Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1750885AbdGYGnJ (ORCPT ); Tue, 25 Jul 2017 02:43:09 -0400 Received: from atrey.karlin.mff.cuni.cz ([195.113.26.193]:46449 "EHLO atrey.karlin.mff.cuni.cz" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750744AbdGYGnG (ORCPT ); Tue, 25 Jul 2017 02:43:06 -0400 Date: Tue, 25 Jul 2017 08:43:04 +0200 From: Pavel Machek To: "Theodore Ts'o" , Reindl Harald , linux-ext4@vger.kernel.org, kernel list , kent.overstreet@gmail.com, linux-bcache@vger.kernel.org Subject: Re: bcache with existing ext4 filesystem Message-ID: <20170725064304.GA11723@amd> References: <20170724185703.GA31422@amd> <64c810cf-a95c-f862-f25a-ebd7419b2632@thelounge.net> <20170724191548.GA32425@amd> <20170724192718.t7n5zgualz5lillg@thunk.org> <20170724200451.GA4318@amd> <20170725045156.kbyaxj4mmi75yyt5@thunk.org> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="qMm9M+Fa2AknHoGS" Content-Disposition: inline In-Reply-To: <20170725045156.kbyaxj4mmi75yyt5@thunk.org> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1731 Lines: 55 --qMm9M+Fa2AknHoGS Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Tue 2017-07-25 00:51:56, Theodore Ts'o wrote: > On Mon, Jul 24, 2017 at 10:04:51PM +0200, Pavel Machek wrote: > > Question for you was... Is the first 1KiB of each ext4 filesystem still > > free and "reserved for a bootloader"? >=20 > Yes. Thanks. > > If I needed more for bcache superblock (8KiB, IIRC), would that be > > easy to accomplish on existing filesystem? >=20 > Huh? Why would the bcache superblock matter when you're talking about > the ext4 layout? The bcache superblock will be on the bcache > device/partition, and the ext4 superblock will be on the ext4 > device/partition. I'd like to enable bcache on already existing ext4 partition. AFAICT normal situation, even on the backing device, is: | 8KiB bcache superblock | 1KiB reserved | ext4 superblock | 400GB data | Unfortunately, that would mean shifting 400GB data 8KB forward, and compatibility problems. So I'd prefer adding bcache superblock into the reserved space, so I can have caching _and_ compatibility with grub2 etc (and avoid 400GB move): | 1KiB (modified) bcache superblock | ext4 superblock | 400GB data | Best regards, Pavel --=20 (english) http://www.livejournal.com/~pavelmachek (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blo= g.html --qMm9M+Fa2AknHoGS Content-Type: application/pgp-signature; name="signature.asc" Content-Description: Digital signature -----BEGIN PGP SIGNATURE----- Version: GnuPG v1 iEYEARECAAYFAll26HgACgkQMOfwapXb+vKPFQCfXSMMm45Xc4MyB3XktnTPl6cs Zv4Anj4AP7It52Ut270Q0/EmqYcN4ral =3CPG -----END PGP SIGNATURE----- --qMm9M+Fa2AknHoGS--