.. include:: ../global.rst.inc .. highlight:: none .. _packs: Pack files ========== Without pack files, each repository chunk is stored as a separate borgstore object. For large repositories this means millions of individual objects, each requiring its own I/O round trip to read or write. On high-latency backends (SFTP, cloud object storage) this overhead dominates backup and restore times. Pack files address this by grouping multiple chunks into a single store object. A reader that needs one chunk does a partial read (range request) at a known offset instead of fetching a separate file. Store object count drops from one-per-chunk to one-per-pack. .. _pack-format: Pack File Format ---------------- There is no separate file header. Each blob starts with the 8-byte ``OBJ_MAGIC`` (``BORG_OBJ``), so a forward scanner can locate blob boundaries and identify each chunk using only the pack file bytes with no external index. Per-blob layout ~~~~~~~~~~~~~~~ Each blob is a self-contained unit:: Offset (relative to blob start) Size Type Field -------------------------------- ---------------- ------- ----- 0 len(OBJ_MAGIC) bytes OBJ_MAGIC = ASCII b"BORG_OBJ" 8 1 uint8 Format version: 0x01 9 32 bytes chunk_id 41 4 uint32le meta_size 45 4 uint32le data_size 49 meta_size bytes encrypted_meta 49 + meta_size data_size bytes encrypted_data ``chunk_id`` is the ID hash of the plaintext data (``id_hash(plaintext_data)``). Storing it in the unencrypted header lets a scanner rebuild the ``chunk_id → location`` index without decrypting any blob. ``chunk_id`` is also written into ``encrypted_meta`` (the meta dict). The header copy enables key-free scanning and recovery; the meta copy lets future code read ``chunk_id`` through the normal meta dict API without parsing the raw header layout. The fixed part of each blob header is 49 bytes (``REPOOBJ_HEADER_SIZE``): ``len(OBJ_MAGIC)`` + 1 version + 32 chunk_id + 4 meta_size + 4 data_size. ``REPOOBJ_HEADER_SIZE = len(OBJ_MAGIC) + 1 + 32 + 4 + 4 = 49`` A reader locates the next blob by advancing:: next_blob_offset = current_blob_offset + REPOOBJ_HEADER_SIZE + meta_size + data_size The per-blob magic limits the blast radius of corrupted length fields: if ``meta_size`` or ``data_size`` is damaged, the scanner loses at most one blob. Once it finds the next ``OBJ_MAGIC`` sequence it resumes. Other corruption (payload bit flips) is caught by AEAD on that blob without losing position. Blobs follow one another contiguously with no padding:: OBJ_MAGIC | version=0x01 | chunk_id_0 | meta_size_0 | data_size_0 | encrypted_meta_0 | encrypted_data_0 OBJ_MAGIC | version=0x01 | chunk_id_1 | meta_size_1 | data_size_1 | encrypted_meta_1 | encrypted_data_1 ... Pack ID ~~~~~~~ The pack ID is the SHA-256 of the pack file's bytes:: pack_id = sha256(pack_bytes) Content-addressing the file by its own bytes makes the name commit to the content, so borgstore can verify and cache it and ``borg check`` can detect silent corruption of the stored file. Namespace ~~~~~~~~~ Pack files are stored under the ``packs/`` namespace in borgstore, using a single directory level keyed on the first byte of the pack ID (hex-encoded):: packs/ 00/ .. ff/ .. _pack-index-entry: Pack Index Entry ---------------- Each pack contains one blob. The pack for a given chunk is always at:: packs/ A ChunkIndex entry maps a chunk to its pack:: chunk_id → pack_id Since each pack holds exactly one blob, the blob is always at offset 0 and its length is the full file size. No offset or length field is stored in the index for this phase. .. _pack-write-order: Write Order and Crash Safety ----------------------------- Pack data must be stored before any archive pointer references it. The required write order is: 1. Store the pack file to ``packs/`` via borgstore. 2. Store the partial index file to ``index/`` (see :ref:`pack-index-namespace`). 3. Write the archive and archive pointer. This is the sole commit point. A crash between steps 1 and 2 leaves orphan pack files in ``packs/``. No archive references these chunks; ``borg compact`` removes them on the next run. A crash between steps 2 and 3 leaves a partial index file covering packs not yet committed to any archive. The extra index entries point to valid, fully-written pack data; they are harmless and will be cleaned up by the next ``borg compact``. A crash after step 3 cannot leave the repository in an inconsistent state. The archive pointer write is the commit point: data not referenced by any archive pointer is unreachable and treated as garbage by ``borg compact``. Only ``borg compact`` and ``borg check --repair`` delete pack files. When compact determines via mark-and-sweep that none of a pack's blobs are referenced by any archive, it removes the whole file. Individual blobs cannot be removed without rewriting the entire pack, so deletion always operates at pack granularity. .. _pack-index-namespace: Index Namespace --------------- Chunk-to-location mappings are stored as a separate set of encrypted partial index files under the ``index/`` namespace. Each partial index file covers the packs written in one backup session. Its name is the SHA-256 digest of its own content. A first backup of a large dataset may produce a large partial index file; using the same medium-sized file writer as compact for ``borg create`` would bound that. That is the intended direction. :: index/ Content-addressed naming makes each partial index file self-verifying and idempotent: writing the same index data twice produces the same filename, so a repeated write is a no-op. Partial index files are write-once. A session stores new partial index files via borgstore; existing files are never modified. On repository open all files under ``index/`` are loaded via borgstore, decrypted, and merged into the in-memory ChunkIndex (a ``borghash`` ``HashTableNT`` keyed on ``chunk_id``). The merge is commutative and idempotent; order does not matter. ``borg compact`` rewrites the ``index/`` namespace: it identifies live chunks via mark-and-sweep, consolidates the surviving mappings into medium-sized replacement files (targeting roughly 10–100 packs per file), and removes the files it supersedes. Medium-sized files keep the open-time merge cost bounded while avoiding the cache-invalidation traffic on other clients that a single all-in-one index would cause. If the entire ``index/`` namespace is lost or corrupt, the ChunkIndex can be rebuilt by scanning pack files directly; see :ref:`pack-recovery`. .. _pack-recovery: Recovery Path ------------- When ``borg check --repair`` detects a missing or incomplete ChunkIndex it rebuilds it by forward-scanning all pack files in ``packs/``. Each blob's unencrypted header supplies the ``OBJ_MAGIC`` (for re-sync after corruption), the ``chunk_id``, and the size fields needed to locate the next blob. The scan produces a complete ``chunk_id → (pack_id, offset, length)`` mapping without decrypting any blob and without the repository key. .. _pack-repo-version: Repository Version and Feature Flags -------------------------------------- Repositories using pack files require repository version **4**. Clients that only accept version 3 refuse to open a version 4 repository with an unsupported-version error before any data is read. In addition, the repository ``config.feature_flags`` must include ``pack_files`` in the mandatory set for all access modes: .. code-block:: python config = { "feature_flags": { "read": {"mandatory": ["pack_files"]}, "write": {"mandatory": ["pack_files"]}, "check": {"mandatory": ["pack_files"]}, } } A client that does not recognise the ``pack_files`` feature flag will refuse to open the repository with a ``MandatoryFeatureUnsupported`` error regardless of the version number. The two guards cover different failure modes: the version bump stops clients that predate feature-flag support entirely; the feature flag gives a clearer error message to clients that understand feature flags but don't know about packs yet. There is no migration path from version 3 repositories to version 4. Users of the version 3 beta format must create a new repository with ``borg repo-create``.