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Re: [Linux-cluster] GFS cluster / DLM locking - Mostly idle but high load

On Wed, 17 Oct 2007, Nikolas Lam wrote:

I have a cluster (3 nodes at the moment, may grow up to 16) for handling a
lot of small files (Maildir). When I test the system by sending around 3-5
messages/second I see the load on the cluster nodes go up to about 20-30,
even though the CPUs on the cluster are about 90% idle at all times.

I am guessing that this is due to the clustered machines waiting for DLM
locks to be established, which causes a lot of processes to be fighting to
run, but since they don't get to run very soon, they back up and cause the
load averages to go up.

Assuming the DLM runs over the interface specified by IP and MAC in
cluster.conf, it is running over gigabit ethernet.

Are there any configuration changes or tuning parameters I can apply to
DLM to alleviate this condition? The machine I'm running the test from
(the one sending messages) is about 1/4 of the spec of each of the cluster
nodes, and it's running a load average of about 0.4. It seems crazy that a
single low-spec node should be able to completely overwhelm a cluster 12x
it's spec several times over.

I don't know alot about GFS but since no one else has replied yet, my
understanding is that it's not suitable for an applications like what
you describe (many small files being opened frequently). I think GFS2,
which is still a tech preview, has been redesigned to improve this

Indeed, I am aware that GFS2 is still broken, but I seem to be getting no worse a performance out of GFS than I get out of NFS. The only penalty is the high load, but the throughput is actually similar. The advantage that makes GFS win is that I don't need an arbitrating server to handle the NFS exports, which makes the clustering and redundancy a bit tidier.


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