[Linux-cluster] Cluster with shared storage on low budget

Gordan Bobic gordan at bobich.net
Tue Feb 15 13:12:58 UTC 2011

Nikola Savic wrote:
> Gordan Bobic wrote:
>>> What is main reason for you not to use LVM on top of DRBD? Is it just
>>> that you didn't require benefits it brings? Or, it makes more
>>> problems by your opinion?
>> Traditionally, CLVM didn't provide any tangible benefits (no
>> snapshots), and I never found myself in a situation where dynamically
>> growing a volume with randomly assembled storage was required. If you
>> are JBOD-ing a bunch of cheap SATA disks, you might as well size the
>> storage correctly to begin with and not have to bother with LVM. I'm
>> assuming this is what you are doing since you are doing it on the
>> cheap (SAN-less). If you are using a SAN, the SAN will provide
>> functionality to grow the exported block device and you can just grow
>> the fs onto that, without needing LVM.
>> So apart from snapshots (non-clustered) or a setup like what was
>> suggested earlier, to have DRBD on top of local LVM to gain
>> local-consistency snapshot capability in a cluster (not sure I'd trust
>> that with my data, but it may be good for non-production
>> environments), I don't really see the advantage. Snapshots also only
>> give you crash-level consistency, which I never felt was good enough
>> for applications like databases. A replicated slave that you can shut
>> down is generally a more reliable solution for backups.
> Thank you for detailed response!
> I generally like idea of removing unneeded levels of technology.
> In case DRBD+GFS2 is used for shared storage, do I need cluster suite?
> Can GFS2 in this setup without cluster setup?

No, it cannot. GFS2's locking is dependant on the cman service being up 
and quorate, so yes, you still need the cluster suite being up and 
running, since that is what handles fencing. You could replace 
DRBD+GFS+RHCS with, say, DRBD+OCFS2+Heartbeat, but that wouldn't gain 
you anything either way - you'd still need fencing configured and working.

Note that DRBD should also have fencing (stonith) configured, and on a 
lower time-out than the rest of the cluster layer to eliminate 
possibility of split-braining.


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