### Please describe the problem.

Running adjust --unlock is unexpectedly slow and seems to use a lot of space, even on BTRFS, suggesting it probably does not use --reflink=auto like most other commands.

### What steps will reproduce the problem?

Run adjust --unlock with very large files.

### What version of git-annex are you using? On what operating system?

6.20170101-1+deb9u1 on Debian Stretch

### Have you had any luck using git-annex before? (Sometimes we get tired of reading bug reports all day and a lil' positive end note does wonders)

Yes I have! I've used it manage lots of video editing disks before, and am now migrating several slightly different copies of 15TB sized documentary footage from random USB3 disks and LTO tapes to a RAID server with BTRFS.
