Merge pull request #2447 from ThomasWaldmann/update-docs4

Update docs

README.rst

@@ -27,6 +27,10 @@ Main features
   of bytes stored: each file is split into a number of variable length chunks
   and only chunks that have never been seen before are added to the repository.
 
+  A chunk is considered duplicate if its id_hash value is identical.
+  A cryptographically strong hash or MAC function is used as id_hash, e.g.
+  (hmac-)sha256.
+
   To deduplicate, all the chunks in the same repository are considered, no
   matter whether they come from different machines, from previous backups,
   from the same backup or even from the same single file.

docs/internals/data-structures.rst

@@ -73,6 +73,9 @@ Normally the keys are computed like this::
 
 The id_hash function depends on the :ref:`encryption mode <borg_init>`.
 
+As the id / key is used for deduplication, id_hash must be a cryptographically
+strong hash or MAC.
+
 Segments
 ~~~~~~~~
 
@@ -247,6 +250,11 @@ The |project_name| chunker uses a rolling hash computed by the Buzhash_ algorith
 It triggers (chunks) when the last HASH_MASK_BITS bits of the hash are zero,
 producing chunks of 2^HASH_MASK_BITS Bytes on average.
 
+Buzhash is **only** used for cutting the chunks at places defined by the
+content, the buzhash value is **not** used as the deduplication criteria (we
+use a cryptographically strong hash/MAC over the chunk contents for this, the
+id_hash).
+
 ``borg create --chunker-params CHUNK_MIN_EXP,CHUNK_MAX_EXP,HASH_MASK_BITS,HASH_WINDOW_SIZE``
 can be used to tune the chunker parameters, the default is:
 

docs/usage.rst

@@ -677,6 +677,11 @@ in ``.ssh/authorized_keys`` ::
     command="borg serve --append-only ..." ssh-rsa <key used for not-always-trustable backup clients>
     command="borg serve ..." ssh-rsa <key used for backup management>
 
+Please note that if you run ``borg init`` via a ``borg serve --append-only``
+server, the repository config will be created with a ``append_only=1`` entry.
+This behaviour is subject to change in a later borg version. So, be aware of
+it for now, but do not rely on it.
+
 Example
 +++++++