Re: Postgresql on software RAID

On Wed, 17 Dec 2003, Adam Witney wrote:

> On 17/12/03 3:45 pm, "scott.marlowe" <scott(dot)marlowe(at)ihs(dot)com> wrote:
>
> > On Tue, 16 Dec 2003, Robert Creager wrote:
> >
> >> When grilled further on (Tue, 16 Dec 2003 22:30:04 -0600),
> >> Patrick Spinler <spinler(at)kmtel(dot)com> confessed:
> >>
> >>>
> >>> According to the theory they expound, a database with any significant
> >>> write activity whatsoever should never be on raid 5, but instead be on
> >>> raid 0+1.
> >>>
> >>
> >> Kind of related and a point of reference. We use ClearCase and have many
> >> multiple Gb vob's(databases). We were using RAID-5, but had to back off to
> >> RAID
> >> 0+1 because of performance reasons (which was indicated in the manual, once
> >> you
> >> read it...). This would happen around 1-2Gb's vob size. Our usage of CC
> >> provides heavy writing activity to the underlying dB.
> >>
> >> I don't know what kind of dB engine Atria->Rational->IBM has implemented
> >> underneath, or even it it would look like a dB to someone who knew the
> >> difference...
> >
> > Just wondering, was that on hardware or software RAID5, and if hardware
> > did it have battery backed cache controllers? Makes a huge difference. I
> > would never use SW RAID5 for heavily written databases.
>
> Hi Scott,
>
> What level of activity would you call "heavily written"?

More than a couple inserts a second. Or more specifically, depending on
the machine, when the CPU usage starts to climb, which really can vary a
lot from machine to machine. A machine with 4 2.8GHz CPUs could probably
handle a much higher write load than a single <1GHz machine.

Things like a data warehouse where you feed in streams slowly or at night
and then do huge selects work well on RAID5 sw. Transactional systems,
like billing or reservation systems should probably not be on sw RAID.