Right now, we're running a RAID 1 for pg_clog, pg_log and pg_xlog and then a RAID 1+0 with 12 disks for the data. Would there be any benefit to running a separate RAID 1+0 with a tablespace for the indexes?
Not really. PostgreSQL doesn't currently support parallel backend fetches, aggregation, or really anything. It's looking like 9.4 will get us a lot closer to that, but right now, everything is serial.
Serial or not, separate backends will have separate read concerns, and PostgreSQL 9.2 and above *do* support index only scans. So theoretically, you might actually see some benefit there. If it were me and I had spindles available, I would just increase the overall size of the pool. It's a lot easier than managing multiple tablespaces.
Ok, that makes sense. Is there a benefit to having the WAL and logs on the separate RAID 1? Or is just having them be part of the larger RAID 1+0 just as good?