Return-Path: Received: from james.kirk.hungrycats.org ([174.142.39.145]:34509 "EHLO james.kirk.hungrycats.org" rhost-flags-OK-FAIL-OK-FAIL) by vger.kernel.org with ESMTP id S1030233AbaLLQpq (ORCPT ); Fri, 12 Dec 2014 11:45:46 -0500 Date: Fri, 12 Dec 2014 11:45:44 -0500 From: Zygo Blaxell To: Robert White Cc: Btrfs BTRFS Subject: Re: mkfs.btrfs limits "odd" [and maybe a "failed" phantom device?] Message-ID: <20141212164544.GB25614@hungrycats.org> References: <5488C6CF.1080608@pobox.com> <20141212035651.GD22023@hungrycats.org> <548A84A2.4000501@pobox.com> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="qlTNgmc+xy1dBmNv" In-Reply-To: <548A84A2.4000501@pobox.com> Sender: linux-btrfs-owner@vger.kernel.org List-ID: --qlTNgmc+xy1dBmNv Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Thu, Dec 11, 2014 at 10:01:06PM -0800, Robert White wrote: > So RAID5 with three media M is >=20 > M MM MMM > D1 D2 P(a) > D3 P(b) D4 > P(c) D5 D6 RAID5 with two media is well defined, and looks like this: M MM D1 P(a) P(b) D2 D3 P(c) With even parity and N disks P(a) ^ D1 [^ D2 ^ ... ^ DN] =3D 0 Simplifying for one data disk and one parity stripe: P(a) ^ D1 =3D 0 therefore P(a) =3D D1 which is effectively (and, in practice, literally) mirroring. --qlTNgmc+xy1dBmNv Content-Type: application/pgp-signature; name="signature.asc" Content-Description: Digital signature -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.12 (GNU/Linux) iEYEARECAAYFAlSLG7cACgkQgfmLGlazG5xcFgCgmvjhgpGQFENaRQBKWTjJBR9J mlUAoLDSUrnEfajRr8PNUZrzkVymdXjZ =Winj -----END PGP SIGNATURE----- --qlTNgmc+xy1dBmNv--