[GENERAL] WAL/pg_xlog on Another Disk: Redundancy?

[Thomas F.O'Connell]

It's part of the conventional wisdom of performance tuning to keep 
pg_xlog on a separate physical disk, and this seems to be common sense 
to me. Am I correct in deducing that the performance gain comes from 
preventing a file being written to sequentially from competing for I/O 
resources with the data presumably experiencing somewhat random access 
in terms of reads/writes? If so, would it be useful to add a few 
explanatory sentences to the docs?

http://www.postgresql.org/docs/7.4/static/wal-internals.html
"It is of advantage if the log is located on another disk than the main 
database files. This may be achieved by moving the directory  pg_xlog 
to another location (while the server is shut down, of course) and 
creating a symbolic link from the original location in the main data 
directory to the new location."

Anyway, I'm wondering what experiences people have had in setting this 
up in production. If I have a server with 5 drive bays, one thought I 
had was to have a RAID 1+0 for the primary partition and then to put 
WAL on the fifth disk, but I assume that failure of that drive if there 
were no redundancy would be a Bad Thing. Another option would be to do 
RAID 5 on three drives and WAL mirrored on the remaining 2 drives. But 
RAID 5 is slower than RAID 1+0 for data I/O in that scenario according 
to what I've read.

Would a more ideal setup be a six-drive server with a RAID 1+0 on four 
drives with 2 drives dedicated to WAL that mirrored one another? Is 
that overkill? Anyone have recommendations for an appropriate setup for 
a 5-bay server?

Thanks!
-tfo
--
Thomas F. O'Connell
Co-Founder, Information Architect
Sitening, LLC
http://www.sitening.com/
110 30th Avenue North, Suite 6
Nashville, TN 37203-6320
615-260-0005
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