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Update documentation
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Alexander Neumann
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doc/Design.md
200
doc/Design.md
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@ -48,8 +48,6 @@ The basic layout of a sample restic repository is shown below:
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├── keys
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│ └── b02de829beeb3c01a63e6b25cbd421a98fef144f03b9a02e46eff9e2ca3f0bd7
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├── locks
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├── maps
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│ └── 3c0721e5c3f5d2d78a12664b568a1bc992d17b993d41079599f8437ed66192fe
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├── snapshots
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│ └── 22a5af1bdc6e616f8a29579458c49627e01b32210d09adb288d1ecda7c5711ec
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├── tmp
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@ -60,8 +58,11 @@ The basic layout of a sample restic repository is shown below:
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│ │ └── 32ea976bc30771cebad8285cd99120ac8786f9ffd42141d452458089985043a5
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│ ├── 95
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│ │ └── 95f75feb05a7cc73e328b2efa668b1ea68f65fece55a93bc65aff6cd0bcfeefc
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│ └── e0
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│ └── e01150928f7ad24befd6ec15b087de1b9e0f92edabd8e5cabb3317f8b20ad044
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│ ├── b8
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│ │ └── b8138ab08a4722596ac89c917827358da4672eac68e3c03a8115b88dbf4bfb59
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│ ├── e0
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│ │ └── e01150928f7ad24befd6ec15b087de1b9e0f92edabd8e5cabb3317f8b20ad044
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│ [...]
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└── version
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A repository can be initialized with the `restic init` command, e.g.:
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@ -124,8 +125,13 @@ pretty-print the contents of a snapshot file:
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Enter Password for Repository:
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{
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"time": "2015-01-02T18:10:50.895208559+01:00",
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"tree": "2da81727b6585232894cfbb8f8bdab8d1eccd3d8f7c92bc934d62e62e618ffdf",
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"map": "3c0721e5c3f5d2d78a12664b568a1bc992d17b993d41079599f8437ed66192fe",
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"tree": "",
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"tree": {
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"id": "2da81727b6585232894cfbb8f8bdab8d1eccd3d8f7c92bc934d62e62e618ffdf",
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"size": 282,
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"sid": "b8138ab08a4722596ac89c917827358da4672eac68e3c03a8115b88dbf4bfb59",
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"ssize": 330
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},
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"dir": "/tmp/testdata",
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"hostname": "kasimir",
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"username": "fd0",
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@ -134,122 +140,118 @@ pretty-print the contents of a snapshot file:
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}
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Here it can be seen that this snapshot represents the contents of the directory
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`/tmp/testdata`.
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The two most important fields are `map` and `tree`.
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Maps
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----
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`/tmp/testdata`. The most important field is `tree`.
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All content within a restic repository is referenced according to its SHA-256
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hash. Before saving, each file is split into variable sized chunks of data. The
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SHA-256 hashes of all chunks are saved in an ordered list which then represents
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the content of the file. In order to relate these plain text hashes to the
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actual encrypted storage hashes (which vary due to random IVs), each snapshot
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references a map.
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the content of the file.
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A map is an encrypted and compressed JSON document which contains a large list
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of plain text hashes and associated storage hashes. This list is sorted by the
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plain text hash in order to speed up lookups.
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Maps are referenced by their storage ID, which is the SHA-256 hash of the
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encrypted file stored in the `maps` directory.
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The command `restic cat map` can be used to inspect the content of a map:
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$ restic -r /tmp/restic-repo cat map 3c0721e5c3f5d2d78a12664b568a1bc992d17b993d41079599f8437ed66192fe
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Enter Password for Repository:
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[
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{
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"id": "1424916fc7279d58e3b2d8b533f481981ea5cb0f21a43932f26475e308e9b599",
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"size": 287,
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"sid": "32ea976bc30771cebad8285cd99120ac8786f9ffd42141d452458089985043a5",
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"ssize": 335
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},
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{
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"id": "160916dec2e9f4597a2cc3f0787ff6b3726c21e056177292eb85281c9c2afaa0",
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"size": 812,
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"sid": "73d04e6125cf3c28a299cc2f3cca3b78ceac396e4fcf9575e34536b26782413c",
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"ssize": 860
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},
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[...]
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]
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In order to relate these plain text hashes to the actual encrypted storage
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hashes (which vary due to random IVs), each object contains a list that maps
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all referenced plaintext hashes to storage hashes. In the case of the snapshot
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data structure listed above, the list only consists of one entry for the
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referenced tree, so the field `tree` consists of such a mapping.
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Trees and Data
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--------------
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The second thing a snapshot references is a tree. Trees are referenced by the
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SHA-256 hash of the JSON string representation of its contents and are saved in
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a subdirectory of the directory `trees`. The sub directory's name is the first
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two characters of the filename the tree object is stored in.
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A snapshot references a tree by the SHA-256 hash of the JSON string
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representation of its contents. Trees are saved in a subdirectory of the
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directory `trees`. The sub directory's name is the first two characters of the
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filename the tree object is stored in.
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The command `restic cat tree` can be used to inspect the tree referenced above:
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$ restic -r /tmp/restic-repo cat tree 2da81727b6585232894cfbb8f8bdab8d1eccd3d8f7c92bc934d62e62e618ffdf
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$ restic -r /tmp/restic-repo cat tree b8138ab08a4722596ac89c917827358da4672eac68e3c03a8115b88dbf4bfb59
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Enter Password for Repository:
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[
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{
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"name": "testdata",
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"type": "dir",
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"mode": 493,
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"mtime": "2014-12-22T14:47:59.912418701+01:00",
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"atime": "2014-12-06T17:49:21.748468803+01:00",
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"ctime": "2014-12-22T14:47:59.912418701+01:00",
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"uid": 1000,
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"gid": 100,
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"user": "fd0",
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"inode": 409704562,
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"content": null,
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"subtree": "a8838fdbf2902095fb1b9de8b0e30d2e4e2a91bbc82fb15f98f6f1535b9ccbe6"
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}
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]
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{
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"nodes": [
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{
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"name": "testdata",
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"type": "dir",
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"mode": 493,
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"mtime": "2014-12-22T14:47:59.912418701+01:00",
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"atime": "2014-12-06T17:49:21.748468803+01:00",
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"ctime": "2014-12-22T14:47:59.912418701+01:00",
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"uid": 1000,
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"gid": 100,
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"user": "fd0",
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"inode": 409704562,
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"content": null,
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"subtree": "b26e315b0988ddcd1cee64c351d13a100fedbc9fdbb144a67d1b765ab280b4dc"
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}
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],
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"map": [
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{
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"id": "b26e315b0988ddcd1cee64c351d13a100fedbc9fdbb144a67d1b765ab280b4dc",
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"size": 910,
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"sid": "8b238c8811cc362693e91a857460c78d3acf7d9edb2f111048691976803cf16e",
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"ssize": 958
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}
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]
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}
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A tree is a list of entries which contain meta data like a name and timestamps.
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When the entry references a directory, the field `subtree` contains the plain
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text ID of another tree object. The associated storage ID can be found in the
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map object.
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A tree contains a list of entries (in the field `nodes`) which contain meta
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data like a name and timestamps. When the entry references a directory, the
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field `subtree` contains the plain text ID of another tree object. The
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associated storage ID can be found in the map object. All referenced plaintext
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hashes are mapped to their corresponding storage hashes in the list containid
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in the field `map`.
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This can also be inspected by using `restic cat tree`, which automatically
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searches all available maps for the storage ID:
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When the command `restic cat tree` is used, the storage hash is needed to print
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a tree. The tree referenced above can be dumped as follows:
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$ restic -r /tmp/restic-repo cat tree a8838fdbf2902095fb1b9de8b0e30d2e4e2a91bbc82fb15f98f6f1535b9ccbe6
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$ restic -r /tmp/restic-repo cat tree 8b238c8811cc362693e91a857460c78d3acf7d9edb2f111048691976803cf16e
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Enter Password for Repository:
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[
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{
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"name": "testfile",
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"type": "file",
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"mode": 420,
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"mtime": "2014-12-06T17:50:23.34513538+01:00",
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"atime": "2014-12-06T17:50:23.338468713+01:00",
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"ctime": "2014-12-06T17:50:23.34513538+01:00",
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"uid": 1000,
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"gid": 100,
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"user": "fd0",
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"inode": 416863351,
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"size": 1234,
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"links": 1,
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"content": [
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"50f77b3b4291e8411a027b9f9b9e64658181cc676ce6ba9958b95f268cb1109d"
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]
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},
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[...]
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]
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{
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"nodes": [
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{
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"name": "testfile",
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"type": "file",
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"mode": 420,
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"mtime": "2014-12-06T17:50:23.34513538+01:00",
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"atime": "2014-12-06T17:50:23.338468713+01:00",
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"ctime": "2014-12-06T17:50:23.34513538+01:00",
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"uid": 1000,
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"gid": 100,
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"user": "fd0",
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"inode": 416863351,
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"size": 1234,
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"links": 1,
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"content": [
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"50f77b3b4291e8411a027b9f9b9e64658181cc676ce6ba9958b95f268cb1109d"
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]
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},
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[...]
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],
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"map": [
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{
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"id": "50f77b3b4291e8411a027b9f9b9e64658181cc676ce6ba9958b95f268cb1109d",
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"size": 1234,
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"sid": "00634c46e5f7c055c341acd1201cf8289cabe769f991d6e350f8cd8ce2a52ac3",
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"ssize": 1282
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},
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[...]
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]
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}
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This tree contains a file entry. In contrast to the entry above, the `subtree`
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field is not present and the `content` field contains a list with one plain
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text SHA-256 hash. The storage ID for this ID can in turn be looked up in the
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map. Data chunks stored as encrypted files in a sub directory of the directory
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`data`, similar to tree objects.
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This tree contains a file entry. This time, the `subtree` field is not present
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and the `content` field contains a list with one plain text SHA-256 hash. The
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storage ID for this ID can in turn be looked up in the map. Data chunks stored
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as encrypted files in a sub directory of the directory `data`, similar to tree
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objects.
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The command `restic cat data` can be used to lookup, extract and decrypt data,
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e.g. for the data mentioned above:
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The command `restic cat data` can be used to extract and decrypt data given a
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storage hash, e.g. for the data mentioned above:
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$ restic -r /tmp/restic-repo cat blob 50f77b3b4291e8411a027b9f9b9e64658181cc676ce6ba9958b95f268cb1109d | sha256sum
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$ restic -r /tmp/restic-repo cat blob 00634c46e5f7c055c341acd1201cf8289cabe769f991d6e350f8cd8ce2a52ac3 | sha256sum
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Enter Password for Repository:
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50f77b3b4291e8411a027b9f9b9e64658181cc676ce6ba9958b95f268cb1109d -
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As can be seen from the output of the program `sha256sum`, the hash is the
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same, so the correct data has been returned.
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As can be seen from the output of the program `sha256sum`, the hash matches the
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plaintext hash from the map included in the tree above, so the correct data has
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been returned.
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Backups and Deduplication
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=========================
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