another newbie question

[Jeffrey Watts]

On Wed, 18 Oct 2000, Tony Hammitt wrote:

> Raid in Linux is best accomplished by using software instead of
> hardware, especially on a newer x86 box.  Software raid is much faster
> than hardware, as well as being cheaper.  You would get a lot more use
> out of extra main memory and processor speed that you can buy instead
> of an expensive raid card.

I'm assuming you miswrote this.  Hardware RAID is ALWAYS faster than
software.  Period.  I suppose you could find a crappy RAID card that
somehow screws up in one of the RAID types, but you can, with certainty,
trust that 99.9% of the hardware RAID implementations out there are better
than software.

Heck, the raison d'ete of hardware RAID controllers is that they are
faster.  Who would buy one if hardware RAID was slower?

Now I agree with you if you are talking about a low-end server from a
_practical_ perspective.  I would state that it would be better worth your
money to buy extra CPU and RAM if you are doing simple mirroring (RAID 1)
or striping (RAID 0).  On a low-end system the performance hit isn't that
bad, and the money saved can be considerable.

But do NOT do RAID 4 or RAID 5 with software.  Skimp on the CPU and/or RAM
if you must, but do RAID 4/5 with hardware only.  RAID 4/5 requires that
for every write made, parity information must be generated.  Now, hardware
implementations do this very fast and very efficient, but software
implementations are just too slow and are an inefficient use of CPU
cycles.

Also, most hardware RAID cards have either built-in cache or have sockets
where you can add some.  This greatly improves performance, since the CPU
sees the RAID volume as a single disk and can fire-and-forget its write to
the RAID controller.  This again greatly improves performance, especially
RAID 4/5 write performance.

Hope this helps,
J.

o-----------------------------------o
| Jeffrey Watts                     |
| watts at jayhawks.net         o-------------------------------------------o
| Systems Programmer         | "Is uniformity [of religion] attainable?  |
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