Re: What exactly is postgres doing during INSERT/UPDATE ?

I've already learned my lesson and will never use raid 5 again.  The 
question is what I do with my 14 drives. Should I use only 1 pair for 
indexes or should I use 4 drives?  The wal logs are already slated for 
an SSD.

Scott Marlowe wrote:
> On Sat, Aug 29, 2009 at 2:46 AM, Greg Stark<gsstark@mit.edu> wrote:
>> On Sat, Aug 29, 2009 at 5:20 AM, Luke Koops<luke.koops@entrust.com> wrote:
>>> Joseph S Wrote
>>>> If I have 14 drives in a RAID 10 to split between data tables
>>>> and indexes what would be the best way to allocate the drives
>>>> for performance?
>>> RAID-5 can be much faster than RAID-10 for random reads and writes.  It is much slower than RAID-10 for sequential writes, but about the same for sequential reads.  For typical access patterns, I would put the data and indexes on RAID-5 unless you expect there to be lots of sequential scans.
>> That's pretty much exactly backwards. RAID-5 will at best slightly
>> slower than RAID-0 or RAID-10 for sequential reads or random reads.
>> For sequential writes it performs *terribly*, especially for random
>> writes. The only write pattern where it performs ok sometimes is
>> sequential writes of large chunks.
> 
> Note that while RAID-10 is theoretically always better than RAID-5,
> I've run into quite a few cheapie controllers that were heavily
> optimised for RAID-5 and de-optimised for RAID-10.  However, if it's
> got battery backed cache and can run in JBOD mode, linux software
> RAID-10 or hybrid RAID-1 in hardware RAID-0 in software will almost
> always beat hardware RAID-5 on the same controller.
>