[Lustre-discuss] Recommended segment size for MDS raid

[Andreas Dilger]

On 2010-02-02, at 09:46, Alexander Oltu wrote:
> I am trying to find information regarding optimal segment size for MDS
> on RAID10 external array. Looking in documentation and searching
> through mailinglist and google didn't help.
>
> I found in Oracle DB documentation
> (http://www.oracle.com/technology/deploy/availability/pdf/Engenio_LCS.pdf 
>  ,
> p.11) that Oracle is using 1MB I/O size. That means that optimal
> segment size for Oracle setup which has 4 data drives will be 1MB/4 =
> 256KB.

While Oracle recently bought Sun, I don't think this information is at  
all relevant. :-)

> But I am not sure what is the maximum I/O size for mdt.
> Our setup MDS will be:
> 6 x 15K FC disks in RAID10 on Engenio 399x controller.
>
> For mdt/mgs formatting we want to use -E stride= and stripe-width= ,
> which we will calculate from RAID segment size.
>
> Can somebody from Lustre guru advise optimal segment size for RAID 10
> external array?

Much of the MDT IO is in 4kB blocks, but whether that implies a 4kB  
RAID chunk size is a separate issue.  Probably 64kB is the smallest  
practical size, given that smaller IOs to the disk don't go  
significantly faster.

Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.