On Aug 20, 2009 09:27 +0200, Arne Wiebalck wrote: >> Is there a reason to do this instead of, say, two 5TB OSTs using MD RAID-0? >> 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 > > What is a 'large' file for you?
Well, this is relative to the size of the OST itself, and the striping. For example, if you have 1TB OSTs and stripecount=1, but your files are 500GB in size, then you might only be able to fit one onto each OST (due to fs-internal overhead), leaving about 50% of each OST unusable. If you have 8TB OSTs then you could store 15 files/OST, leaving about 6% of each OST unusable. This is "external" free space fragmentation. Similarly, if you have, say, 5% of space free in each OST, then for an 8TB OST the free space would likely be in 8x larger chunks of space, vs a 1TB OST. Other benefits of fewer, larger OSTs: - less configuration - more bandwidth per stripe - more space per file (if you have > 160 OSTs) The reasons for NOT going to a larger single OST are: - more resource contention - larger point of failure - longer e2fsck time Cheers, Andreas -- Andreas Dilger Sr. Staff Engineer, Lustre Group Sun Microsystems of Canada, Inc. _______________________________________________ Lustre-discuss mailing list [email protected] http://lists.lustre.org/mailman/listinfo/lustre-discuss
