Adaptec 2400A Performance

[Charles Swiger]

On Jan 24, 2004, at 12:51 AM, Rishi Chopra wrote:
> I was rather disappointed with the results.  Can anyone suggest what 
> might be causing such slow disk speeds, or whether these speeds are 
> out of the ordinary for a 4-disk FreeBSD RAID5 installation?  I have 
> done nothing to configure the card aside from striping the array in 
> BIOS; FreeBSD seems to automatically detect the disks.

For us to be able to comment beyond generalizations, it's necessary to 
also benchmark how a single disk performs.  I can still answer your 
question, though:

RAID-5 is slow.  RAID-5 trades availability against performance and 
hardware costs.  With RAID-0, n drives gives n drives' worth of usable 
space.  With RAID-5, n drives gives n-1 drives' worth of usable space.  
The performance is between RAID-0 and RAID-1 is comparible for large 
accesses.  For small accesses, particularly small writes, RAID-5 
performance is much worse than plain RAID-0 or a plain disk.

-- 
-Chuck