Re: performance drop on RAID5

> Hello,
> i have a pg-8.0.3 running on Linux  kernel  2.6.8,  CPU  Sempron
2600+,
> 1Gb RAM on IDE HD ( which could be called a "heavy desktop" ),
measuring
> this performance with pgbench ( found on /contrib ) it gave me an
> average ( after several runs ) of 170 transactions per second;

170 tps is not plausible no a single platter IDE disk without using
write caching of some kind.  For a 7200 rpm drive any result much over
100 tps is a little suspicious. (my 10k sata raptor can do about 120).

> for the sake of experimentation ( actually, i'm scared this IDE drive
> could fail at any time, hence i'm looking for an alternative, more
> "robust", machine ), i've installed on an aging Compaq Proliant server
(
> freshly compiled SMP kernel 2.6.12.5  with preemption ), dual Pentium
> III Xeon 500Mhz, 512Mb RAM, (older) SCSI-2 80pin drives, and
re-tested,
> when the database was on a single SCSI drive, pgbench gave me an
average
> of 90 transactions per second, but, and that scared me most, when the
> database was on a RAID-5 array ( four 9Gb disks, using linux software
> RAID mdadm and LVM2, with the default filesystem cluster size of 32Kb
),
> the performance dropped to about 55 transactions per second.

Is natural to see a slight to moderate drop in write performance moving
to RAID 5.  The only raid levels that are faster than single disk levels
for writing are the ones with '0' in it or caching raid controllers.
Even for 0+1, expect modest gains in tps vs. single disk if not using
write caching.

Merlin