linux-ext4 - Re: bcache with existing ext4 filesystem

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Message-ID: <20170725161054.dbf6ily5jb7gjvdg@thunk.org>
Date:   Tue, 25 Jul 2017 12:10:54 -0400
From:   Theodore Ts'o <tytso@....edu>
To:     Pavel Machek <pavel@....cz>
Cc:     Vojtech Pavlik <vojtech@....cz>,
        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, Jul 25, 2017 at 01:12:10PM +0200, Pavel Machek wrote:
> 
> 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).

Both Grub and Linux's implementation of ext4 expect the superblock to
be at offset 1024 bytes from the beginning of the block device.

>From looking at Documentation/bcache.txt, the problem is that bcache
works much like LVM or device mapper.  That is, you have to create the
file system on /dev/bcacheN.  That simplies that grub needs to
understand bcache, which as far as I understand, it doesn't today.

      	     	  	      	 	 - Ted