doc: Improve check documentation

Make it clearer what the difference is between regular check and check with --read-data.

doc/045_working_with_repos.rst

@@ -82,54 +82,85 @@ Furthermore you can group the output by the same filters (host, paths, tags):
     1 snapshots
 
 
-Checking a repo's integrity and consistency
-===========================================
+Checking integrity and consistency
+==================================
 
 Imagine your repository is saved on a server that has a faulty hard
-drive, or even worse, attackers get privileged access and modify your
-backup with the intention to make you restore malicious data:
+drive, or even worse, attackers get privileged access and modify the
+files in your repository with the intention to make you restore
+malicious data:
 
 .. code-block:: console
 
-    $ echo "boom" >> backup/index/d795ffa99a8ab8f8e42cec1f814df4e48b8f49129360fb57613df93739faee97
+    $ echo "boom" > /srv/restic-repo/index/de30f3231ca2e6a59af4aa84216dfe2ef7339c549dc11b09b84000997b139628
 
-In order to detect these things, it is a good idea to regularly use the
-``check`` command to test whether everything is alright, your precious
-backup data is consistent and the integrity is unharmed:
+Trying to restore a snapshot which has been modified as shown above
+will yield an error:
+
+.. code-block:: console
+
+    $ restic -r /srv/restic-repo --no-cache restore c23e491f --target /tmp/restore-work
+    ...
+    Fatal: unable to load index de30f323: load <index/de30f3231c>: invalid data returned
+
+In order to detect these things before they become a problem, it's a
+good idea to regularly use the ``check`` command to test whether your
+repository is healthy and consistent, and that your precious backup
+data is unharmed. There are two types of checks that can be performed:
+
+- Structural consistency and integrity, e.g. snapshots, trees and pack files (default)
+- Integrity of the actual data that you backed up (enabled with flags, see below)
+
+To verify the structure of the repository, issue the ``check`` command.
+If the repository is damaged like in the example above, ``check`` will
+detect this and yield the same error as when you tried to restore:
 
 .. code-block:: console
 
     $ restic -r /srv/restic-repo check
-    Load indexes
-    ciphertext verification failed
+    ...
+    load indexes
+    error: error loading index de30f323: load <index/de30f3231c>: invalid data returned
+    Fatal: LoadIndex returned errors
 
-Trying to restore a snapshot which has been modified as shown above will
-yield the same error:
+If the repository structure is intact, restic will show that no errors were found:
 
 .. code-block:: console
 
-    $ restic -r /srv/restic-repo restore 79766175 --target /tmp/restore-work
-    Load indexes
-    ciphertext verification failed
+    $ restic -r /src/restic-repo check
+    ...
+    load indexes
+    check all packs
+    check snapshots, trees and blobs
+    no errors were found
 
-By default, ``check`` command does not check that repository data files 
-are unmodified. Use ``--read-data`` parameter to check all repository
-data files:
+By default, the ``check`` command does not verify that the actual data files
+on disk in the repository are unmodified, because doing so requires reading
+a copy of every data file in the repository. To tell restic to also verify the
+integrity of the data files in the repository, use the ``--read-data`` flag:
 
 .. code-block:: console
 
     $ restic -r /srv/restic-repo check --read-data
+    ...
     load indexes
     check all packs
     check snapshots, trees and blobs
     read all data
+    [0:00] 100.00%  3 / 3 items
+    duration: 0:00
+    no errors were found
 
-Use the ``--read-data-subset=n/t`` parameter to check only a subset of the
-repository data files at a time. The parameter takes two values, ``n`` and
-``t``. When the check command runs, all data files in the repository are
-logically divided in ``t`` (roughly equal) groups, and only files that
-belong to the group number ``n`` are checked. For example, the following
-commands check all repository data files over 5 separate invocations:
+.. note:: Since ``--read-data`` has to download all data files in the
+    repository, beware that it might incur higher bandwidth costs than usual
+    and also that it takes more time than the default ``check``.
+
+Alternatively, use the ``--read-data-subset=n/t`` parameter to check only a
+subset of the repository data files at a time. The parameter takes two values,
+``n`` and ``t``. When the check command runs, all data files in the repository
+are logically divided in ``t`` (roughly equal) groups, and only files that
+belong to group number ``n`` are checked. For example, the following commands
+check all repository data files over 5 separate invocations:
 
 .. code-block:: console
 
@@ -138,4 +169,3 @@ commands check all repository data files over 5 separate invocations:
     $ restic -r /srv/restic-repo check --read-data-subset=3/5
     $ restic -r /srv/restic-repo check --read-data-subset=4/5
     $ restic -r /srv/restic-repo check --read-data-subset=5/5
-