Re: Hardware related question: 3ware 9500S

On Wed, 2006-04-12 at 13:10, Ted Byers wrote:
> > ----- Original Message -----
> > From: "Merlin Moncure" <mmoncure(at)gmail(dot)com>
> > To: "Janning Vygen" <vygen(at)gmx(dot)de>
> > Cc: <pgsql-general(at)postgresql(dot)org>
> > Sent: Wednesday, April 12, 2006 12:31 PM
> > Subject: Re: [GENERAL] Hardware related question: 3ware 9500S
> > [snip]
>
> > > - I want to know if 3ware 9500 S is recommended or if its one of those
> > > controllers which sucks.
> >
> > escalade is a fairly full featured raid controller for the price.
> > consider it the ford taurus of raid controllers, it's functional and
> > practical but not sexy. Their S line is not native sata but operates
> > over a pata->sata bridge. Stay away from raid 5.
> >
> Hi Merlin
>
> Why? What's wrong with raid 5? I could well be wrong (given how little
> attention I have paid to hardware over the past few years because of a focus
> on developing software), but I was under the impression that of the raid
> options available, raid 5 with hot swappable drives provided good data
> protection and performance at a reasonably low cost. Is the problem with
> the concept of raid 5, or the common implementations?
>
> Do you have a recommendation regarding whether the raid array is built into
> the server running the RDBMS (in our case PostgreSQL), or located in a
> network appliance dedicated to storing the data managed by the RDBMS? If
> you were asked to design a subnet that provides the best possible
> performance and protection of the data, but without gold-plating anything,
> what would you do? How much redundancy would you build in, and at what
> granularity?

There have been NUMEROUS discussions of RAID-5 versus RAID 1+0 in the
perform group in the last year or two. Short version:

RAID 5 is useful, with large numbers of drives, for OLAP type databases,
where you're trying to get as much storage as possible from your
drives. RAID 5 pretty much REQUIRES battery backed cache for decent
write performance, and even then, will saturate faster than RAID 1+0.
RAID-5 cannot survive multiple simultaneous drive failures.

RAID 1+0 requires better than average controllers, since many serialize
and lockstep data through the various layers of RAID on them. It
provides less storage for a given number of drives. It is faster for
OLTP workloads than RAID-5. RAID 1+0 can survive multiple drive
failures as long as two drives in the same mirror set do not fail at
once.

With increasing number of drives, the chances of a RAID 5 failing go up
linearly, while the chances of RAID 1+0 failing due to multiple drive
failure stay the same.