[Lustre-discuss] Limit of OSTs per OSS?

[Arne Wiebalck]

Andreas Dilger wrote:
> On Aug 19, 2009  13:55 +0200, Arne Wiebalck wrote:
>> Brian J. Murrell wrote:
>>> On Wed, 2009-08-19 at 11:45 +0200, Arne Wiebalck wrote:
>>> Unless you are making lots and lots of small OSTs -- which is not
>>> usually beneficial anyway -- typically, you will run into resource
>>> limitations (memory, bus bandwidth, etc.) on an OSS before you hit a
>>> limit on the number of OSTs.
>> I just thought I remembered there was something like 8 OSTs per OSS,
>> but apparently I was wrong.
>>
>>> Just how many OSTs are you considering, and how big will they be?
>> My OSSs will have 10 OSTs with 1TB each.
> 
> Is there a reason to do this instead of, say, two 5TB OSTs using MD RAID-0?

I thought a higher number of independent spindles was better. I should
add that the 1TB OSTs are HW RAID-1s already.

> Or for that matter one 8+2 8TB OST with MD RAID-6?  That will give
> better space utilization if you have large files, otherwise you will
> have a lot of smaller chunks of free space on each OST that cannot
> be utilized well when the OSTs are nearly full.
> 
> If you are looking at straight performance it may be that 10x 1TB OSTs
> is the fastest, since each one can be seeked independently.  Reducing
> the journal size from the default 400MB is probably not harmful if you
> have correspondingly more OSTs.

OK, thanks for the hints.

Cheers,
  Arne
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