At 02:50 PM 12/24/2005, Frank Wiles wrote:
>On Wed, 21 Dec 2005 22:31:54 -0500
>Juan Casero <caseroj@comcast.net> wrote:
>
> > Sorry folks. I had a couple of glasses of wine as I wrote this.
> > Anyway I originally wanted the box to have more than two drives so I
> > could do RAID 5 but that is going to cost too much. Also, contrary
> > to my statement below it seems to me I should run the 32 bit
> > postgresql server on the 64 bit kernel. Would you agree this will
> > probably yield the best performance? I know it depends alot on the
> > system but for now this database is about 20 gigabytes. Not too large
> > right now but it may grow 5x in the next year.
>
> You definitely DO NOT want to do RAID 5 on a database server. That
> is probably the worst setup you could have, I've seen it have lower
> performance than just a single hard disk.
>
> RAID 1 and RAID 1+0 are optimal, but you want to stay far away from
> RAID 5. IMHO RAID 5 is only useful on near line backup servers or
> Samba file servers where space is more important than speed.
That's a bit misleading. RAID 5 excels when you want read speed but
don't care as much about write speed. Writes are typical ~2/3 the
speed of reads on a typical decent RAID 5 set up.
Side Note: Some years ago Mylex had a family of fast (for the time)
RAID 5 HW controllers that actually read and wrote at the same
speed. IBM bought them to kill them and protect LSI Logic. Mylex
X24's (?IIRC the model number correctly?) are still reasonable HW.
So if you have tables that are read often and written to rarely or
not at all, putting them on RAID 5 is optimal. In both data mining
like and OLTP like apps there are usually at least some such tables.
RAID 1 is good for stuff where speed doesn't matter and all you are
looking for is an insurance policy.
RAID 10 is the best way to get high performance on both reads and
writes, but it has a significantly greater cost for the same amount
of usable physical media.
If you've got the budget or are dealing with small enough physical
storage needs, by all means use RAID 10. OTOH, if you are dealing
with large enterprise class apps like Sarbanes Oxley compliance,
medical and/or insurance, etc, etc, the storage needs can get so
large that RAID 10 for everything or even most things is not
possible. Even if economically feasible.
RAID levels are like any other tool. Each is useful in the proper
circumstances.
Happy holidays,
Ron Peacetree