Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751406AbdGYLAy (ORCPT ); Tue, 25 Jul 2017 07:00:54 -0400 Received: from jablonecka.jablonka.cz ([91.219.244.36]:40033 "EHLO jablonecka.jablonka.cz" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750851AbdGYLAw (ORCPT ); Tue, 25 Jul 2017 07:00:52 -0400 X-Greylist: delayed 1680 seconds by postgrey-1.27 at vger.kernel.org; Tue, 25 Jul 2017 07:00:51 EDT Date: Tue, 25 Jul 2017 12:32:48 +0200 From: Vojtech Pavlik To: Pavel Machek Cc: "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: <20170725103248.GA12869@suse.com> 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> <20170725064304.GA11723@amd> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170725064304.GA11723@amd> X-Bounce-Cookie: It's a lemon tree, dear Watson! User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1446 Lines: 37 On Tue, Jul 25, 2017 at 08:43:04AM +0200, Pavel Machek wrote: > 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"? > > > > Yes. > > Thanks. > > > > If I needed more for bcache superblock (8KiB, IIRC), would that be > > > easy to accomplish on existing filesystem? > > > > 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): The common way to do that is to move the beginning of the partition, assuming your ext4 lives in a partition. I don't see how overlapping the ext4 and the bcache backing device starts would give you what you want, because bcache assumes the backing device data starts with an offset. -- Vojtech Pavlik