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

[yu song]

thanks Ryan.

So in the linux cluster, there is no concept about odd number coordinate
disks, which is used to deal with this issue?

anyway, probably I have to use power fencing.


cheers

Yu

On Mon, Jun 15, 2009 at 1:36 PM, Ryan O'Hara <rohara at redhat.com> wrote:

> 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
>
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