Re: Btrfs send with parent different size depending on source of files.

[Chris Murphy <lists@colorremedies.com>]

On Thu, Feb 14, 2019 at 4:37 AM André Malm <admin@sheepa.org> wrote:
>
> Hello,
>
> I'm not sure this is the right forum to ask on but I'll try and if its
> not I do apologize. I have also created a stack overflow question
> without success (
> https://stackoverflow.com/questions/54634703/btrfs-send-with-parent-different-size-depending-on-source-of-files
> ) but ill paste the question here too. Thank you.
>
> What i'm trying to achieve is sending only the diff of the parent with
> btrfs send -p
>
> Running this will produce a file 'out' with size 639 bytes, i.e only
> diff sent.
>
> ====================================================
>
> btrfs subvolume create A
> btrfs subvolume create B
> mkdir A/dir
>
> dd if=/dev/urandom of=A/dir/server.jar bs=1024 count=40K
> cp --reflink=always A/dir/server.jar B/server.jar
>
> btrfs subvolume snapshot -r A a
> btrfs subvolume snapshot -r B b
> btrfs send -p a b > out

It doesn't work this way. The snapshots a and b are not based on the
same underlying subvolume. The gist is that you would keep changing A,
and take additional snapshots of A, such as a.1 a.2 a.3, and you can
do incremental send with 'btrfs send -p a.1 a.2' which describes the
difference between those two snapshots of A at their respective
moments in time. You could also do 'btrfs send -p a.2 a.3' or even
'btrfs send -p a.1 a.3'

But as there's no relationship between snapshots a and b, I consider
it a bug/missing error handling feature, that btrfs send doesn't fail
in this case. By using -p you're claiming there is a parent-child
relationship between a and b, but there plainly isn't.

Read this:
https://btrfs.wiki.kernel.org/index.php/Incremental_Backup#Doing_it_by_hand.2C_step_by_step

Depending on your use case if you can describe it, might be tolerated
with some adjustments by using the -c clone option instead.

-- 
Chris Murphy