
> 
> Could anyone kindly explain it why? (never had this issue when use veritas
> cluster)

In a 2 node cluster, fencing becomes a race -- the node fences the
other node first wins. This works well with power fencing, but not so
well with SAN fencing (eg. fence_scsi).

The problem with fence_scsi in a 2 node cluster is this:

Suppose we have 2 node, call them A and B. Also assume we have multple
LUNs, which we will call lun1, lun2, lun3. Consider what happens when
a network partition occurs -- both nodes attempt to fence one
another. It is possible that A could remove B's key from lun1 and
lun2, but node B could remove node A's key from lun3. This is
inconsistent and there is no clear "winner".

Ryan