[Linux-cluster] Starter Cluster / GFS

Gordan Bobic gordan at bobich.net
Wed Nov 10 12:17:14 UTC 2010

Nicolas Ross wrote:
> Thanks
>> 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.

>> If you are only ever going to have the SAN volume mounted on one 
>> device at a time, don't bother with GFS and make the SAN block device 
>> a fail-over resource so that only one node can mount it at a time, and 
>> put a normal non-shared FS on it. You will get better performance.
> I do need a shared file-system, I am aware of the added latency, we 
> currently have some latency on our xSan setup. But we do also need on 
> some services an additional block-device that is accessed only by one 
> node and is indeed failed-over another node when a node fail.

So handle the file system failover for the ones where only one node 
accesses them at a time and have a shared file system for the areas 
where multiple nodes need concurrent access.

>> The number of journals needs to be equal to or greater than the number 
>> of nodes you have in a cluster. e.g. if you have 5 nodes in a cluster, 
>> you need at least 5 journals. If you think you might upgrade your 
>> cluster to 10 nodes at some point in the future, then create 10 
>> journals, as this needs to be done at FS creation time.
> That I got. It's the size that I don't know how to figure out. Will 32 
> megs will be enough ? 64 ? 128 ?

That depends largely on how big your operations are. I cannot remember 
what the defaults are, but they are reasonable. In general, big journals 
can help if you do big I/O operations. In practice, block group sizes 
can be more important for performance (bigger can help on very large 
file systems or big files).


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