linux-kernel - Re: bcache with existing ext4 filesystem

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Message-ID: <20170725111210.GA5667@amd>
Date:   Tue, 25 Jul 2017 13:12:10 +0200
From:   Pavel Machek <pavel@....cz>
To:     Vojtech Pavlik <vojtech@....cz>
Cc:     Theodore Ts'o <tytso@....edu>,
        Reindl Harald <h.reindl@...lounge.net>,
        linux-ext4@...r.kernel.org,
        kernel list <linux-kernel@...r.kernel.org>,
        kent.overstreet@...il.com, linux-bcache@...r.kernel.org
Subject: Re: bcache with existing ext4 filesystem

On Tue 2017-07-25 12:32:48, Vojtech Pavlik wrote:
> 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.

Well... if I move the partition, grub2 (etc) will be unable to access
data on it. (Plus I do not have free space before some of the
partitions I'd like to be cached).

> 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.

My plan is to make offset 0. AFAICT bcache superblock can be shrunk.

									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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