1 processor vs. 2

[Jan Grant]

On Thu, 4 Mar 2004, Chuck Swiger wrote:

> The parity calculations for RAID-5 are a lot of work and that work scales
> linearly with the number of drives in the array.  The longer you make the
> array, the worse the performance becomes for small writes in particular.

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.

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.


-- 
jan grant, ILRT, University of Bristol. http://www.ilrt.bris.ac.uk/
Tel +44(0)117 9287088 Fax +44 (0)117 9287112 http://ioctl.org/jan/
There's no convincing English-language argument that this sentence is true.