[Linux-cluster] Starter Cluster / GFS

Gordan Bobic gordan at bobich.net
Wed Nov 10 16:09:54 UTC 2010

Digimer wrote:
> On 10-11-10 07:17 AM, Gordan Bobic wrote:
>>>> If you want the FS mounted on all nodes at the same time then all
>>>> those nodes must be a part of the cluster, and they have to be
>>>> quorate (majority of nodes have to be up). You don't need a quorum
>>>> block device, but it can be useful when you have only 2 nodes.
>>> At term, I will have 7 to 10 nodes, but 2 at first for initial setup
>>> and testing. Ok, so if I have a 3 nodes cluster for exemple, I need at
>>> least 2 nodes for the cluster, and thus the gfs, to be up ? I cannot
>>> have a running gfs with only one node ?
>> In a 2-node cluster, you can have running GFS with just one node up. But
>> in that case it is advisble to have a quorum block device on the SAN.
>> With a 3 node cluster, you cannot have quorum with just 1 node, and thus
>> you cannot have GFS running. It will block until quorum is re-established.
> With a quorum disk, you can in fact have one node left and still have
> quorum. This is because the quorum drive should have (node-1) votes,
> thus always giving the last node 50%+1 even with all other nodes being dead.

I've never tried testing that use-case extensively, but I suspect that 
it is only safe to do with SAN-side fencing. Otherwise two nodes could 
lose contact with each other and still both have access to the SAN and 
thus both be individually quorate.


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