Computing size of snapshots approximatly

[Jan-Hendrik Palic <billgotchy@ki.tng.de>]

Hi,

we using on a server several lvm volumes with btrfs. We want to use 
nightly build snapshots for some days as an alternative to backups.

Now I want to get the size of the snapshots in detail. Therefore I 
played with

   btrfs subvolume find-new $snapshot $gen-id.

And I know, that this is quite complicated and not implemented. 
Therefore I try to go my own way:

Now assume there are two snapshots of one subvolume, snap1 and snap2. 
Further get the find-new informations of these snapshots with $gen-id=1 
and save them into different files. A diff of these files shows the 
changes between snap1 and snap2, right?

Ok.

There are three operations on a filesystem, I think,

1. copy a file on the filesystem
2. change a file on the filesystem
3. delete a file on the filesystem

Am I right to assume, that operation 1 and 2 are not change much the 
size of a snapshot and the delete operation let increase the size of a 
snapshot in the size of the deleted files?

If it is so, it would be enough for me to get the deletions of files 
between two snapshots and their size. But is there another way to get 
these informations beside btrfs subvolume find-new? Perhaps it makes 
sense to use ioctl for it? What about the send/receive feature, which is 
upcoming?

Are there any hints?

Many thanks in advance.

Jan