cloning files on BTRFS read-only snapshots doesn't work

[saintger]

Trying to deduplicate/clone files which are on BTRFS read-only snapshots, return an error:

ERROR: btrfs clone: failed to open dest file.

If it helps, duperemove has a special flag ("-A") to manage this situation:
https://github.com/markfasheh/duperemove/blob/master/filerec.c#L405-L434

[sahib]

I guess you used a command like this?

$ rmlint -c sh:link subvol/

And tried to execute rmlint.sh afterwards?

$ ./rmlint.sh

If so, what would your expected behaviour be?
If not, which commands did you use exactly?

I see that the error message is not very nice, but, as I understand you, you're suggesting that
this message is silenced and becomes a no-op in case of readonly subvolumes?
(Sorry, hadn't time to look into duperemove yet)

[saintger]

Indeed I used this command line:

rmlint --config=sh:handler=clone subvolume subvolume_read_only

The rmlint "clone" command, should be able to operate on read-only snapshots and not giving an error, but perhaps with an option/flag to make sure that the user knows what he is doing (but cloning is quite safe, from what I understand).
Duperemove managed to deduplicate read-only snapshots without any problem.

[SeeSpotRun]

Thanks @saintger for finding this.

It doesn't seem well documented anywhere but here's what I found by trial-and-error and reference to markfasheh/duperemove@fe67b7c:

• Read-only snapshots can only be opened read-only (makes sense)
• To clone identical files in btrfs, the target must generally be opened for writing (makes sense)
• However, the BTRFS_IOC_FILE_EXTENT_SAME ioctl will also work with a read-only target if called with elevated priveleges (sounds scary but is reasonable when you think it through).

Branch https://github.com/SeeSpotRun/rmlint/tree/btrfs-clone-readonly should be able to deduplicate read-only snapshots.
Usage for deduping a single file (source and dest must have same data; requires elevated priveleges):

$ sudo rmlint --btrfs-clone -r <source> <dest>

Usage via rmlint.sh:

$ rmlint --config=sh:handler=clone subvolume subvolume_read_only
$ ./rmlint.sh -r   # note: will ask for sudo password

[saintger]

@SeeSpotRun Thanks it seems to work perfectly !

[sahib]

I merged @SeeSpotRun's changes into the master branch.
This issue can probably be closed now, feel free to re-open if there are other problems.

[SeeSpotRun]

Have updated docs to include --btrfs-clone [-r](probably breaks translations)