borg/docs/faq.rst

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

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

Can I backup VM disk images?
----------------------------

Yes, the `deduplication`_ technique used by
|project_name| makes sure only the modified parts of the file are stored.
Also, we have optional simple sparse file support for extract.

Can I backup from multiple servers into a single repository?
------------------------------------------------------------

Yes, but in order for the deduplication used by |project_name| 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/borg/``.  If |project_name| 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
|project_name| will keep an exclusive lock on the repository while creating
or deleting archives, which may make *simultaneous* backups fail.

Which file types, attributes, etc. are preserved?
-------------------------------------------------

    * Directories
    * Regular files
    * Hardlinks (considering all files in the same archive)
    * Symlinks (stored as symlink, the symlink is not followed)
    * Character and block device files
    * FIFOs ("named pipes")
    * Name
    * Contents
    * Time of last modification (nanosecond precision with Python >= 3.3)
    * IDs of owning user and owning group
    * Names of owning user and owning group (if the IDs can be resolved)
    * Unix Mode/Permissions (u/g/o permissions, suid, sgid, sticky)
    * Extended Attributes (xattrs) on Linux, OS X and FreeBSD
    * Access Control Lists (ACL_) on Linux, OS X and FreeBSD
    * BSD flags on OS X and FreeBSD

Which file types, attributes, etc. are *not* preserved?
-------------------------------------------------------

    * UNIX domain sockets (because it does not make sense - they are
      meaningless without the running process that created them and the process
      needs to recreate them in any case). So, don't panic if your backup
      misses a UDS!
    * The precise on-disk representation of the holes in a sparse file.
      Archive creation has no special support for sparse files, holes are
      backed up as (deduplicated and compressed) runs of zero bytes.
      Archive extraction has optional support to extract all-zero chunks as
      holes in a sparse file.

Why is my backup bigger than with attic? Why doesn't |project_name| do compression by default?
----------------------------------------------------------------------------------------------

Attic was rather unflexible when it comes to compression, it always
compressed using zlib level 6 (no way to switch compression off or
adjust the level or algorithm).

|project_name| offers a lot of different compression algorithms and
levels. Which of them is the best for you pretty much depends on your
use case, your data, your hardware - so you need to do an informed
decision about whether you want to use compression, which algorithm
and which level you want to use. This is why compression defaults to
none.

How can I specify the encryption passphrase programmatically?
-------------------------------------------------------------
      
The encryption passphrase can be specified programmatically using the
`BORG_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.

.. _password_env:
.. note:: Be careful how you set the environment; using the ``env``
          command, a ``system()`` call or using inline shell scripts
          might expose the credentials in the process list directly
          and they will be readable to all users on a system. Using
          ``export`` in a shell script file should be safe, however, as
          the environment of a process is `accessible only to that
          user
          <http://security.stackexchange.com/questions/14000/environment-variable-accessibility-in-linux/14009#14009>`_.

When backing up to remote encrypted repos, is encryption done locally?
----------------------------------------------------------------------
     
Yes, file and directory metadata and data is locally encrypted, before
leaving the local machine. We do not mean the transport layer encryption
by that, but the data/metadata itself. Transport layer encryption (e.g.
when ssh is used as a transport) applies additionally.

When backing up to remote servers, do I have to trust the remote server?
------------------------------------------------------------------------

Yes and No.

No, as far as data confidentiality is concerned - if you use encryption,
all your files/dirs data and metadata are stored in their encrypted form
into the repository.

Yes, as an attacker with access to the remote server could delete (or
otherwise make unavailable) all your backups.

If a backup stops mid-way, does the already-backed-up data stay there?
----------------------------------------------------------------------

Yes, |project_name| supports resuming backups.

During a backup a special checkpoint archive named ``<archive-name>.checkpoint``
is saved every checkpoint interval (the default value for this is 5
minutes) containing all the data backed-up until that point. This means
that at most <checkpoint interval> worth of data needs to be retransmitted
if a backup needs to be restarted.

Once your backup has finished successfully, you can delete all ``*.checkpoint``
archives.

If it crashes with a UnicodeError, what can I do?
-------------------------------------------------

Check if your encoding is set correctly. For most POSIX-like systems, try::

  export LANG=en_US.UTF-8  # or similar, important is correct charset

I can't extract non-ascii filenames by giving them on the commandline!?
-----------------------------------------------------------------------

This might be due to different ways to represent some characters in unicode
or due to other non-ascii encoding issues.

If you run into that, try this:

- avoid the non-ascii characters on the commandline by e.g. extracting
  the parent directory (or even everything)
- mount the repo using FUSE and use some file manager

Can |project_name| add redundancy to the backup data to deal with hardware malfunction?
---------------------------------------------------------------------------------------

No, it can't. While that at first sounds like a good idea to defend against
some defect HDD sectors or SSD flash blocks, dealing with this in a
reliable way needs a lot of low-level storage layout information and
control which we do not have (and also can't get, even if we wanted).

So, if you need that, consider RAID or a filesystem that offers redundant
storage or just make backups to different locations / different hardware.

See also `ticket 225 <https://github.com/borgbackup/borg/issues/225>`_.

Can |project_name| verify data integrity of a backup archive?
-------------------------------------------------------------

Yes, if you want to detect accidental data damage (like bit rot), use the
``check`` operation. It will notice corruption using CRCs and hashes.
If you want to be able to detect malicious tampering also, use a encrypted
repo. It will then be able to check using CRCs and HMACs.

.. _a_status_oddity:

I am seeing 'A' (added) status for a unchanged file!?
-----------------------------------------------------

The files cache is used to determine whether |project_name| already
"knows" / has backed up a file and if so, to skip the file from
chunking. It does intentionally *not* contain files that:

- have >= 10 as "entry age" (|project_name| has not seen this file for a while)
- have a modification time (mtime) same as the newest mtime in the created
  archive

So, if you see an 'A' status for unchanged file(s), they are likely the files
with the most recent mtime in that archive.

This is expected: it is to avoid data loss with files that are backed up from
a snapshot and that are immediately changed after the snapshot (but within
mtime granularity time, so the mtime would not change). Without the code that
removes these files from the files cache, the change that happened right after
the snapshot would not be contained in the next backup as |project_name| would
think the file is unchanged.

This does not affect deduplication, the file will be chunked, but as the chunks
will often be the same and already stored in the repo (except in the above
mentioned rare condition), it will just re-use them as usual and not store new
data chunks.

Since only the files cache is used in the display of files status,
those files are reported as being added when, really, chunks are
already used.

Why was Borg forked from Attic?
-------------------------------

Borg was created in May 2015 in response to the difficulty of getting new
code or larger changes incorporated into Attic and establishing a bigger
developer community / more open development.

More details can be found in `ticket 217
<https://github.com/jborg/attic/issues/217>`_ that led to the fork.

Borg intends to be:

* simple:

  * as simple as possible, but no simpler
  * do the right thing by default, but offer options
* open:

  * welcome feature requests
  * accept pull requests of good quality and coding style
  * give feedback on PRs that can't be accepted "as is"
  * discuss openly, don't work in the dark
* changing:

  * Borg is not compatible with Attic
  * do not break compatibility accidentally, without a good reason
    or without warning. allow compatibility breaking for other cases.
  * if major version number changes, it may have incompatible changes