What is best practice for using a SAN with ASM in an 11gR2 RAC installation

Question

I'm setting up a RAC environment. Planning on using Oracle 11g release 2 for RAC & ASM, although the db will initially be 10g r2. OS: RedHat. I have a SAN available to me and want to know the best way to utilise that via ASM.
I've chosen ASM as it allows me to store everything, including the voting and cluster registry files.
So I think I'll need three disk groups: Data (+spfile, control#1, redo#1, cluster files#1), Flashback (+control#2, redo#2, archived redo, backups, cluster files#2) and Cluster - Cluster files#3. So that last one in tiny.
The SAN and ASM are both capable of doing lots of the same work, and it's a waste to get them both to stripe & mirror.
If I let the SAN do the redundancy work, then I can minimize the data transfer to the SAN. The administrative load of managing the discs is up to the Sys Admin, rather than the DBA, so that's attractive as well.
If I let ASM do the work, it can be intelligent about the data redundacy it uses.
It looks like I should have LUN (Logical Unit Numbers) with RAID 0+1. And then mark the disk groups as extrenal redundancy.
Does this seem the best option ?
Can I avoid this third disk group just for the voting and cluster registry files ?
Am I OK to have this lower version of Oracle 10gr2 DB on a RAC 11gr2 and ASM 11gr2 ?
TIA, Duncan

Answer 1

Hi Duncan,
if your storage uses SAN RAID 0+1 and you use "External" redundancy, then ASM will not mirror (only stripe).
Hence theoretically 1 LUN per diskgroup would be enough. (External redundancy will also only create 1 voting disk, hence only one LUN is needed).
However there are 2 things to note:
-> Tests have shown that for the OS it is better to have multiple LUNs, since the I/O can be better handled. Therefore it is recommended to have 4 disks in a diskgroup.
-> LUNs in a diskgroup should be the same size and should have same I/O characteristica. If you now have in mind, that maybe your database one time will need more space (more disks) than you should use a disk size, which can easily be added, without waisting too much space.
E.g:
If you have a 900GB database then does it make sense to only use 1 lun with 1TB?
What happens if the database grows, but only grows slightly above 1TB? Then you should add another disk with 1TB.... You loose a lot of space.
Hence it does make more sence to use 4 disks á 250GB, since the "disks" needed to grow the disk groups can be extended more easily. (just add another 250G disk).
O.k. there is also the possibility to resize a disk in ASM, but it is a lot easier to simple add an additional lun.
PS: If you use a "NORMAL" redundancy diskgroup, then you need at least 3 disks in your diskgroup (in 3 failgroups) to be able to handle the 3 voting disks.
Hope that helps a little.
Sebastian