Re: bcache with existing ext4 filesystem

[Vojtech Pavlik <vojtech@ucw.cz>]

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.

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.

-- 
Vojtech Pavlik