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improve internals docs
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@ -6,38 +6,43 @@ Internals
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This page documents the internal data structures and storage
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This page documents the internal data structures and storage
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mechanisms of |project_name|. It is partly based on `mailing list
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mechanisms of |project_name|. It is partly based on `mailing list
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discussion about internals`_ and also on static code analysis. It may
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discussion about internals`_ and also on static code analysis.
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not be exactly up to date with the current source code.
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It may not be exactly up to date with the current source code.
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Repository and Archives
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-----------------------
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|project_name| stores its data in a `Repository`. Each repository can
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|project_name| stores its data in a `Repository`. Each repository can
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hold multiple `Archives`, which represent individual backups that
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hold multiple `Archives`, which represent individual backups that
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contain a full archive of the files specified when the backup was
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contain a full archive of the files specified when the backup was
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performed. Deduplication is performed across multiple backups, both on
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performed. Deduplication is performed across multiple backups, both on
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data and metadata, using `Segments` chunked with the Buzhash_
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data and metadata, using `Chunks` created by the chunker using the Buzhash_
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algorithm. Each repository has the following file structure:
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algorithm.
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Each repository has the following file structure:
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README
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README
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simple text file describing the repository
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simple text file telling that this is a |project_name| repository
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config
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config
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description of the repository, includes the unique identifier. also
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repository configuration and lock file
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acts as a lock file
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data/
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data/
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directory where the actual data (`segments`) is stored
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directory where the actual data is stored
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hints.%d
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hints.%d
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undocumented
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hints for repository compaction
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index.%d
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index.%d
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cache of the file indexes. those files can be regenerated with
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repository index
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``check --repair``
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Config file
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Config file
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-----------
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-----------
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Each repository has a ``config`` file which which is a ``INI``
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Each repository has a ``config`` file which which is a ``INI``-style file
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formatted file which looks like this::
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and looks like this::
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[repository]
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[repository]
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version = 1
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version = 1
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@ -48,20 +53,35 @@ formatted file which looks like this::
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This is where the ``repository.id`` is stored. It is a unique
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This is where the ``repository.id`` is stored. It is a unique
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identifier for repositories. It will not change if you move the
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identifier for repositories. It will not change if you move the
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repository around so you can make a local transfer then decide to move
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repository around so you can make a local transfer then decide to move
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the repository in another (even remote) location at a later time.
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the repository to another (even remote) location at a later time.
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|project_name| will do a POSIX read lock on that file when operating
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|project_name| will do a POSIX read lock on the config file when operating
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on the repository.
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on the repository.
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Keys
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----
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The key to address the key/value store is usually computed like this:
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key = id = id_hash(unencrypted_data)
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The id_hash function is:
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* sha256 (no encryption keys available)
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* hmac-sha256 (encryption keys available)
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Segments and archives
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Segments and archives
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---------------------
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---------------------
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|project_name| is a "filesystem based transactional key value
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A |project_name| repository is a filesystem based transactional key/value
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store". It makes extensive use of msgpack_ to store data and, unless
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store. It makes extensive use of msgpack_ to store data and, unless
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otherwise noted, data is stored in msgpack_ encoded files.
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otherwise noted, data is stored in msgpack_ encoded files.
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Objects referenced by a key (256bits id/hash) are stored inline in
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Objects referenced by a key are stored inline in files (`segments`) of approx.
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files (`segments`) of size approx 5MB in ``repo/data``. They contain:
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5MB size in numbered subdirectories of ``repo/data``.
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They contain:
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* header size
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* header size
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* crc
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* crc
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@ -77,21 +97,26 @@ Tag is either ``PUT``, ``DELETE``, or ``COMMIT``. A segment file is
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basically a transaction log where each repository operation is
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basically a transaction log where each repository operation is
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appended to the file. So if an object is written to the repository a
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appended to the file. So if an object is written to the repository a
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``PUT`` tag is written to the file followed by the object id and
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``PUT`` tag is written to the file followed by the object id and
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data. And if an object is deleted a ``DELETE`` tag is appended
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data. If an object is deleted a ``DELETE`` tag is appended
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followed by the object id. A ``COMMIT`` tag is written when a
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followed by the object id. A ``COMMIT`` tag is written when a
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repository transaction is committed. When a repository is opened any
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repository transaction is committed. When a repository is opened any
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``PUT`` or ``DELETE`` operations not followed by a ``COMMIT`` tag are
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``PUT`` or ``DELETE`` operations not followed by a ``COMMIT`` tag are
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discarded since they are part of a partial/uncommitted transaction.
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discarded since they are part of a partial/uncommitted transaction.
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The manifest is an object with an id of only zeros (32 bytes), that
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references all the archives. It contains:
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The manifest
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------------
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The manifest is an object with an all-zero key that references all the
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archives.
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It contains:
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* version
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* version
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* list of archives
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* list of archive infos
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* timestamp
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* timestamp
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* config
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* config
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Each archive contains:
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Each archive info contains:
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* name
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* name
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* id
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* id
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@ -102,21 +127,21 @@ each time.
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The archive metadata does not contain the file items directly. Only
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The archive metadata does not contain the file items directly. Only
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references to other objects that contain that data. An archive is an
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references to other objects that contain that data. An archive is an
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object that contain metadata:
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object that contains:
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* version
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* version
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* name
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* name
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* items list
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* list of chunks containing item metadata
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* cmdline
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* cmdline
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* hostname
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* hostname
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* username
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* username
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* time
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* time
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Each item represents a file or directory or
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Each item represents a file, directory or other fs item and is stored as an
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symlink is stored as an ``item`` dictionary that contains:
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``item`` dictionary that contains:
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* path
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* path
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* list of chunks
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* list of data chunks
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* user
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* user
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* group
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* group
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* uid
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* uid
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@ -135,124 +160,136 @@ it and it is reset every time an inode's metadata is changed.
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All items are serialized using msgpack and the resulting byte stream
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All items are serialized using msgpack and the resulting byte stream
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is fed into the same chunker used for regular file data and turned
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is fed into the same chunker used for regular file data and turned
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into deduplicated chunks. The reference to these chunks is then added
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into deduplicated chunks. The reference to these chunks is then added
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to the archive metadata. This allows the archive to store many files,
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to the archive metadata.
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beyond the ``MAX_OBJECT_SIZE`` barrier of 20MB.
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A chunk is an object as well, of course. The chunk id is either
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A chunk is stored as an object as well, of course.
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HMAC-SHA256_, when encryption is used, or a SHA256_ hash otherwise.
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Hints are stored in a file (``repo/hints``) and contain:
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* version
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* list of segments
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* compact
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Chunks
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Chunks
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------
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------
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|project_name| uses a rolling checksum with Buzhash_ algorithm, with
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|project_name| uses a rolling hash computed by the Buzhash_ algorithm, with a
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window size of 4095 bytes (`0xFFF`), with a minimum of 1024, and triggers when
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window size of 4095 bytes (`0xFFF`), with a minimum chunk size of 1024 bytes.
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the last 16 bits of the checksum are null, producing chunks of 64kB on
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It triggers (chunks) when the last 16 bits of the hash are zero, producing
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average. All these parameters are fixed. The buzhash table is altered
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chunks of 64kiB on average.
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by XORing it with a seed randomly generated once for the archive, and
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stored encrypted in the keyfile.
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Indexes
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The buzhash table is altered by XORing it with a seed randomly generated once
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-------
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for the archive, and stored encrypted in the keyfile.
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There are two main indexes: the chunk lookup index and the repository
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index. There is also the file chunk cache.
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The chunk lookup index is stored in ``cache/chunk`` and is indexed on
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Indexes / Caches
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the ``chunk hash``. It contains:
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----------------
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* reference count
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The files cache is stored in ``cache/files`` and is indexed on the
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* size
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``file path hash``. At backup time, it is used to quickly determine whether we
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* ciphered size
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need to chunk a given file (or whether it is unchanged and we already have all
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its pieces).
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The repository index is stored in ``repo/index.%d`` and is also
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It contains:
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indexed on ``chunk hash`` and contains:
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* segment
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* offset
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The repository index files are random access but those files can be
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recreated if damaged or lost using ``check --repair``.
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Both indexes are stored as hash tables, directly mapped in memory from
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the file content, with only one slot per bucket, but that spreads the
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collisions to the following buckets. As a consequence the hash is just
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a start position for a linear search, and if the element is not in the
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table the index is linearly crossed until an empty bucket is
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found. When the table is full at 90% its size is doubled, when it's
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empty at 25% its size is halfed. So operations on it have a variable
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complexity between constant and linear with low factor, and memory
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overhead varies between 10% and 300%.
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The file chunk cache is stored in ``cache/files`` and is indexed on
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the ``file path hash`` and contains:
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* age
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* age
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* inode number
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* file inode number
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* size
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* file size
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* mtime_ns
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* file mtime_ns
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* chunks hashes
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* file content chunk hashes
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The inode number is stored to make sure we distinguish between
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The inode number is stored to make sure we distinguish between
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different files, as a single path may not be unique across different
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different files, as a single path may not be unique across different
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archives in different setups.
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archives in different setups.
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The file chunk cache is stored as a python associative array storing
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The files cache is stored as a python associative array storing
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python objects, which generate a lot of overhead. This takes around
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python objects, which generates a lot of overhead.
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240 bytes per file without the chunk list, to be compared to at most
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64 bytes of real data (depending on data alignment), and around 80
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bytes per chunk hash (vs 32), with a minimum of ~250 bytes even if
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only one chunk hash.
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Indexes memory usage
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The chunks cache is stored in ``cache/chunks`` and is indexed on the
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--------------------
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``chunk id_hash``. It is used to determine whether we already have a specific
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chunk, to count references to it and also for statistics.
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It contains:
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Here is the estimated memory usage of |project_name| when using those
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* reference count
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indexes.
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* size
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* encrypted/compressed size
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Repository index
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The repository index is stored in ``repo/index.%d`` and is indexed on the
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40 bytes x N ~ 200MB (If a remote repository is
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``chunk id_hash``. It is used to determine a chunk's location in the repository.
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used this will be allocated on the remote side)
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It contains:
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Chunk lookup index
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* segment (that contains the chunk)
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44 bytes x N ~ 220MB
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* offset (where the chunk is located in the segment)
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File chunk cache
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The repository index file is random access.
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probably 80-100 bytes x N ~ 400MB
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Hints are stored in a file (``repo/hints.%d``).
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It contains:
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* version
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* list of segments
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* compact
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hints and index can be recreated if damaged or lost using ``check --repair``.
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The chunks cache and the repository index are stored as hash tables, with
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only one slot per bucket, but that spreads the collisions to the following
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buckets. As a consequence the hash is just a start position for a linear
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search, and if the element is not in the table the index is linearly crossed
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until an empty bucket is found.
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When the hash table is almost full at 90%, its size is doubled. When it's
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almost empty at 25%, its size is halved. So operations on it have a variable
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complexity between constant and linear with low factor, and memory overhead
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varies between 10% and 300%.
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Indexes / Caches memory usage
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-----------------------------
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Here is the estimated memory usage of |project_name|:
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chunk_count ~= total_file_size / 65536
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repo_index_usage = chunk_count * 40
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chunks_cache_usage = chunk_count * 44
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files_cache_usage = total_file_count * 240 + chunk_count * 80
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mem_usage ~= repo_index_usage + chunks_cache_usage + files_cache_usage
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= total_file_count * 240 + total_file_size / 400
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All units are Bytes.
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It is assuming every chunk is referenced exactly once and that typical chunk size is 64kiB.
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If a remote repository is used the repo index will be allocated on the remote side.
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E.g. backing up a total count of 1Mi files with a total size of 1TiB:
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mem_usage = 1 * 2**20 * 240 + 1 * 2**40 / 400 = 2.8GiB
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Note: there is a commandline option to switch off the files cache. You'll save
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some memory, but it will need to read / chunk all the files then.
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In the above we assume 350GB of data that we divide on an average 64KB
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chunk size, so N is around 5.3 million.
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Encryption
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Encryption
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----------
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----------
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AES_ is used with CTR mode of operation (so no need for padding). A 64
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AES_ is used in CTR mode (so no need for padding). A 64bit initialization
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bits initialization vector is used, a `HMAC-SHA256`_ is computed
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vector is used, a `HMAC-SHA256`_ is computed on the encrypted chunk with a
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on the encrypted chunk with a random 64 bits nonce and both are stored
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random 64bit nonce and both are stored in the chunk.
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in the chunk. The header of each chunk is : ``TYPE(1)`` +
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The header of each chunk is : ``TYPE(1)`` + ``HMAC(32)`` + ``NONCE(8)`` + ``CIPHERTEXT``.
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``HMAC(32)`` + ``NONCE(8)`` + ``CIPHERTEXT``. Encryption and HMAC use
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Encryption and HMAC use two different keys.
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two different keys.
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In AES CTR mode you can think of the IV as the start value for the
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In AES CTR mode you can think of the IV as the start value for the counter.
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counter. The counter itself is incremented by one after each 16 byte
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The counter itself is incremented by one after each 16 byte block.
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block. The IV/counter is not required to be random but it must NEVER be
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The IV/counter is not required to be random but it must NEVER be reused.
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reused. So to accomplish this |project_name| initializes the encryption counter
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So to accomplish this |project_name| initializes the encryption counter to be
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to be higher than any previously used counter value before encrypting
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higher than any previously used counter value before encrypting new data.
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new data.
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To reduce payload size only 8 bytes of the 16 bytes nonce is saved in
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To reduce payload size, only 8 bytes of the 16 bytes nonce is saved in the
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the payload, the first 8 bytes are always zeroes. This does not affect
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payload, the first 8 bytes are always zeros. This does not affect security but
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security but limits the maximum repository capacity to only 295
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limits the maximum repository capacity to only 295 exabytes (2**64 * 16 bytes).
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exabytes (2**64 * 16 bytes).
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Encryption keys are either a passphrase, passed through the
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Encryption keys are either derived from a passphrase or kept in a key file.
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``BORG_PASSPHRASE`` environment or prompted on the commandline, or
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The passphrase is passed through the ``BORG_PASSPHRASE`` environment variable
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stored in automatically generated key files.
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or prompted for interactive usage.
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Key files
|
Key files
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---------
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---------
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|
@ -274,22 +311,20 @@ enc_key
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the key used to encrypt data with AES (256 bits)
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the key used to encrypt data with AES (256 bits)
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enc_hmac_key
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enc_hmac_key
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the key used to HMAC the resulting AES-encrypted data (256 bits)
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the key used to HMAC the encrypted data (256 bits)
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id_key
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id_key
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the key used to HMAC the above chunks, the resulting hash is
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the key used to HMAC the plaintext chunk data to compute the chunk's id
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stored out of band (256 bits)
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chunk_seed
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chunk_seed
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the seed for the buzhash chunking table (signed 32 bit integer)
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the seed for the buzhash chunking table (signed 32 bit integer)
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Those fields are processed using msgpack_. The utf-8 encoded phassphrase
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Those fields are processed using msgpack_. The utf-8 encoded passphrase
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is encrypted with PBKDF2_ and SHA256_ using 100000 iterations and a
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is processed with PBKDF2_ (SHA256_, 100000 iterations, random 256 bit salt)
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random 256 bits salt to give us a derived key. The derived key is 256
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to give us a derived key. The derived key is 256 bits long.
|
||||||
bits long. A `HMAC-SHA256`_ checksum of the above fields is generated
|
A `HMAC-SHA256`_ checksum of the above fields is generated with the derived
|
||||||
with the derived key, then the derived key is also used to encrypt the
|
key, then the derived key is also used to encrypt the above pack of fields.
|
||||||
above pack of fields. Then the result is stored in a another msgpack_
|
Then the result is stored in a another msgpack_ formatted as follows:
|
||||||
formatted as follows:
|
|
||||||
|
|
||||||
version
|
version
|
||||||
currently always an integer, 1
|
currently always an integer, 1
|
||||||
|
@ -315,3 +350,9 @@ The resulting msgpack_ is then encoded using base64 and written to the
|
||||||
key file, wrapped using the standard ``textwrap`` module with a header.
|
key file, wrapped using the standard ``textwrap`` module with a header.
|
||||||
The header is a single line with a MAGIC string, a space and a hexadecimal
|
The header is a single line with a MAGIC string, a space and a hexadecimal
|
||||||
representation of the repository id.
|
representation of the repository id.
|
||||||
|
|
||||||
|
|
||||||
|
Compression
|
||||||
|
-----------
|
||||||
|
|
||||||
|
Currently, zlib level 6 is used as compression.
|
||||||
|
|
Loading…
Reference in New Issue