Re: RAID-6 support in kernel?

[Vojtech Pavlik <vojtech@suse.cz>]

On Mon, Jun 03, 2002 at 02:25:22AM -0700, Derek Vadala wrote:
> On Mon, 3 Jun 2002, Roy Sigurd Karlsbakk wrote:
> 
> > 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.
> 
> > 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