noob storage pools and LUN setup

[mckingsley]

I have not actually set anything up yet. Just doing a lot of research.

Lets say you have a bunch of RAID boxes and to keep things simple you just want to do a bunch of RAID 5 for Data.

I've been reading a lot about setting up even amounts of LUN's per storage pool, even about using 4 as a magic #. Any definitive word on why? I guess it makes sense if XSan generally writes in a RAID 0 over the LUN's so that is why it should be even. Any notable exceptions?

If it is true that you should not go above 4 (or some number) in a new ideal setup, then I guess you need multiple storage pools. How does the round robin with multiple storage pools relate to the above. Is it similar to a RAID 0 situation?

Last if you already have your SAN setup and need to add more storage, is it preferable to make another storage pool or add more LUN's to a current storage pool. As far as space allocation it would seem adding more LUNS would add a disparity of space allocation unless you recopy files on it. Adding another storage pool would kind of do the same with round robin?

Thanks, if I don't make sense...sorry

[PaulRS]

Hi there.

Yes, LUNs are best added to storage pools in binary multiples- 2, 4, 8, 16, or 32 (the max). With Promise storage the practical number is considered 4 because you will not get more than about 1GB/s from a single storage pool.

If files written to a volume are approximately the same size and bandwidth requirement, then round-robin will result in adding the bandwidth of each storage pool.

It is possible to expand a volume by adding LUNs to a storage pool (called a bandwidth expansion), but it rarely works if there is a significant amount of data on the volume. The filesystem will see if it can re-distribute the data from the existing LUNs to the new- in most cases it comes back and says "nope I'm leaving everything the way it is". You don't loose anything, but you're kind of stuck!

-Paul