Re: Extents for a particular subvolume

[Adam Borowski <kilobyte@angband.pl>]

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.

And that can be done with FIEMAP on any filesystem, not just btrfs.

> 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".


Meow!
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