path: root/docs/faq.rst

.. include:: global.rst.inc
.. highlight:: none
.. _faq:

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

Usage & Limitations
###################

What is the difference between a repo on an external hard drive vs. repo on a server?
-------------------------------------------------------------------------------------

If Borg is running in client/server mode, the client uses SSH as a transport to
talk to the remote agent, which is another Borg process (Borg is installed on
the server, too) started automatically by the client. The Borg server is doing
storage-related low-level repo operations (get, put, commit, check, compact),
while the Borg client does the high-level stuff: deduplication, encryption,
compression, dealing with archives, backups, restores, etc., which reduces the
amount of data that goes over the network.

When Borg is writing to a repo on a locally mounted remote file system, e.g.
SSHFS, the Borg client only can do file system operations and has no agent
running on the remote side, so *every* operation needs to go over the network,
which is slower.

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

In order for the deduplication used by Borg 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 Borg 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
Borg will keep an exclusive lock on the repository while creating
or deleting archives, which may make *simultaneous* backups fail.

Can I back up to multiple, swapped backup targets?
--------------------------------------------------

It is possible to swap your backup disks if each backup medium is assigned its
own repository by creating a new one with :ref:`borg_rcreate`.

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

If you want to have redundant backup repositories (preferably at separate
locations), the recommended way to do that is like this:

- ``borg rcreate repo1 --encryption=X``
- ``borg rcreate repo2 --encryption=X --other-repo=repo1``
- maybe do a snapshot to have stable and same input data for both borg create.
- client machine ---borg create---> repo1
- client machine ---borg create---> repo2

This will create distinct (different repo ID), but related repositories.
Related means using the same chunker secret and the same id_key, thus producing
the same chunks / the same chunk ids if the input data is the same.

The 2 independent borg create invocations mean that there is no error propagation
from repo1 to repo2 when done like that.

An alternative way would be to use ``borg transfer`` to copy backup archives
from repo1 to repo2. Likely a bit more efficient and the archives would be identical,
but suffering from potential error propagation.

Warning: using borg with multiple repositories with identical repository ID (like when
creating 1:1 repository copies) is not supported and can lead to all sorts of issues,
like e.g. cache coherency issues, malfunction, data corruption.

"this is either an attack or unsafe" warning
--------------------------------------------

About the warning:

  Cache, or information obtained from the security directory is newer than
  repository - this is either an attack or unsafe (multiple repos with same ID)

"unsafe": If not following the advice from the previous section, you can easily
run into this by yourself by restoring an older copy of your repository.

"attack": maybe an attacker has replaced your repo by an older copy, trying to
trick you into AES counter reuse, trying to break your repo encryption.

If you'ld decide to ignore this and accept unsafe operation for this repository,
you could delete the manifest-timestamp and the local cache:

::

  borg config id   # shows the REPO_ID
  rm ~/.config/borg/security/REPO_ID/manifest-timestamp
  borg rdelete --cache-only

This is an unsafe and unsupported way to use borg, you have been warned.

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.
    * Some filesystem specific attributes, like btrfs NOCOW, see :ref:`platforms`.

Are there other known limitations?
----------------------------------

- borg extract only supports restoring into an empty destination. After that,
  the destination will exactly have the contents of the extracted archive.
  If you extract into a non-empty destination, borg will (for example) not
  remove files which are in the destination, but not in the archive.
  See :issue:`4598` for a workaround and more details.

.. _checkpoints_parts:

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

Yes, Borg 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 30
minutes) containing all the data backed-up until that point.

This checkpoint archive is a valid archive,
but it is only a partial backup (not all files that you wanted to backup are
contained in it). 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. This means that in case of an interruption, you only need to
retransfer the data since the last checkpoint.

If a backup was interrupted, you normally do not need to do anything special,
just invoke ``borg create`` as you always do. If the repository is still locked,
you may need to run ``borg break-lock`` before the next backup. 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.

Borg 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.

Note: the checkpointing mechanism creates hidden, partial files in an archive,
so that checkpoints even work while a big file is being processed.
They are named ``<filename>.borg_part_<N>`` and all operations usually ignore
these files, but you can make them considered by giving the option
``--consider-part-files``. You usually only need that option if you are
really desperate (e.g. if you have no completed backup of that file and you'ld
rather get a partial file extracted than nothing). You do **not** want to give
that option under any normal circumstances.

How can I backup huge file(s) over a unstable connection?
---------------------------------------------------------

Yes. For more details, see :ref:`checkpoints_parts`.

How can I restore huge file(s) over an unstable connection?
-----------------------------------------------------------

If you cannot manage to extract the whole big file in one go, you can extract
all the part files and manually concatenate them together.

For more details, see :ref:`checkpoints_parts`.

How can I switch append-only mode on and off?
-----------------------------------------------------------------------------------------------------------------------------------

You could do that (via borg config REPO append_only 0/1), but using different
ssh keys and different entries in ``authorized_keys`` is much easier and also
maybe has less potential of things going wrong somehow.

My machine goes to sleep causing `Broken pipe`
----------------------------------------------

While backing up your data over the network, your machine should not go to sleep.
On macOS you can use `caffeinate` to avoid that.

How can I compare contents of an archive to my local filesystem?
-----------------------------------------------------------------

You can instruct ``export-tar`` to send a tar stream to the stdout, and
then use ``tar`` to perform the comparison:

::

    borg export-tar archive-name - | tar --compare -f - -C /path/to/compare/to


Can Borg 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 Borg 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.

Can I use Borg on SMR hard drives?
----------------------------------

SMR (shingled magnetic recording) hard drives are very different from
regular hard drives. Applications have to behave in certain ways or
performance will be heavily degraded.

Borg ships with default settings suitable for SMR drives,
and has been successfully tested on *Seagate Archive v2* drives
using the ext4 file system.

Some Linux kernel versions between 3.19 and 4.5 had various bugs
handling device-managed SMR drives, leading to IO errors, unresponsive
drives and unreliable operation in general.

For more details, refer to :issue:`2252`.

.. _faq-integrityerror:

I get an IntegrityError or similar - what now?
----------------------------------------------

A single error does not necessarily indicate bad hardware or a Borg
bug. All hardware exhibits a bit error rate (BER). Hard drives are typically
specified as exhibiting fewer than one error every 12 to 120 TB
(one bit error in 10e14 to 10e15 bits). The specification is often called
*unrecoverable read error rate* (URE rate).

Apart from these very rare errors there are two main causes of errors:

(i) Defective hardware: described below.
(ii) Bugs in software (Borg, operating system, libraries):
     Ensure software is up to date.
     Check whether the issue is caused by any fixed bugs described in
     :ref:`important_notes`.

.. rubric:: Finding defective hardware

.. note::

   Hardware diagnostics are operating system dependent and do not
   apply universally. The commands shown apply for popular Unix-like
   systems. Refer to your operating system's manual.

Checking hard drives
  Find the drive containing the repository and use *findmnt*, *mount* or *lsblk*
  to learn the device path (typically */dev/...*) of the drive.
  Then, smartmontools can retrieve self-diagnostics of the drive in question::

      # smartctl -a /dev/sdSomething

  The *Offline_Uncorrectable*, *Current_Pending_Sector* and *Reported_Uncorrect*
  attributes indicate data corruption. A high *UDMA_CRC_Error_Count* usually
  indicates a bad cable.

  I/O errors logged by the system (refer to the system journal or
  dmesg) can point to issues as well. I/O errors only affecting the
  file system easily go unnoticed, since they are not reported to
  applications (e.g. Borg), while these errors can still corrupt data.

  Drives can corrupt some sectors in one event, while remaining
  reliable otherwise. Conversely, drives can fail completely with no
  advance warning. If in doubt, copy all data from the drive in
  question to another drive -- just in case it fails completely.

  If any of these are suspicious, a self-test is recommended::

      # smartctl -t long /dev/sdSomething

  Running ``fsck`` if not done already might yield further insights.

Checking memory
  Intermittent issues, such as ``borg check`` finding errors
  in