• Features include:
    1. Ability to deduplicate data on a variable-sized, content-based blocking
      • Just renaming a 1.5 gigabyte file doesn't cause a second copy of that same file to be stored, unlike rsync-based schemes
      • Storing 3 identical copies of your fambly jpeg's on 3 different computers results in only a single copy of the pictures being stored in the repository
      • Changing one byte in the middle of a 6 gigabyte file doesn't result in a distinct copy of the whole file in the repository - only the changed block is stored again
    2. Compresses deduplicated chunks with xz compression (falling back on bzip2 if necessary)
    3. Compresses almost all metadata, again with xz compression (again falling back on bzip2 if necessary)
    4. Few to no arbitrary limits on how big files can be - even if you're backing up to a file-size-limited filesystem.
    5. Ability to expire old data for the repo as a whole.
    6. Safe, concurrent operation over local or remote filesystems, including but not limited to: NFS, CIFS and sshfs. The only operation that isn't (designed to be) concurrent-safe, is expiring old files.
    7. No big filelist inhale at the outset is necessary, but if you allow one, you'll get a nice progress report as a result.
    8. Hybrid fullsaves/incrementals, much like what one gets with an rsync --link-dest backup script - so an interrupted backup can in a significant sense be subsequently resumed
    9. Ability to not mess up your buffer cache during a backup (planned, not yet fully implemented)
    10. A far smaller number of directory entries than a year's worth of daily snapshots with an rsync-based backup script would give
    11. Copying a backup repository with 1 year of daily snapshots from one host to another is far more practical with backshift than rsync --link-dest
    12. Input files are selected in a manner similar to cpio, using GNU find with -print0 or -printf
    13. Output is created in GNU tar format; a restore is a matter of piping tar output into a tar process for extraction. This means there's no restore application to worry about race conditions in other than tar itself
    14. No temporary files are necessary on the client system for backups or restores; even a system with (nearly?) full disks can be backed up (except on Cygwin, where a large number of small temporary files are written and read, but there's only one on disk at a given time).
    15. Easy, no-temp-files (except on Cygwin) backup verification using a pipe to GNU tar's --diff
    16. Runs on a wide assortment of Python interpreters, including:
      • CPython 2.[567] (with or without Cython, with or without Psyco - Psyco is not available for 2.7 or x86-64 though)
      • CPython 3.[0123] (with or without Cython)
      • PyPy 1.4.x, 1.5, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2. 1.6 had a problem.
      • Jython 2.5.3 or 2.7b1, but not Jython 2.5.2.
    17. Backshift is known not to work on IronPython, due to IronPython's lack of a python standard library.
    18. The backup process is cautious about symlink races, at least if the Python interpreter has an os.fstat (noteably, Jython does not have an os.fstat. CPython 2 and 3, and PyPy do have os.fstat)
    19. Backshift compresses data pretty hard through its deduplication and use of xz. EG on 2014-10-05, I calculated that I have a smattering of gig over 2.3 terabytes of data in use that I'm backing up at home, and 1 year of backshift snapshots of that data came to only 2.4 terabytes, including metadata.
  • Misfeatures:
    1. There's currently no way for a user to restore their own files without requiring excessive trust in users; the administrator needs to get involved.
    2. During a backup, users can see each others' files; data is not saved in an encrypted format (but note that sshfs restricts who can see a mount)
    3. It could conceivably be nice to have host- or filesystem- granularity on expires, but this would require that quite a bit more metadata be saved
    4. Disk-to-disk only - Disk-to-tape is not supported
    5. It's not super fast - especially the first time you backup a system.