btrfs-send format that contains binary diffs

[Claudius Heine <ch@denx.de>]

Hi,

I am currently investigating the possibility to use `btrfs-stream` files 
(generated by `btrfs send`) for deploying a image based update to 
systems (probably embedded ones).

One of the issues I encountered here is that btrfs-send does not use any 
diff algorithm on files that have changed from one snapshot to the next.

One way to implement this would be to add some sort of 'patch' command 
to the `btrfs-stream` format.

Is this something upstream would be interested in?

Lets say we introduce a new `btrfs-send` format, lets call it 
`btrfs-delta-stream`, which could can be created from a `btrfs-stream`:

1. For all `write` commands, check the requirements:
    - Does the file already exists in the old snapshot?
    - Is the file smaller than xMiB (this depends on the diff-algo and 
the available resources)
2. If the file fulfills those requirements, replace 'write' command with 
'patch' command, and calculate the binary delta.  Also check if the 
delta is actually smaller than the data of the new file.  Possible add 
the used binary diff algo as well as a checksum of the 'old' file to the 
command as well.

This file format can of course be converted back to `btrfs-stream` and 
then applied with `btrfs-receive`.

I would probably start with `bsdiff` for the diff algorithm, but maybe 
we want to be flexible here.

Of course if `btrfs-delta-stream` is implemented in `btrfs-progs` then, 
we can create and apply this format directly.

regards,
Claudius