From: Pavel Machek <pavel@ucw.cz>
Subject: Re: bcache with existing ext4 filesystem
Date: Tue, 25 Jul 2017 08:43:04 +0200
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To: Theodore Ts'o <tytso@mit.edu>,
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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

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