[H-GEN] Disk transfer speeds

[Robert Brockway]

[ Humbug *General* list - semi-serious discussions about Humbug and     ]
[ Unix-related topics. Posts from non-subscribed addresses will vanish. ]

On Sat, 9 Aug 2003, Michael Anthon wrote:

> Yes, but the point still remains the same, it should be reading from the
> 5 disks simultaneously with 20% of the data being redundant.  I think in
> terms of reading speed the performance of 4 and 5 is the same.  If I

Under certain conditions I believe that RAID 5 would provide faster reads
than RAID 4.  Here is my logic:

For an arbitrary read we find that the same 4 drives are always hit in
RAID 4 (ie, all the drives except the parity drive).  In RAID 5 we find
that 4 out of the 5 drives are hit for an arbitrary read.  The actual 4
drives hit varies.  On a loaded system, particularly one in which the
drives are striped across controllers, RAID 5 should provide better read
performance because bandwidth from more drives (and possibly controllers)
is coming into play.

For "controller" please insert the words "host adapter" is appropriate.

> understand things correctly the main (possibly only) advantage of 5 or 4

There are other advantages of 5 over 4.  The biggest being bandwidth of
the parity drive.  In RAID 4 _each_ write requires a hit to the lone
parity drive.  In RAID 5 the parity bit is striped across the 5 (or 3)
drives, so each drive is hit equally, providing better write performance
during normal operation (not just during a disk recovery).

One would think RAID 5 would provide better capacity usage as well.
Surely with a single parity bit, RAID 4 must result in half of the parity
drive being unused (assuming all drives are equal) or have I forgotten
some important characteristic of RAID 4?

Rob

-- 
Robert Brockway B.Sc. email: robert at timetraveller.org, zzbrock at uqconnect.net
Linux counter project ID #16440 (http://counter.li.org)
"The earth is but one country and mankind its citizens" -Baha'u'llah

--
* This is list (humbug) general handled by majordomo at lists.humbug.org.au .
* Postings to this list are only accepted from subscribed addresses of
* lists 'general' or 'general-post'.  See http://www.humbug.org.au/