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docs: internals: edited obj graph related sections a bit

This commit is contained in:
Marian Beermann 2017-06-04 18:17:38 +02:00
parent 2b0e7bc924
commit 89d8f54afb
4 changed files with 43 additions and 21 deletions

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@ -278,10 +278,21 @@ If the quota shall be enforced accurately in these cases, either
- edit the msgpacked ``hints.N`` file (not recommended and thus not
documented further).
The object graph
----------------
On top of the simple key-value store offered by the Repository_,
Borg builds a much more sophisticated data structure that is essentially
a completely encrypted object graph. Objects, such as archives_, are referenced
by their chunk ID, which is cryptographically derived from their contents.
More on how this helps security in :ref:`security_structural_auth`.
.. figure:: object-graph.png
.. _manifest:
The manifest
------------
~~~~~~~~~~~~
The manifest is an object with an all-zero key that references all the
archives. It contains:
@ -303,24 +314,33 @@ each time an archive is added, modified or deleted.
.. _archive:
Archives
--------
~~~~~~~~
The archive metadata does not contain the file items directly. Only
references to other objects that contain that data. An archive is an
object that contains:
Each archive is an object referenced by the manifest. The archive object
itself does not store any of the data contained in the archive it describes.
* version
* name
* list of chunks containing item metadata (size: count * ~40B)
* cmdline
* hostname
* username
* time
Instead, it contains a list of chunks which form a msgpacked stream of items_.
The archive object itself further contains some metadata:
* *version*
* *name*, which might differ from the name set in the manifest.
When :ref:`borg_check` rebuilds the manifest (e.g. if it was corrupted) and finds
more than one archive object with the same name, it adds a counter to the name
in the manifest, but leaves the *name* field of the archives as it was.
* *items*, a list of chunk IDs containing item metadata (size: count * ~31B)
* *cmdline*, the command line which was used to create the archive
* *hostname*
* *username*
* *time* and *time_end* are the start and end timestamps, respectively
* *comment*, a user-specified archive comment
* *chunker_params* are the :ref:`chunker-params <chunker-params>` used for creating the archive.
This is used by :ref:`borg_recreate` to determine whether a given archive needs rechunking.
* Some other pieces of information related to recreate.
.. _archive_limitation:
Note about archive limitations
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
++++++++++++++++++++++++++++++
The archive is currently stored as a single object in the repository
and thus limited in size to MAX_OBJECT_SIZE (20MiB).
@ -349,10 +369,10 @@ also :issue:`1452`.
.. _item:
Items
-----
~~~~~
Each item represents a file, directory or other fs item and is stored as an
``item`` dictionary that contains:
Each item represents a file, directory or other file system item and is stored as a
dictionary created by the ``Item`` class that contains:
* path
* list of data chunks (size: count * ~40B)
@ -361,12 +381,12 @@ Each item represents a file, directory or other fs item and is stored as an
* uid
* gid
* mode (item type + permissions)
* source (for links)
* rdev (for devices)
* source (for symlinks, and for hardlinks within one archive)
* rdev (for device files)
* mtime, atime, ctime in nanoseconds
* xattrs
* acl
* bsdfiles
* acl (various OS-dependent fields)
* bsdflags
All items are serialized using msgpack and the resulting byte stream
is fed into the same chunker algorithm as used for regular file data
@ -381,7 +401,7 @@ A chunk is stored as an object as well, of course.
.. _chunker_details:
Chunks
------
~~~~~~
The |project_name| chunker uses a rolling hash computed by the Buzhash_ algorithm.
It triggers (chunks) when the last HASH_MASK_BITS bits of the hash are zero,

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@ -37,6 +37,8 @@ Under these circumstances Borg guarantees that the attacker cannot
The attacker can always impose a denial of service per definition (he could
forbid connections to the repository, or delete it entirely).
.. _security_structural_auth:
Structural Authentication
-------------------------