Re: Building a fast pc (1)

Lehi Gracia wrote:
> 
> i would benchmark software raid0 also, hardware raid uses the
> processor of the raid adapter. how fast can that be? if you use
> software RAID then you will be using the processor of your system that
> is a whole lot faster than the raid adapter's so if you have some
> cycles to spare on your processor just go with software RAID, it is
> readily available in windows 2000 and 2003, and NT for that matter.

    If speed is the objective, hardware RAID will usually
do better than software RAID.  True, the processors in the
hardware RAID controller are slower than those in the host
machine -- but they don't need to be tremendously fast!
The disks themselves are slow devices ("slow" in comparison
to a CPU), so a faster processor in the controller would
merely spend more time in its idle loop.  In rush-hour traffic,
mopeds and Maseratis travel at the same speed.

    Meanwhile, though, the RAID controller probably contains
a write cache: a bunch of RAM with battery backup.  This can
dramatically lower the apparent time for a write operation,
because the controller can signal "done" to the host as soon
as the data has made it across the bus (SCSI, FC, whatever)
and into the cache.  The host can then go on about its business
without waiting for seek time and rotational delays on the
physical disk; the RAID controller will take care of all that
at a convenient time and without disturbing the host.

    However, if the I/O load is random and mostly "I," hardware
RAID doesn't help much.  You'll only be fetching data from the
disk when you've already failed to find it in the O/S' own caches.
The O/S usually has far more memory at its disposal than does the
RAID controller, so the chance of having the desired block in the
controller's cache given that it's already vanished from the O/S'
cache is pretty small; most reads must wait for the physical drive.
(Note the assumption of a random as opposed to sequential I/O load
here: for sequential reads, the controller's cache can be very
helpful.)

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