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docs: update the parent pointers documentation to the final version

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Now that we've decided on the ondisk format of parent pointers, update
the documentation to reflect that.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
This commit is contained in:
Darrick J. Wong 2024-04-15 14:55:07 -07:00
parent b0ffe661fa
commit d85fe250f2
1 changed files with 53 additions and 41 deletions
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94
Documentation/filesystems/xfs/xfs-online-fsck-design.rst
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@ -4465,10 +4465,10 @@ reconstruction of filesystem space metadata.
The parent pointer feature, however, makes total directory reconstruction
possible.
XFS parent pointers include the dirent name and location of the entry within
the parent directory.
XFS parent pointers contain the information needed to identify the
corresponding directory entry in the parent directory.
In other words, child files use extended attributes to store pointers to
parents in the form ``(parent_inum, parent_gen, dirent_pos) → (dirent_name)``.
parents in the form ``(dirent_name) → (parent_inum, parent_gen)``.
The directory checking process can be strengthened to ensure that the target of
each dirent also contains a parent pointer pointing back to the dirent.
Likewise, each parent pointer can be checked by ensuring that the target of
@ -4476,8 +4476,6 @@ each parent pointer is a directory and that it contains a dirent matching
the parent pointer.
Both online and offline repair can use this strategy.
**Note**: The ondisk format of parent pointers is not yet finalized.
+--------------------------------------------------------------------------+
| **Historical Sidebar**: |
+--------------------------------------------------------------------------+
@ -4519,8 +4517,58 @@ Both online and offline repair can use this strategy.
| Chandan increased the maximum extent counts of both data and attribute |
| forks, thereby ensuring that the extended attribute structure can grow |
| to handle the maximum hardlink count of any file. |
| |
| For this second effort, the ondisk parent pointer format as originally |
| proposed was ``(parent_inum, parent_gen, dirent_pos) → (dirent_name)``. |
| The format was changed during development to eliminate the requirement |
| of repair tools needing to to ensure that the ``dirent_pos`` field |
| always matched when reconstructing a directory. |
| |
| There were a few other ways to have solved that problem: |
| |
| 1. The field could be designated advisory, since the other three values |
| are sufficient to find the entry in the parent. |
| However, this makes indexed key lookup impossible while repairs are |
| ongoing. |
| |
| 2. We could allow creating directory entries at specified offsets, which |
| solves the referential integrity problem but runs the risk that |
| dirent creation will fail due to conflicts with the free space in the |
| directory. |
| |
| These conflicts could be resolved by appending the directory entry |
| and amending the xattr code to support updating an xattr key and |
| reindexing the dabtree, though this would have to be performed with |
| the parent directory still locked. |
| |
| 3. Same as above, but remove the old parent pointer entry and add a new |
| one atomically. |
| |
| 4. Change the ondisk xattr format to |
| ``(parent_inum, name) → (parent_gen)``, which would provide the attr |
| name uniqueness that we require, without forcing repair code to |
| update the dirent position. |
| Unfortunately, this requires changes to the xattr code to support |
| attr names as long as 263 bytes. |
| |
| 5. Change the ondisk xattr format to ``(parent_inum, hash(name)) → |
| (name, parent_gen)``. |
| If the hash is sufficiently resistant to collisions (e.g. sha256) |
| then this should provide the attr name uniqueness that we require. |
| Names shorter than 247 bytes could be stored directly. |
| |
| 6. Change the ondisk xattr format to ``(dirent_name) → (parent_ino, |
| parent_gen)``. This format doesn't require any of the complicated |
| nested name hashing of the previous suggestions. However, it was |
| discovered that multiple hardlinks to the same inode with the same |
| filename caused performance problems with hashed xattr lookups, so |
| the parent inumber is now xor'd into the hash index. |
| |
| In the end, it was decided that solution #6 was the most compact and the |
| most performant. A new hash function was designed for parent pointers. |
+--------------------------------------------------------------------------+
Case Study: Repairing Directories with Parent Pointers
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@ -4569,42 +4617,6 @@ The proposed patchset is the
<https://git.kernel.org/pub/scm/linux/kernel/git/djwong/xfs-linux.git/log/?h=pptrs-online-dir-repair>`_
series.
**Unresolved Question**: How will repair ensure that the ``dirent_pos`` fields
match in the reconstructed directory?
*Answer*: There are a few ways to solve this problem:
1. The field could be designated advisory, since the other three values are
sufficient to find the entry in the parent.
However, this makes indexed key lookup impossible while repairs are ongoing.
2. We could allow creating directory entries at specified offsets, which solves
the referential integrity problem but runs the risk that dirent creation
will fail due to conflicts with the free space in the directory.
These conflicts could be resolved by appending the directory entry and
amending the xattr code to support updating an xattr key and reindexing the
dabtree, though this would have to be performed with the parent directory
still locked.
3. Same as above, but remove the old parent pointer entry and add a new one
atomically.
4. Change the ondisk xattr format to ``(parent_inum, name) → (parent_gen)``,
which would provide the attr name uniqueness that we require, without
forcing repair code to update the dirent position.
Unfortunately, this requires changes to the xattr code to support attr
names as long as 263 bytes.
5. Change the ondisk xattr format to ``(parent_inum, hash(name)) →
(name, parent_gen)``.
If the hash is sufficiently resistant to collisions (e.g. sha256) then
this should provide the attr name uniqueness that we require.
Names shorter than 247 bytes could be stored directly.
Discussion is ongoing under the `parent pointers patch deluge
<https://www.spinics.net/lists/linux-xfs/msg69397.html>`_.
Case Study: Repairing Parent Pointers
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^