improve cache / index docs, esp. files cache docs, fixes #1825

docs/internals.rst

@@ -252,44 +252,94 @@ For some more general usage hints see also ``--chunker-params``.
 Indexes / Caches
 ----------------
 
-The **files cache** is stored in ``cache/files`` and is indexed on the
-``file path hash``. At backup time, it is used to quickly determine whether we
-need to chunk a given file (or whether it is unchanged and we already have all
-its pieces).
-It contains:
+The **files cache** is stored in ``cache/files`` and is used at backup time to
+quickly determine whether a given file is unchanged and we have all its chunks.
+
+The files cache is a key -> value mapping and contains:
+
+* key:
+
+  - full, absolute file path id_hash
+* value:
+
+  - file inode number
+  - file size
+  - file mtime_ns
+  - list of file content chunk id hashes
+  - age (0 [newest], 1, 2, 3, ..., BORG_FILES_CACHE_TTL - 1)
+
+To determine whether a file has not changed, cached values are looked up via
+the key in the mapping and compared to the current file attribute values.
 
-* age
-* file inode number
-* file size
-* file mtime_ns
-* file content chunk hashes
+If the file's size, mtime_ns and inode number is still the same, it is
+considered to not have changed. In that case, we check that all file content
+chunks are (still) present in the repository (we check that via the chunks
+cache).
 
-The inode number is stored to make sure we distinguish between
+If everything is matching and all chunks are present, the file is not read /
+chunked / hashed again (but still a file metadata item is written to the
+archive, made from fresh file metadata read from the filesystem). This is
+what makes borg so fast when processing unchanged files.
+
+If there is a mismatch or a chunk is missing, the file is read / chunked /
+hashed. Chunks already present in repo won't be transferred to repo again.
+
+The inode number is stored and compared to make sure we distinguish between
 different files, as a single path may not be unique across different
 archives in different setups.
 
-The files cache is stored as a python associative array storing
-python objects, which generates a lot of overhead.
+Not all filesystems have stable inode numbers. If that is the case, borg can
+be told to ignore the inode number in the check via --ignore-inode.
 
-The **chunks cache** is stored in ``cache/chunks`` and is indexed on the
-``chunk id_hash``. It is used to determine whether we already have a specific
-chunk, to count references to it and also for statistics.
-It contains:
+The age value is used for cache management. If a file is "seen" in a backup
+run, its age is reset to 0, otherwise its age is incremented by one.
+If a file was not seen in BORG_FILES_CACHE_TTL backups, its cache entry is
+removed. See also: :ref:`always_chunking` and :ref:`a_status_oddity`
 
-* reference count
-* size
-* encrypted/compressed size
+The files cache is a python dictionary, storing python objects, which
+generates a lot of overhead.
 
-The **repository index** is stored in ``repo/index.%d`` and is indexed on the
-``chunk id_hash``. It is used to determine a chunk's location in the repository.
-It contains:
+Borg can also work without using the files cache (saves memory if you have a
+lot of files or not much RAM free), then all files are assumed to have changed.
+This is usually much slower than with files cache.
+
+The **chunks cache** is stored in ``cache/chunks`` and is used to determine
+whether we already have a specific chunk, to count references to it and also
+for statistics.
 
-* segment (that contains the chunk)
-* offset (where the chunk is located in the segment)
+The chunks cache is a key -> value mapping and contains:
+
+* key:
+
+  - chunk id_hash
+* value:
+
+  - reference count
+  - size
+  - encrypted/compressed size
+
+The chunks cache is a hashindex, a hash table implemented in C and tuned for
+memory efficiency.
+
+The **repository index** is stored in ``repo/index.%d`` and is used to
+determine a chunk's location in the repository.
+
+The repo index is a key -> value mapping and contains:
+
+* key:
+
+  - chunk id_hash
+* value:
+
+  - segment (that contains the chunk)
+  - offset (where the chunk is located in the segment)
+
+The repo index is a hashindex, a hash table implemented in C and tuned for
+memory efficiency.
 
-The repository index file is random access.
 
 Hints are stored in a file (``repo/hints.%d``).
+
 It contains:
 
 * version