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document hlid, fixes #2388

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Thomas Waldmann 2022-07-05 22:27:08 +02:00
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docs/internals/data-structures.rst
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@ -568,6 +568,7 @@ dictionary created by the ``Item`` class that contains:
* gid
* mode (item type + permissions)
* source (for symlinks)
* hlid (for hardlinks)
* rdev (for device files)
* mtime, atime, ctime in nanoseconds
* xattrs
@ -1246,3 +1247,28 @@ For example, using 1.1 on a repository, noticing corruption or similar issues an
``borg-1.0 check --repair``, which rewrites the index and hints, results in this situation.
Borg 1.1 would erroneously report checksum errors in the hints and/or index files and trigger
an automatic rebuild of these files.
HardLinkManager and the hlid concept
------------------------------------
Dealing with hard links needs some extra care, implemented in borg within the HardLinkManager
class:
- At archive creation time, fs items with st_nlink > 1 indicate that they are a member of
a group of hardlinks all pointing to the same inode. For such fs items, the archived item
includes a hlid attribute (hardlink id), which is computed like H(st_dev, st_ino). Thus,
if archived items have the same hlid value, they pointed to the same inode and form a
group of hardlinks. Besides that, nothing special is done for any member of the group
of hardlinks, meaning that e.g. for regular files, each archived item will have a
chunks list.
- At extraction time, the presence of a hlid attribute indicates that there might be more
hardlinks coming, pointing to the same content (inode), thus borg will remember the "hlid
to extracted path" mapping, so it will know the correct path for extracting (hardlinking)
the next hardlink of that group / with the same hlid.
- This symmetric approach (each item has all the information, e.g. the chunks list)
simplifies dealing with such items a lot, especially for partial extraction, for the
FUSE filesystem, etc.
- This is different from the asymmetric approach of old borg versions (< 2.0) and also from
tar which have the concept of a main item (first hardlink, has the content) and content-less
secondary items with by-name back references for each subsequent hardlink, causing lots
of complications when dealing with them.