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.

Can I copy or synchronize my repo to another location?
------------------------------------------------------

Yes, you could just copy all the files. Make sure you do that while no
backup is running. So what you get here is this:

- client machine ---borg create---> repo1
- repo1 ---copy---> repo2

There is no special borg command to do the copying, just use cp or rsync if
you want to do that.

But think about whether that is really what you want. If something goes
wrong in repo1, you will have the same issue in repo2 after the copy.

If you want to have 2 independent backups, it is better to do it like this:

- client machine ---borg create---> repo1
- client machine ---borg create---> repo2

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
    * Timestamps in nanosecond precision: mtime, atime, ctime
    * 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 (or rather: not-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.
    * filesystem specific attributes, like ext4 immutable bit, see :issue:`618`.

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
          <https://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.

How can I protect against a hacked backup client?
-------------------------------------------------

Assume you backup your backup client machine C to the backup server S and
C gets hacked. In a simple push setup, the attacker could then use borg on
C to delete all backups residing on S.

These are your options to protect against that:

- Do not allow to permanently delete data from the repo, see :ref:`append-only-mode`.
- Use a pull-mode setup using ``ssh -R``, see :issue:`900`.
- Mount C's filesystem on another machine and then create a backup of it.
- Do not give C filesystem-level access to S.

How can I protect against a hacked backup server?
-------------------------------------------------

Just in case you got the impression that pull-mode backups are way more safe
than push-mode, you also need to consider the case that your backup server S
gets hacked. In case S has access to a lot of clients C, that might bring you
into even bigger trouble than a hacked backup client in the previous FAQ entry.

These are your options to protect against that:

- Use the standard push-mode setup (see also previous FAQ entry).
- Mount (the repo part of) S's filesystem on C.
- Do not give S file-system level access to C.
- Have your backup server at a well protected place (maybe not reachable from
  the internet), configure it safely, apply security updates, monitor it, ...

How can I protect against theft, sabotage, lightning, fire, ...?
----------------------------------------------------------------

In general: if your only backup medium is nearby the backupped machine and
always connected, you can easily get into trouble: they likely share the same
fate if something goes really wrong.

Thus:

- have multiple backup media
- have media disconnected from network, power, computer
- have media at another place
- have a relatively recent backup on your media

Why do I get "connection closed by remote" after a while?
---------------------------------------------------------

When doing a backup to a remote server (using a ssh: repo URL), it sometimes
stops after a while (some minutes, hours, ... - not immediately) with
"connection closed by remote" error message. Why?

That's a good question and we are trying to find a good answer in :issue:`636`.

The borg cache eats way too much disk space, what can I do?
-----------------------------------------------------------

There is a temporary (but maybe long lived) hack to avoid using lots of disk
space for chunks.archive.d (see :issue:`235` for details):

::

    # this assumes you are working with the same user as the backup.
    # you can get the REPOID from the "config" file inside the repository.
    cd ~/.cache/borg/<REPOID>
    rm -rf chunks.archive.d ; touch chunks.archive.d

This deletes all the cached archive chunk indexes and replaces the directory
that kept them with a file, so borg won't be able to store anything "in" there
in future.

This has some pros and cons, though:

- much less disk space needs for ~/.cache/borg.
- chunk cache resyncs will be slower as it will have to transfer chunk usage
  metadata for all archives from the repository (which might be slow if your
  repo connection is slow) and it will also have to build the hashtables from
  that data.
  chunk cache resyncs happen e.g. if your repo was written to by another
  machine (if you share same backup repo between multiple machines) or if
  your local chunks cache was lost somehow.

The long term plan to improve this is called "borgception", see :issue:`474`.

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 checkpoint
archive is a valid archive, but it is only a partial backup. Having it
in the repo until a successful, full backup is completed is useful because it
references all the transmitted chunks up to the checkpoint time. This means
that at most <checkpoint interval> worth of data needs to be retransmitted
if you restart the backup.

If a backup was interrupted, you do not need to do any special considerations,
just invoke ``borg create`` as you always do. You may use the same archive name
as in previous attempt or a different one (e.g. if you always include the current
datetime), it does not matter.
|project_name| always does full single-pass backups, so it will start again
from the beginning - but it will be much faster, because some of the data was
already stored into the repo (and is still referenced by the checkpoint
archive), so it does not need to get transmitted and stored again.

Once your backup has finished successfully, you can delete all
``<archive-name>.checkpoint`` archives. If you run ``borg prune``, it will
also care for deleting unneeded checkpoints.

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 :issue:`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 an 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.

If you want to avoid unnecessary chunking, just create or touch a small or
empty file in your backup source file set (so that one has the latest mtime,
not your 50GB VM disk image) and, if you do snapshots, do the snapshot after
that.

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.


Is there a way to limit bandwidth with |project_name|?
------------------------------------------------------

There is no command line option to limit bandwidth with |project_name|, but
bandwidth limiting can be accomplished with pipeviewer_:

Create a wrapper script:  /usr/local/bin/pv-wrapper  ::

    #!/bin/bash
        ## -q, --quiet              do not output any transfer information at all
        ## -L, --rate-limit RATE    limit transfer to RATE bytes per second
    export RATE=307200
    pv -q -L $RATE  | "$@"

Add BORG_RSH environment variable to use pipeviewer wrapper script with ssh. ::

    export BORG_RSH='/usr/local/bin/pv-wrapper.sh ssh'

Now |project_name| will be bandwidth limited. Nice thing about pv is that you can change rate-limit on the fly: ::

    pv -R $(pidof pv) -L 102400

.. _pipeviewer: http://www.ivarch.com/programs/pv.shtml


I am having troubles with some network/FUSE/special filesystem, why?
--------------------------------------------------------------------

|project_name| is doing nothing special in the filesystem, it only uses very
common and compatible operations (even the locking is just "mkdir").

So, if you are encountering issues like slowness, corruption or malfunction
when using a specific filesystem, please try if you can reproduce the issues
with a local (non-network) and proven filesystem (like ext4 on Linux).

If you can't reproduce the issue then, you maybe have found an issue within
the filesystem code you used (not with |project_name|). For this case, it is
recommended that you talk to the developers / support of the network fs and
maybe open an issue in their issue tracker. Do not file an issue in the
|project_name| issue tracker.

If you can reproduce the issue with the proven filesystem, please file an
issue in the |project_name| issue tracker about that.


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