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Re: RAID & Hard disk performance

On Tue, 6 Nov 2001 07:26, Dave Watkins wrote:
> Not to start a holy war, but there are real reasons to use SCSI.
> The big ones are
> Much larger MTBF,

Mean Time Between Failures is not such a big deal when you run RAID.  As long 
as you don't have two drives fail at the same time.  Cheaper IDE disks make 
RAID-10 more viable, RAID-10 allows two disks to fail at the same time as 
long as they aren't a matched pair.  So a RAID-10 of IDE disks should give 
you more safety than a RAID-5 of SCSI.

> faster access times due to higher spindle speeds, better

When doing some tests on a Mylex DAC 960 controller and a Dual P3-800 machine 
I found speed severely limited by the DAC.  The performance on bulk IO for 
the 10K rpm Ultra2 SCSI drives was much less than that of ATA-66 drives.

> bus management (eg 2 drives can perform tasks at once unlike IDE), Hot

See http://www.coker.com.au/~russell/hardware/46g.png for a graph of 
performance of an ATA disk on it's own, two ATA disks running on separate 
busses, and two disks on the same bus.  From that graph I conclude that most 
of the performance hit of running two such drives comes from the motherboard 
bus performance not from an IDE cable.  That graph was done with an old 
kernel (about 2.4.1), I'll have to re-do it with the latest results from the 
latest kernel.

Anyway motherboards with 4 IDE buses on the motherboard are common now, most 
servers don't have more than 4 drives.

> Swapable (This is HUGE) and more cache on the drive.

NO!  SCSI hard drives are no more swappable than ATA drives!  If you unplug 
an active SCSI bus you run the same risks of hardware damage as you do for 

Hardware support for hot-swap is more commonly available for SCSI drives than 
for ATA, but it is very pricey.

http://www.coker.com.au/bonnie++/     Bonnie++ hard drive benchmark
http://www.coker.com.au/postal/       Postal SMTP/POP benchmark
http://www.coker.com.au/projects.html Projects I am working on
http://www.coker.com.au/~russell/     My home page

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