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Re: [Linux-cluster] More GFS2 tuning...

On Mon, Feb 16, 2009 at 7:09 PM, Gordan Bobic <gordan bobich net> wrote:
>> When a large multi user,  multi file, multi thread simulation of a
>> total file output of 18GB is run, I plot the output of vmstat 1 and
>> see a definite pattern with is very periodic. The bo values start at
>> around 200MB, then drop down to 0 in most cases for a few seconds,
>> then spike to ~700MB/s then eases back down to 200, 150 and back down
>> to 0. It looks very much like a cacheing issue to me. These numbers
>> are almost identical on the FC switches.
> How are your test files distributed across directories, and what is your
> ratio of reads to writes? Are you mounting with noatime,nodiratime,noquota?
> What is your clustering network connection?

The tests are from a single node in the cluster, using a single
filesystem/mount point. I am doing writes only. I mount using
noatime,nodiratime. When I tried noquota, I got an error for invalid

> If all your files are in the same directory (or a small number of
> subdirectories) and the access is distributed across all the nodes, then I
> have to say that you may well be out of luck and what you are seeing is
> normal. Bouncing directory locks between the nodes on each access will
> introduce enough latency to kill the performance. Also remember that no two
> nodes can have a lock on the same file at the same time, and for file
> creation/deletion, that means a directory lock, which in turn means only one
> file creation/deletion per directory at any one time.

I'll try it with a single file and check the difference.



> I can well believe the 900MB/s figure if you are just reading back one big
> file from multiple nodes. But the performance will fall off a cliff on
> random I/O involving writes.
> Gordan
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