[Linux-cluster] Cluster among geographically separated nodes ?

[Gordan Bobic]

On Thu, 18 Jun 2009 17:11:42 +0530, Brahadambal Srinivasan
<brahadambal at gmail.com> wrote:
> Hi,
> 
> I am trying to figure out if it is possible to create an RHCS cluster
among
> nodes that are in remote locations? If yes, then how are the following
> handled? :
> 
> 1. Storage - how is the shared storage acheived?

Same as it is achieved locally. It is up to your SAN to handle this in a
real-time, consistent way. You may want to look into DRBD
(http://www.drbd.org) for the block device level replication. Be aware,
however, that performance on the disk access front will be terrible,
because the latency will end up being limited by your ping time on the WAN.
So instead of it having 0.1ms added via a local gigabit interconnect, it'll
have 50-100ms added to it. Most applications will not produce usable
performance with this kind of disk I/O speed.

You may, instead, want to look into something like GlusterFS
(http://www.gluster.org) or PeerFS (http://www.radiantdata.com).

> 2. Fencing - any special methods to fence ?

Just be aware that if your site interconnect goes down, you'll end up with
a hung cluster, since the nodes will disconnect and be unable to fence each
other. You could offset that by having separate cluster and fencing
interconnects, but you would also need to look into quorum - you need n/2+1
nodes for quorum, so to make this work sensibly you'd need at least three
sites - otherwise if you lose the bigger site you lose the whole cluster
anyway.

> 3. Max. number of nodes possible in such a setup

I don't think there is a difference in this regard between LAN and WAN
clusters.

Gordan