1 processor vs. 2

[Charles Swiger]

On Mar 5, 2004, at 5:57 AM, Jan Grant wrote:
> How did you come to this conclusion? For a RAID 5 with a single parity
> drive, the reason you zero the disks out completely on initialisation 
> is
> to set up the integrity of the parity check. Then any update to any
> RAID5 with single parity requires a read of two drives (the target
> sector and the corresponding parity drive), an in-memory exclusive or
> against the new data, and two writes. Reads and writes can be in
> parallel.

You're right, which means I came to my conclusion wrongly, I guess.  
:-)  Part of this was because I was also thinking about how the array 
behaves after a failure, as you mention next:

> The "work" for parity updates only scales linearly with number of disks
> if you use a naive parity algorithm. Or, obviously, if a drive fails.

Even using a non-naive :-) algorithm, RAID-5 writes still take somewhat 
more work than RAID-1 writes do in terms of I/O ops.

-- 
-Chuck