Hi,
I configured a similar architecture some months ago and this is the best choice after some pgbench and Bonnie++ tests.
Server: DELL R720d
CPU: dual Xeon 8-core
RAM: 32GB ECC
Controller PERC H710
Disks:
2xSSD (MLC) Raid1 for Operating System (CentOS 6.4)
4xSSD (SLC) Raid10 for WAL archive and a dedicated "fast tablespace", where we have most UPDATE actions (+ Hot spare).
10xHDD 15kRPM Raid5 for "default tablespace" (optimized for space, instead of Raid10) (+ Hot spare).
Our application have above 200 UPDATE /sec. (on the "fast tablespace") and above 15GB per die of records (on the "default tablespace").
After the testing phase I had the following conclusion:
4xSSD (SLC) RAID 10 vs. 10xHDD RAID 5 have comparable I/O performance in the sequential Read and Write, but much more performance on the Random scan (obviously!!), BUT as far I know the postgresql I/O processes are not heavily involved in a random I/O, so at same price I will prefer to buy 10xHDD instead of 4xSSD (SLC) using above 10x of available space at the same price!!
10xHDD RAID 10 vs. 10xHDD RAID 5 : with Bonnie++ I noticed a very small difference in I/O performance so I decided to use RAID 5 + a dedicated Hot Spare instead of a RAID10.
If I could go back, I would have spent the money of the SLC in other HDDs.
regards.