[Lustre-discuss] external journals

[Andreas Dilger]

On May 29, 2008  16:30 -0400, Brock Palen wrote:
> Whats a good way to find out if your (our) workload would benefit  
> from external journals?  Our OST's are x4500's and I get little if  
> any activity on the journals from my regular benchmarks.

The best way to know is to run your workload with an external journal...
I can't really say any more than this given the information here.

> What are the benefits?  Does ldiskfs do anything intelligent with  
> external journals? What should we see the most help with?  Or should  
> be just devote these disks to being another OST?

The benefit of an external journal is that the journal IO (for all
metadata operations (file block allocations, bitmaps, etc) go to
a separate device instead of causing seeking between the journal
and the rest of the filesystem.

Whether this is of use to you depends on many factors, whether it
is better to use these disks for the journal and reduce seeking,
or to add IO bandwidth to a RAID set or make a new OST...

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