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

On Sat, Aug 29, 2009 at 2:46 AM, Greg Stark<gsstark(at)mit(dot)edu> wrote:
> On Sat, Aug 29, 2009 at 5:20 AM, Luke Koops<luke(dot)koops(at)entrust(dot)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.