Re: bcache with existing ext4 filesystem

From	Pavel Machek <pavel@ucw.cz>
Newsgroups	linux.kernel
Subject	Re: bcache with existing ext4 filesystem
Date	2017-07-25 08:50 +0200
Message-ID	<u71Pj-7Qu-1@gated-at.bofh.it> (permalink)
References	(1 earlier) <u6R3A-KM-9@gated-at.bofh.it> <u6R3z-KM-7@gated-at.bofh.it> <u6Rdh-OM-25@gated-at.bofh.it> <u6RPY-1lt-7@gated-at.bofh.it> <u706S-6JM-13@gated-at.bofh.it>
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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"?
> 
> 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):

| 1KiB (modified) bcache superblock | ext4 superblock | 400GB data |

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

Thread

bcache with existing ext4 filesystem Pavel Machek <pavel@ucw.cz> - 2017-07-24 21:00 +0200
  Re: bcache with existing ext4 filesystem Pavel Machek <pavel@ucw.cz> - 2017-07-24 21:20 +0200
    Re: bcache with existing ext4 filesystem Theodore Ts'o <tytso@mit.edu> - 2017-07-24 21:30 +0200
      Re: bcache with existing ext4 filesystem Pavel Machek <pavel@ucw.cz> - 2017-07-24 22:10 +0200
        Re: bcache with existing ext4 filesystem Theodore Ts'o <tytso@mit.edu> - 2017-07-25 07:00 +0200
          Re: bcache with existing ext4 filesystem Pavel Machek <pavel@ucw.cz> - 2017-07-25 08:50 +0200
            Re: bcache with existing ext4 filesystem Vojtech Pavlik <vojtech@ucw.cz> - 2017-07-25 13:10 +0200
              Re: bcache with existing ext4 filesystem Pavel Machek <pavel@ucw.cz> - 2017-07-25 13:20 +0200
                Re: bcache with existing ext4 filesystem Theodore Ts'o <tytso@mit.edu> - 2017-07-25 18:20 +0200
                Re: bcache with existing ext4 filesystem Eric Wheeler <bcache@lists.ewheeler.net> - 2017-07-25 20:20 +0200
                Re: bcache with existing ext4 filesystem Pavel Machek <pavel@ucw.cz> - 2017-07-26 00:10 +0200
                Re: bcache with existing ext4 filesystem Eric Wheeler <bcache@lists.ewheeler.net> - 2017-07-26 19:50 +0200
                Re: bcache with existing ext4 filesystem "Austin S. Hemmelgarn" <ahferroin7@gmail.com> - 2017-07-26 21:10 +0200
                Re: bcache with existing ext4 filesystem Eric Wheeler <bcache@lists.ewheeler.net> - 2017-07-26 21:20 +0200
                Re: bcache with existing ext4 filesystem Pavel Machek <pavel@ucw.cz> - 2017-07-26 22:10 +0200
          Re: bcache with existing ext4 filesystem Pavel Machek <pavel@ucw.cz> - 2017-07-25 15:50 +0200
            Re: bcache with existing ext4 filesystem Theodore Ts'o <tytso@mit.edu> - 2017-07-25 20:10 +0200
              Re: bcache with existing ext4 filesystem Pavel Machek <pavel@ucw.cz> - 2017-07-25 23:00 +0200
  Re: bcache with existing ext4 filesystem Reindl Harald <h.reindl@thelounge.net> - 2017-07-24 21:30 +0200

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