To make it work, one should be able to add sort of external resource in the cluster monitoring the synchronization status between the source LUNs and the target ones, and by the way automatically invert the synchronization in case your resource or service fails over another node on the other site.
This can be tricky and your SAN arrays must allow you to do this (HDS/HP command devices, etc...)
IMHO, LVM mirror is the simplest way to achieve this if latency constraints are acceptable.
When I say partially, there is always the quorum issue, as on a 4 nodes cluster, equally located on 2 sites, in case of a site failure, the 2 remaining nodes are not quorate.
Brem
2009/6/10 Tom Lanyon <tom@xxxxxxxxxxxxxx>
On 05/06/2009, at 6:52 PM, brem belguebli wrote:I have been wondering whether the same could be done (cross-site RHCS) using SAN replication and multipath, avoiding LVM mirroring. This is going to depend strongly on the storage replication failover time; if the IO to shared storage devices is queued for too long, the cluster will stop. Does anyone have any experience with how quick this would need to happen for RHCS to tolerate it?
Hello,
That sounds pretty much to the question I've asked to this mailing-list last May (https://www.redhat.com/archives/linux-cluster/2009-May/msg00093.html).
We are in the same setup, already doing "Geo-cluster" with other technos and we are looking at RHCS to provide us the same service level.
Latency could be a problem indeed if too high , but in a lot of cases (many companies for which I've worked), datacenters are a few tens of kilometers far, with a latency max close to 1 ms, which is not a problem.
Let's consider this kind of setup, 2 datacenters far from each other by 1 ms delay, each hosting a SAN array, each of them connected to 2 SAN fabrics extended between the 2 sites.
What reason would prevent us from building Geo-clusters without having to rely on a database replication mechanism, as the setup I would like to implement would also be used to provide NFS services that are disaster recovery proof.
Obviously, such setup should rely on LVM mirroring to allow a node hosting a service to be able to write to both local and distant SAN LUN's.
Brem
I have been meaning to test this but have not had a chance...
Tom
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