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 - To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
