From mboxrd@z Thu Jan 1 00:00:00 1970 From: Gordan Bobic Subject: Re: Update to Project_ideas wiki page Date: Thu, 18 Nov 2010 15:06:30 +0000 Message-ID: <4CE540F6.7060701@bobich.net> References: <20101117143103.GA2401@selene> <20101117175657.GB2401@selene> <4CE419ED.3020209@bobich.net> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed To: linux-btrfs@vger.kernel.org Return-path: In-Reply-To: List-ID: Bart Noordervliet wrote: > On Wed, Nov 17, 2010 at 19:07, Gordan Bobic wrote: >> Since BTRFS is already doing some relatively radical things, I would like to >> suggest that RAID5 and RAID6 be deemed obsolete. RAID5 isn't safely usable >> for arrays bigger than about 5TB with disks that have a specified error rate >> of 10^-14. RAID6 pushes that problem a little further away, but in the >> longer term, I would argue that RAID (n+m) would work best. We specify that >> of (n+m) disks in the array, we want n data disks and m redundancy disks. If >> this is implemented in a generic way, then there won't be a need to >> implement additional RAID modes later. > > I presume you're talking about the uncaught read errors that makes > many people avoid RAID5. Btrfs actually enables us to use it with > confidence again, since using checksums it's able to detect these > errors and prevent corruption of the array. So to the contrary, I see > a lot of potential for parity-based redundancy in combination with > btrfs. No. What I'm talking about the the probability of finding an error during the process of rebuilding a degraded array. With a 6TB (usable) array and disks with 10^-14 error rate, the probability of getting an unrecoverable read error exceeds 50%. n+1 RAID isn't fit for use with the current generation of drives where n > 1-5TB depending on how important your data and downtime are and how good your backups are. And I don't put much stock in the manufacturer figures, either, so assume that 10^-14 is optimistic of it is reported. On high capacity drives (especially 1TB Seagates, both 3 and 4 platter variants) I am certainly seeing a higher error rate than that on a significant fraction of the disks. Gordan