From mboxrd@z Thu Jan 1 00:00:00 1970 From: Gregory Leblanc Subject: Re: RAID-6 support in kernel? Date: 03 Jun 2002 10:33:24 -0700 Sender: linux-raid-owner@vger.kernel.org Message-ID: <1023125615.1051.1283.camel@peecee> References: Mime-Version: 1.0 Content-Type: text/plain Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: To: linux-raid@vger.kernel.org List-Id: linux-raid.ids On Mon, 2002-06-03 at 02:25, 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. Hot spares are quite a nice way to increase the reliability of your arrays, somewhat. You can still be in trouble if a second disk fails before the resync finishes, but at that point you're probably talking about something of a more catastrophic failure, perhaps outside of the machine itself. > 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. The issue isn't so much CPU load, but latency. I'm too lazy to go read a summary on RAID 6, but with RAID 5, blocks to be written as part of a stripe often need to be read from the disk in order to generate the parity. Parity calculations are pretty trivial on modern CPUs, but disk latency certainly isn't. HTH, Greg -- Portland, Oregon, USA. Please don't copy me on replies to the list.