Re: Extents for a particular subvolume

[Graham Cobb <g.btrfs@cobb.uk.net>]

On 03/08/16 21:37, Adam Borowski wrote:
> On Wed, Aug 03, 2016 at 08:56:01PM +0100, Graham Cobb wrote:
>> Are there any btrfs commands (or APIs) to allow a script to create a
>> list of all the extents referred to within a particular (mounted)
>> subvolume?  And is it a reasonably efficient process (i.e. doesn't
>> involve backrefs and, preferably, doesn't involve following directory
>> trees)?
> 
> Since the size of your output is linear to the number of extents which is
> between the number of files and sum of their sizes, I see no gain in
> trying to avoid following the directory tree.

Thanks for the help, Adam.  There are a lot of files and a lot of
directories - find, "ls -R" and similar operations take a very long
time. I was hoping that I could query some sort of extent tree for the
subvolume and get the answer back in seconds instead of multiple minutes.

But I can follow the directory tree if I need to.

>> I am not looking to relate the extents to files/inodes/paths.  My
>> particular need, at the moment, is to work out how much of two snapshots
>> is shared data, but I can think of other uses for the information.
> 
> Thus, unlike the question you asked above, you're not interested in _all_
> extents, merely those which changed.
> 
> You may want to look at "btrfs subv find-new" and "btrfs send --no-data".

Unfortunately, the subvolumes do not have an ancestor-descendent
relationship (although they do have some common ancestors), so I don't
think find-new is much help (as far as I can see).

But just looking at the size of the output  from "send -c" would work
well enough for the particular problem I am trying to solve tonight!
Although I will need to take read-only snapshots of the subvolumes to
allow send to work. Thanks for the suggestion.

I would still be interested in the extent list, though.  The main
problem with find-new and send is that they don't tell me how much has
been deleted, only added.  I am thinking about using the extents to get
a much better handle on what is using up space and what I could recover
if I removed (or moved to another volume) various groups of related
subvolumes.

Thanks again for the help.