From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from frost.carfax.org.uk ([85.119.82.111]:50848 "EHLO frost.carfax.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754352Ab2LRMNP (ORCPT ); Tue, 18 Dec 2012 07:13:15 -0500 Date: Tue, 18 Dec 2012 12:13:11 +0000 From: Hugo Mills To: Brendan Hide Cc: Sebastien Luttringer , linux-btrfs@vger.kernel.org Subject: Re: Feeback on RAID1 feature of Btrfs Message-ID: <20121218121311.GL19051@carfax.org.uk> References: <20121217162350.GJ19051@carfax.org.uk> <50D05174.4060109@swiftspirit.co.za> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="Y+Z5jE7Arku/2GrR" In-Reply-To: <50D05174.4060109@swiftspirit.co.za> Sender: linux-btrfs-owner@vger.kernel.org List-ID: --Y+Z5jE7Arku/2GrR Content-Type: text/plain; charset=us-ascii Content-Disposition: inline On Tue, Dec 18, 2012 at 01:20:20PM +0200, Brendan Hide wrote: > On 2012/12/17 06:23 PM, Hugo Mills wrote: > >On Mon, Dec 17, 2012 at 04:51:33PM +0100, Sebastien Luttringer wrote: > >>Hello, > snip > >>I get the feeling that RAID1 only allow one disk removing. Which is more > >>a RAID5 feature. > > The RAID-1 support in btrfs makes exactly two copies of each item > >of data, so you can lose at most one disk from the array safely. Lose > >any more, and you're likely to have lost data, as you've found out. > >>I'm afraid Btrfs raid1 will not be working before the end of the world. > > It does work (as you demonstrated with the first disk being > >removed) -- but just not as you thought it should. Now, you can argue > >that "RAID-1" isn't a good name to use here, but there's no good name > >in RAID terminology to describe what we actually have here. > Technically, btrfs's "RAID1" implementation is much closer to RAID1E > than traditional RAID1. See > http://en.wikipedia.org/wiki/Non-standard_RAID_levels#RAID_1E or http://pic.dhe.ibm.com/infocenter/director/v5r2/index.jsp?topic=/serveraid_9.00/fqy0_craid1e.html > > Perhaps a new name, as with ZFS, might be appropriate. RAID-Z and > RAID-Z2, for example, could not adequately be described by any > existing RAID terminology and, technically, RAID-Z still isn't a > RAID in the classical sense. Yeah, we did have a naming scheme proposed, with combinations of nCmSpP, where n is the number of copies held, m the number of stripes, and p the number of parity stripes. So btrfs RAID-1 is 2C, RAID-5 on 5 disks would be 4S1P, and RAID-10 on 4 disks would be 2C2S. I'd prefer to see that than some non-"standard" RAID-18KTHXBYE formulation. Plenty of room for shed-painting here, though. Hugo. -- === Hugo Mills: hugo@... carfax.org.uk | darksatanic.net | lug.org.uk === PGP key: 515C238D from wwwkeys.eu.pgp.net or http://www.carfax.org.uk --- I believe that it's closely correlated with --- the aeroswine coefficient. --Y+Z5jE7Arku/2GrR Content-Type: application/pgp-signature; name="signature.asc" Content-Description: Digital signature -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.12 (GNU/Linux) iQIVAwUBUNBd179z9OVl50rAAQK1/Q//SEYkT42rjPI3hjrtbk7oG4gEE6aPMxjK 3AJBuUoQmP3a14GXwhV9y/bstfhmztPGsJ2X09duFSLrX6DKa9vmSzYmWBUv6dc8 S4yyRmsdpYZDse2pUveg/a6fywasMJGyocGZEtOkLZ6JqiAhKnVvoZ/98sNKIdDa hHtaYBpERAmA7TYhHCEykC9b7LqtBeaH1dRPZ3LXsC2/jNAD+ZvwUzF+5S+ZDCUC KTdAf3MQeyZV8WJlOj3T1cInqw2NTgxVVmDfzj+Vy8Z7ZAqpp1RcmvDqISatby46 IAuNRag8Yjx82p9/CVkJGFTK7uocHbyv9FQUpVgR4I+i0XBuMU5ZJClfNlxqX2u+ dT7k2CG/AHAUn0reYWS9uJfQJU2WfM2JzwXlIxvcegNvf/Iv8Kw5Zwvmo8itNw7I jVMFxJYfoe+g2k0MFy/5PgSQmLhhfbDLdbAi0IuZquFpQFVV/cROCsrFaNlk0VFJ Ke+EXqIG9iiDkHbTsRAGUiFLujb/bFsl7nEcFkSWrOLROufgj7LO4VTO2Jvc25cE SbLrnSm/GX0llHDV9VesS0Uqk7vrPrdByc+bTgyjUh/7Zb4Bwzi8MnJAHCLgMLW0 jdsenk2Xn5Fxktmdb37uZ+99W3OARzvPVlBiY95HONh0xfbg0xdKlUvHwPn+QL7/ 6YVECcAXvpE= =8VwB -----END PGP SIGNATURE----- --Y+Z5jE7Arku/2GrR--