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doc: rewrite troubleshooting section

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Troubleshooting
#########################
Being a backup software, the repository format ensures that the data saved in the repository
is verifiable and error-restistant. Restic even implements some self-healing functionalities.
The repository format used by restic is designed to be error resistant. In
particular, commands like, for example, ``backup`` or ``prune`` can be interrupted
at *any* point in time without damaging the repository. You might have to run
``unlock`` manually though, but that's it.
However, situations might occur where your repository gets in an incorrect state and measurements
need to be done to get you out of this situation. These situations might be due to hardware failure,
accidentially removing files directly from the repository or bugs in the restic implementation.
However, a repository might be damaged if some of its files are damaged or lost.
This can occur due to hardware failures, accidentally removing files from the
repository or bugs in the implementation of restic.
This document is meant to give you some hints about how to recover from such situations.
The following steps will help you recover a repository. This guide does not cover
all possible types of repository damages. Thus, if the steps do not work for you
or you are unsure how to proceed, then ask for help. Please always include the
check output discussed in the next section and what steps you've taken to repair
the repository so far.
1. Stay calm and don't over-react
********************************************
* `Forum <https://forum.restic.net/>`_
* Our IRC channel ``#restic`` on ``irc.libera.chat``
The most important thing if you find yourself in the situation of a damaged repository is to
stay calm and don't do anything you might regret later.
The following point should be always considered:
- Make a copy of you repository and try to recover from that copy. If you suspect a storage failure,
it may be even better, to make *two* copies: one to get all data out of the possibly failing storage
and another one to try the recovery process.
- Pause your regular operations on the repository or let them run on a copy. You will especially make
sure that no `forget` or `prune` is run as these command are supposed to remove data and may result
in data loss.
- Search if your issue is already known and solved. Good starting points are the restic forum and the
github issues.
- Get you some help if you are unsure what to do. Find a colleage or friend to discuss what should be done.
Also feel free to consult the restic forum.
- When using the commands below, make sure you read and understand the documentation. Some of the commands
may not be your every-day commands, so make sure you really understand what they are doing.
Make sure that you **use the latest available restic version**. It can contain
bugfixes, and improvements to simplify the repair of a repository. It might also
contain a fix for your repository problems!
2. `check` is your friend
********************************************
1. Find out what is damaged
***************************
Run `restic check` to find out what type of error you have. The results may be technical but can give you
a good hint what's really wrong.
The first step is always to check the repository.
Moreover, you can always run a `check` to ensure that your repair really was sucessful and your repository
is in a sane state again.
But make sure that your needed data is also still contained in your repository ;-)
Note that `check` also prints out warning in some cases. These warnings point out that the repo may be
optimized but is still in perfect shape and does not need any troubleshooting.
.. code-block:: console
3. Index trouble -> `repair index`
********************************************
$ restic check --read-data
A common problem with broken repostories is that the index does no longer correctly represent the contents
of your pack files. This is especially the case if some pack files got lost.
`repair index` recovers this situation and ensures that the index exactly represents the pack files.
using temporary cache in /tmp/restic-check-cache-1418935501
repository 12345678 opened (version 2, compression level auto)
created new cache in /tmp/restic-check-cache-1418935501
create exclusive lock for repository
load indexes
check all packs
check snapshots, trees and blobs
error for tree 7ef8ebab:
id 7ef8ebabc59aadda1a237d23ca7abac487b627a9b86508aa0194690446ff71f6 not found in repository
[0:02] 100.00% 7 / 7 snapshots
read all data
[0:05] 100.00% 25 / 25 packs
Fatal: repository contains errors
You might even need to manually remove corrupted pack files. In this case make sure, you run
`restic repair index` after.
.. note::
Also if you encounter problems with the index files itselves, `repair index` will solve these problems
immediately.
This will download the whole repository. If retrieving data from the backend is
expensive, then omit the ``--read-data`` option. Keep a copy of the check output
as it might be necessary later on!
However, rebuilding the index does not solve every problem, e.g. lost pack files.
If the output contains warnings that the ``ciphertext verification failed`` for
some blobs in the repository, then please ask for help in the forum or our IRC
channel. These errors are often caused by hardware problems which **must** be
investigated and fixed. Otherwise, the backup will be damaged again and again.
4. Delete unneeded defect snapshots -> `forget`
********************************************
Similarly, if a repository is repeatedly damaged, please open an `issue on Github
<https://github.com/restic/restic/issues/new/choose>`_ as this could indicate a bug
somewhere. Please include the check output and additional information that might
help locate the problem.
If you encounter defect snapshots but realize you can spare them, it is often a good idea to simply
delete them using `forget`. In case that your repository remains with just sane snapshots (including
all trees and files) the next `prune` run will put your repository in a sane state.
This can be also used if you manage to create new snapshots which can replace the defect ones, see
below.
2. Backup the repository
************************
5. No fear to `backup` again
********************************************
Create a full copy of the repository if possible. Or at the very least make a
copy of the ``index`` and ``snapshots`` folders. This will allow you to roll back
the repository if the repair procedure fails. If your repository resides in a
cloud storage, then you can for example use `rclone <https://rclone.org/>`_ to
make such a copy.
There are quite some self-healing mechanisms withing the `backup` command. So it is always a good idea to
backup again and check if this did heal your repository.
If you realize that a specific file is broken in your repository and you have this file, any run of
`backup` which includes that file will be able to heal the situation.
Please disable all regular operations on the repository to prevent unexpected
changes. Especially, ``forget`` or ``prune`` must be disabled as they could
remove data unexpectedly.
Note that `backup` relies on a correct index state, so make sure your index is fine or run `repair index`
before running `backup`.
.. warning::
6. Unreferenced tree -> `recover`
********************************************
If you suspect hardware problems, then you *must* investigate those first.
Otherwise, the repository will soon be damaged again.
If for some reason you have unreferenced trees in your repository but you actually need them, run
`recover` it will generate a new snapshot which allows access to all trees that you have in your
repository.
Please take the time to understand what the commands described in the following
do. If you are unsure, then ask for help in the forum or our IRC channel. Search
whether your issue is already known and solved. Please take a look at the
`forum`_ and `Github issues <https://github.com/restic/restic/issues>`_.
Note that `recover` relies on a correct index state, so make sure your index is fine or run `repair index`
before running `recover`.
7. Repair defect snapshots using `repair`
********************************************
3. Repair the index
*******************
If all other things did not help, you can repair defect snapshots with `repair`. Note that the repaired
snapshots will miss data which was referenced in the defect snapshot.
Restic relies on its index to contain correct information about what data is
stored in the repository. Thus, the first step to repair a repository is to
repair the index:
.. code-block:: console
$ restic repair index
repository a14e5863 opened (version 2, compression level auto)
loading indexes...
getting pack files to read...
removing not found pack file 83ad44f59b05f6bce13376b022ac3194f24ca19e7a74926000b6e316ec6ea5a4
rebuilding index
[0:00] 100.00% 27 / 27 packs processed
deleting obsolete index files
[0:00] 100.00% 3 / 3 files deleted
done
This ensures that no longer existing files are removed from the index. All later
steps to repair the repository rely on a correct index. That is, you must always
repair the index first!
Please note that it is not recommended to repair the index unless the repository
is actually damaged.
4. Run all backups (optional)
*****************************
With a correct index, the ``backup`` command guarantees that newly created
snapshots can be restored successfully. It can also heal older snapshots,
if the missing data is also contained in the new snapshot.
Therefore, it is recommended to run all your ``backup`` tasks again. In some
cases, this is enough to fully repair the repository.
5. Remove missing data from snapshots
*************************************
If your repository is still missing data, then you can use the ``repair snapshots``
command to remove all inaccessible data from the snapshots. That is, this will
result in a limited amount of data loss. Using the ``--forget`` option, the
command will automatically remove the original, damaged snapshots.
.. code-block:: console
$ restic repair snapshots --forget
snapshot 6979421e of [/home/user/restic/restic] at 2022-11-02 20:59:18.617503315 +0100 CET)
file "/restic/internal/fuse/snapshots_dir.go": removed missing content
file "/restic/internal/restorer/restorer_unix_test.go": removed missing content
file "/restic/internal/walker/walker.go": removed missing content
saved new snapshot 7b094cea
removed old snapshot 6979421e
modified 1 snapshots
If you did not add the ``--forget`` option, then you have to manually delete all
modified snapshots using the ``forget`` command. In the example above, you'd have
to run ``restic forget 6979421e``.
6. Check the repository again
*****************************
Phew, we're almost done now. To make sure that the repository has been successfully
repaired please run ``check`` again.
.. code-block:: console
$ restic check --read-data
using temporary cache in /tmp/restic-check-cache-2569290785
repository a14e5863 opened (version 2, compression level auto)
created new cache in /tmp/restic-check-cache-2569290785
create exclusive lock for repository
load indexes
check all packs
check snapshots, trees and blobs
[0:00] 100.00% 7 / 7 snapshots
read all data
[0:00] 100.00% 25 / 25 packs
no errors were found
If the ``check`` command did not complete with ``no errors were found``, then
the repository is still damaged. At this point, please ask for help at the
`forum`_ or our IRC channel ``#restic`` on ``irc.libera.chat``.