Re: Losing data from Postgres

On Wed, 15 Nov 2000, Alfred Perlstein wrote:

> * Serge Canizares <serge@ephilosopher.com> [001115 08:23] wrote:
> >
> > Of course, if someone sees a reason that RAID 5 would be better than RAID 1+0,
> > I'd appreciate an explanation!
>
> Cost. :)
>

A little bit more explanation: :-)

RAID 1+0 gives you only half of the installed space but gives you (n/2)
times speed of individual disks. For example for 4 hard disks 1G each, you
get only 2G of space but double the speed of individual disks.

RAID 5 gives you space equal to (n-1) times individual disks. This is far
better than RAID 1+0. For example for 4 hard disks 1G each, you get 3G
space (this is only 25% waste compared to 50% for RAID 1+0). As you add
hard disks to the array, the 25% ratio of RAID 5 get lower and lower, but
that of RAID 1+0 stays at 50%. For 10 hard disks for example, the ratio
gets down to 10%.
Speed of RAID 5 however is very much dependant of a few factors: speed of
the controller (or CPU speed in case of software RAID), type of data, and
how the array is setup (how many blocks of data per strip, etc.). In
theory, it can exceed speed of RAID 1+0, but I have never seen it in real
life (but it does approach that of RAID 1+0 if you spend $$$ on the
controller or CPU).


Hope it helps,
Hossein