From mboxrd@z Thu Jan 1 00:00:00 1970 From: NeilBrown Subject: Re: [PATCH v2] DM RAID: Add support for MD RAID10 Date: Fri, 13 Jul 2012 11:27:17 +1000 Message-ID: <20120713112717.3b15647c@notabene.brown> References: <1342057001.22214.6.camel@f16> <20120712162205.GA13485@www5.open-std.org> <331FD2B6-AD96-4513-AF37-4E1B9EE7A34F@redhat.com> <20120713011505.GA3099@www5.open-std.org> Mime-Version: 1.0 Content-Type: multipart/signed; micalg=PGP-SHA1; boundary="Sig_/x1u9VTXBkgrjFq0Wrb2dbd8"; protocol="application/pgp-signature" Return-path: In-Reply-To: <20120713011505.GA3099@www5.open-std.org> Sender: linux-raid-owner@vger.kernel.org To: keld@keldix.com Cc: Brassow Jonathan , dm-devel@redhat.com, linux-raid@vger.kernel.org, agk@redhat.com List-Id: linux-raid.ids --Sig_/x1u9VTXBkgrjFq0Wrb2dbd8 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: quoted-printable On Fri, 13 Jul 2012 03:15:05 +0200 keld@keldix.com wrote: > I think the layout you described should not be promoted at all, > and only kept for backward compatibility. As there is no backward=20 > compatibility in your case I think it is an error to implement it. > I understand that you do not reuse any of the MD code here? Not correct. The whole point of this exercise is to reuse md code. > The flaw is worse than Neil described, as far as I understand. > With n=3D2 you can in the current implementation only have 1 disk failing, > for any numbers of drives in the array. With the suggested layout > then for 4 drives you have the probability of surviving 66 %=20 > of 2 drives failing. This get even better for 6, 8 .. disks in the array. > And you may even survive 3 or more disk failures, dependent on the number > of drives employed. The probability is the same as for raid-1+0 Also not correct. You can certainly have more than one failed device providing you don't have 'n' adjacent devices all failed. So e.g. if you have 2 drives in a far-2 layout then you can survive the failure of three devices if they are 0,2,4 or 1,3,5. >=20 > > When it is available to MD, I'll make it available to dm-raid also. >=20 > Please dont implement it in the flawed way. It will just create a number= of problems > for when to switch over and convert between the two formats, and then whi= ch should > be the default (I fear some would say the old flawed should be the defaul= t), and we need > to explain the two formats and implement two sets of repairs and so on. This "flawed" arrangement is the only one that makes sense for an odd number of devices (assuming 2 copies). NeilBrown --Sig_/x1u9VTXBkgrjFq0Wrb2dbd8 Content-Type: application/pgp-signature; name=signature.asc Content-Disposition: attachment; filename=signature.asc -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.18 (GNU/Linux) iQIVAwUBT/95dTnsnt1WYoG5AQIgrRAAwMpvHtWG22m3rONhyoAnYwFXP1iXe+Pt WBsLXoZIPfqz8MrXlrpmQyKgjv5zarU3G9E1jmjD4k+ubKEUMRW+1ftFv4xIGTWZ rdPgAPyRotV9vVj2JczsU/mr7KA+S0k/e10/4OiZ2WMEn2RP6V2Ismdjv64LPrwj NEiUl+XkpgyJfRojVzBlPZJAlk+mPWXBa9W9dREEJq3LfZ9nYP6+dSKU9XzxFNmC bXdb+mVCEEqDIGqGyR+wBAdvs6cgllN/Oy3GNkYjcTW6DM7PHK+e6rT0KYPexubV vrB2GF2W5iXMyoqaFz/lxC7s4DxVzLLRkE0Srf2Rm9BWEwnPyDYr+tco4hbreG+U NqynORDkec+tEi9kQj3s6HuMb8JpN/2jWNgmZ5v8cVJ3/D7AwNKHvtCUUX6AFKqv tiHndL4/f6rkc/ORKUNf7RxLTa+Norhe113W2/GFivhOK1RNX7VMJENBqtgPXSVY xZKNIG+CwpQgonwqr5a4GLdErAGKkERIoinVV6KU3vbRmfJLubT2OwesA5MAl8NU wzAmPHhSNr24t8G3MImt4YSWgC32QuMpD0uKoMsAu7R5GvXBdNBW6XMMArPbfQyx /OODtrl+iiAPwN03k2PgFwBSqpYzsGVRQBRNh4HbTFcLb91MvKLb9vFZ6tSeMYbU OFiAz+cHbZ0= =TRGZ -----END PGP SIGNATURE----- --Sig_/x1u9VTXBkgrjFq0Wrb2dbd8--