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

[Gordan Bobic]

Thomas Sjolshagen wrote:
> On Tue, 15 Feb 2011 12:49:38 +0100, Nikola Savic wrote:
> 
>>    This is interesting approach. I understand that DRBD with GFS2 
>> doesn't require LVM between, but it does bring some inflexibility:
>>
>>     * For each logical volume, one has to setup separate DRBD
>>     * Cluster wide logical volume resizing not easy
>>     * No snapshot - this is very important to me for MySQL backups.
>>
>>   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?
> 
> Just so you realize; If you intend to use clvm (i.e. lvme in a cluster 
> where you expect to be able to write to the volume from more than one 
> node at/around the same time w/o a full-on failover), you will _not_ 
> have snapshot support. And no, this isn't "not supported" as in "nobody 
> to call if you encounter a problem", it's "not supported" as in "the 
> tools will not let you create a snapshot of the LV".
> 
> However, you ought to be able to configure one of the DRBD mirror 
> members as part of a split/mount read-only/merge based equivalent and 
> thus get a similar result, I think.

Indeed, that is right - you can drop a server out of the cluster, stop 
drbd replication and mount it read-only (lock_nolock) and use that as a 
"snapshot". The added benefit is that it won't cause massive cluster 
slow-down through lock-bouncing during the backup.

Gordan