On 2016-08-03 17:55, Graham Cobb wrote:
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.
FWIW, if you're not using any files in the subvolumes, you can run:
btrfs property set <subvolume> ro true
to mark them read-only so you don't need the snapshots, and then run the
same command with 'false' at the end instead of true to mark them
writable again.
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.
You may want to look into 'btrfs filesystem usage' and 'btrfs filesystem
du' commands. I'm not sure if they'll cover what you need, but they can
show info about how much is shared.
--
To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
