From: Theodore Ts'o <tytso@mit.edu>
Subject: Re: bcache with existing ext4 filesystem
Date: Tue, 25 Jul 2017 12:10:54 -0400
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Cc: Vojtech Pavlik <vojtech@ucw.cz>,
        Reindl Harald <h.reindl@thelounge.net>,
        linux-ext4@vger.kernel.org,
        kernel list <linux-kernel@vger.kernel.org>,
        kent.overstreet@gmail.com, linux-bcache@vger.kernel.org
To: Pavel Machek <pavel@ucw.cz>
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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