.. _faq:
.. include:: global.rst.inc

Frequently asked questions
==========================

Which platforms are supported?
    Currently Linux, FreeBSD and MacOS X are supported.


Can I backup VM disk images?
    Yes, the :ref:`deduplication <deduplication_def>` technique used by |project_name|
    makes sure only the modified parts of the file are stored.

Can I backup from multiple servers into a single repository?
    Yes, but in order for the deduplication used by Attic to work, it
    needs to keep a local cache containing checksums of all file
    chunks already stored in the repository. This cache is stored in
    ``~/.cache/attic/``.  If Attic detects that a repository has been
    modified since the local cache was updated it will need to rebuild
    the cache. This rebuild can be quite time consuming.

    So, yes it's possible. But it will be most efficient if a single
    repository is only modified from one place. Also keep in mind that
    Attic will keep an exclusive lock on the repository while creating
    or deleting archives, which may make *simultaneous* backups fail.

Which file attributes are preserved?
    The following attributes are preserved:

    * Name
    * Contents
    * Time of last modification (nanosecond precision with Python >= 3.3)
    * User ID of owner
    * Group ID of owner
    * Unix Permission
    * Extended attributes (xattrs)
    * Access Control Lists (ACL_) on Linux, OS X and FreeBSD
    * BSD flags on OS X and FreeBSD

How can I specify the encryption passphrase programmatically?
    The encryption passphrase can be specified programmatically using the
    `ATTIC_PASSPHRASE` environment variable. This is convenient when setting up
    automated encrypted backups. Another option is to use
    key file based encryption with a blank passphrase. See
    :ref:`encrypted_repos` for more details.

When backing up to remote servers, is data encrypted before leaving the local machine, or do I have to trust that the remote server isn't malicious?
    Yes, everything is encrypted before leaving the local machine.

If a backup stops mid-way, does the already-backed-up data stay there? I.e. does Attic resume backups?
    Yes, during a backup a special checkpoint archive named ``<archive-name>.snapshot`` is saved every 5 minutes
    containing all the data backed-up until that point. This means that at most 5 minutes worth of data needs to be
    retransmitted if a backup needs to be restarted.