From mboxrd@z Thu Jan 1 00:00:00 1970 From: Gordan Bobic Subject: Re: Update to Project_ideas wiki page Date: Wed, 17 Nov 2010 18:07:41 +0000 Message-ID: <4CE419ED.3020209@bobich.net> References: <20101117143103.GA2401@selene> <20101117175657.GB2401@selene> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed To: linux-btrfs@vger.kernel.org Return-path: In-Reply-To: <20101117175657.GB2401@selene> List-ID: On 11/17/2010 05:56 PM, Hugo Mills wrote: > On Wed, Nov 17, 2010 at 04:12:29PM +0100, Bart Noordervliet wrote: >> Can I suggest we combine this new RAID level management with a >> modernisation of the terminology for storage redundancy, as has been >> discussed previously in the "Raid1 with 3 drives" thread of March this >> year? I.e. abandon the burdened raid* terminology in favour of >> something that makes more sense for a filesystem. > > Well, our current RAID modes are: > > * 1 Copy ("SINGLE") > * 2 Copies ("DUP") > * 2 Copies, different spindles ("RAID1") > * 1 Copy, 2 Stripes ("RAID0") > * 2 Copies, 2 Stripes [each] ("RAID10") > > The forthcoming RAID5/6 code will expand on that, with > > * 1 Copy, n Stripes + 1 Parity ("RAID5") > * 1 Copy, n Stripes + 2 Parity ("RAID6") > > (I'm not certain how "n" will be selected -- it could be a config > option, or simply selected on the basis of the number of > spindles/devices currently in the FS). > > We could further postulate a RAID50/RAID60 mode, which would be > > * 2 Copies, n Stripes + 1 Parity > * 2 Copies, n Stripes + 2 Parity 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. Gordan