[Lustre-discuss] MDS disk recmendations

Andreas Dilger adilger at sun.com
Mon Jun 16 12:10:20 PDT 2008


On Jun 16, 2008  14:50 -0400, Brock Palen wrote:
> On Jun 16, 2008, at 2:31 PM, Andreas Dilger wrote:
> > Note - It is better to have the MDS on RAID1+0 than on RAID5 or RAID6.
> >
> > RAID1 with an internal journal and two disks from different controllers.
> > If you need a larger MDT, create multiple RAID1 devices from pairs of
> > disks, and then make a RAID0 array of the RAID1 devices.  This ensures
> > maximum reliability because multiple disk failures only have a small
> > chance of hitting both disks in the same RAID1 device.
> 
> We are going to have several unused disks in the MGS/MDS array.   
> Would it be helpful for the MDS (less the MGS I would think) to use  
> and external journal on a pair of disks in raid 1?  I am not sure it  
> would help that much, but I could be wrong.

Yes, having an external RAID1 journal helps spread the load over more
spindles and avoids seeking the "filesystem" heads to write into the
journal area.  The journal itself is mostly written linearly.  Note
also that "journal size ~= RAM usage" so don't go making it too huge
unless you have enough RAM to back it up.  Some customers use journals
up to 2GB (on a 32GB RAM machine) for maximum performance.

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




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