[Lustre-discuss] Rule of thumb for setting up lustre resources...

[Brian J. Murrell]

On Tue, 2008-06-17 at 09:40 -0400, Mark True wrote:
> Thanks so much for the prompt response, I do have a couple of
> questions for clarification:
> 
> Does the hardware makeup of the OSS affect the speed of the OSTs?

Of course.  An OST is only going to go as fast as the hardware that it's
made up of.  If you put a slow disk in an OSS, the OST is going to be
slow.

> If so, what is likely to be the bottleneck in an OSS.

There is no one right answer to that.  You have to get together with
your hardware vendor and explain your use scenario (i.e. for an OSS) and
have them spec out some hardware that meets the use-case.  If you will
be spec'ing your own hardware then you need to grab the technical
specifications for all of the hardware you are proposing using and
understand their performance aspects.  If you don't feel confident in
being able to do the latter, then I would suggest you do the former.

In general, an OSS is I/O bound.  You need to provide enough I/O
capacity between the disk and network through which the data will
travel.

> Say we have an OSS with 3 OSTs attatched, is that different than
> having
> three OSSs with 1 OST apiece?

That depends on whether that OSS with the 3 OSTs attached has the I/O
capacity to do full-out I/O to all three disks.  As I've said before,
this is basically an exercise in understanding the capacity of your
entire I/O path from OST to client and sizing to meet that capacity.

> Also, does the OSS have as much of a performance impact as the speed
> of the
> OST.

The OSS hosts the OST, so you can't really compare the performance
impact of one vs. the other.

> What is the recommended max number of OSTs per OSS?

As you have probably gathered, that is *completely* dependent on the
hardware you are using for OSSes and OSTs and there is no one answer
that fits all hardware.  You just want to make sure you don't create
bottlenecks, again by understanding the capacity of the various paths
from OST (disk) to client.

> If I am able to determine the max capabilities of an OST/OSS is it
> safe to
> assume that the increase in performance scales linearly as I increase
> the
> number of OSS/OSTs?

Yes, raw bandwidth will grow pretty linearly.  It is up to your
applications and use-cases to take advantage of that though.

b.


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