From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andre Noll Subject: Re: Why do I need 4 disks for a raid6? Date: Wed, 18 Mar 2009 13:18:25 +0100 Message-ID: <20090318121825.GL32416@skl-net.de> References: <87ljr4hsk9.fsf@frosties.localdomain> Mime-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="S6vg04ofUPzW4qJg" Return-path: Content-Disposition: inline In-Reply-To: <87ljr4hsk9.fsf@frosties.localdomain> Sender: linux-raid-owner@vger.kernel.org To: Goswin von Brederlow Cc: linux-raid@vger.kernel.org List-Id: linux-raid.ids --S6vg04ofUPzW4qJg Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On 20:12, Goswin von Brederlow wrote: > I'm wondering why the kernel requires a raid6 to have at least 4 > disks (of which at most 2 can be missing). Why not 3 disks? Yes, this limitation looks a bit arbitrary. I can not see any reason why raid6 requires at least four disks. Probably it even works without any significant changes if some of the checks are being relaxed. > Now for the raid6 case. With only 1 data disk and 2 parity disks all 3 > disks should end up with identical data on them. In effect this should > be a 3 disk raid1, a cpu intensive one. In fact, it wouldn't be much more CPU intensive than raid5 because the math to "calculate" the Q parity would obviously not involve any GF multiplications at all. > So back to my original question: Why does the kernel require 4 disks > for a raid6 instead of allowing 3? Dunno. Maybe Dan, Neil or HPA can tell the reason for imposing this limitation. Andre --=20 The only person who always got his work done by Friday was Robinson Crusoe --S6vg04ofUPzW4qJg Content-Type: application/pgp-signature; name="signature.asc" Content-Description: Digital signature Content-Disposition: inline -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.1 (GNU/Linux) iD8DBQFJwOaRWto1QDEAkw8RArqpAJ9sI6C0jGd+DbGKnL6/ZJ2axj1f2wCgqkMH fdW9d4U1OmwF9B8U+FX1KiQ= =FNh0 -----END PGP SIGNATURE----- --S6vg04ofUPzW4qJg--