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Re: [Linux-cluster] Starter Cluster / GFS

Nicolas Ross wrote:

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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