Re: RAID-6 support in kernel?

On Mon, Jun 03, 2002 at 02:25:22AM -0700, Derek Vadala wrote:

On Mon, 3 Jun 2002, Roy Sigurd Karlsbakk wrote:

quoted

It'll waste 9 drives, giving me a total capacity of 7n instead of 14n. 
And, by definition, RAID-6 _can_ withstand _any_ two-drive failure.

This is certainly not true. 

Combining N RAID-5 into a stripe wastes on N disks. 

If you combine two it wastes 2 disks, etc.

That is, for each RAID-5 you waste a single disk worth of storage for
partiy. I don't know what equation you're using where you get 9 drives
from.

He was thinking "mirror", not "stripe". Mirror of 2 RAID-5 arrays (would
be probably called RAID-15 (when there is a RAID-10 for mirrored stripe
arrays)), can withstand any two disks failing anytime. Even more for
certain combinations. But it is terribly inefficient.

As far as it's ability to withstand _any_ 2-disk failure... I'm not sure
what you mean by definition. RAID-6 implemations don't follow a standard
because there isn't one. Depending on how it's implemented, RAID-6 is not
necessarily able to withstand a filaure of any two disks. We can argue as
much as you want, but I'm not willing to invest the time.

quoted

With a 1500MHz Athlon on a typical file server where there's not much 
writes, the CPU is sitting there chrunching RC5-64 som 99,95 % of the 
time. I don't think it'll make much differnce with today's CPUs

It's up to you to decide if the performance trade-off is worthwhile. I
merely trying to point out that system with 2 RAID-5 is likely to incur
the same CPU hit as a single RAID-6, implemented in the kernel. 

-- 
Vojtech Pavlik
SuSE Labs