diff options
Diffstat (limited to 'Documentation/filesystems/ext4/orphan.rst')
| -rw-r--r-- | Documentation/filesystems/ext4/orphan.rst | 52 |
1 files changed, 52 insertions, 0 deletions
diff --git a/Documentation/filesystems/ext4/orphan.rst b/Documentation/filesystems/ext4/orphan.rst new file mode 100644 index 000000000000..bb19ecd1b626 --- /dev/null +++ b/Documentation/filesystems/ext4/orphan.rst @@ -0,0 +1,52 @@ +.. SPDX-License-Identifier: GPL-2.0 + +Orphan file +----------- + +In unix there can inodes that are unlinked from directory hierarchy but that +are still alive because they are open. In case of crash the filesystem has to +clean up these inodes as otherwise they (and the blocks referenced from them) +would leak. Similarly if we truncate or extend the file, we need not be able +to perform the operation in a single journalling transaction. In such case we +track the inode as orphan so that in case of crash extra blocks allocated to +the file get truncated. + +Traditionally ext4 tracks orphan inodes in a form of single linked list where +superblock contains the inode number of the last orphan inode (s\_last\_orphan +field) and then each inode contains inode number of the previously orphaned +inode (we overload i\_dtime inode field for this). However this filesystem +global single linked list is a scalability bottleneck for workloads that result +in heavy creation of orphan inodes. When orphan file feature +(COMPAT\_ORPHAN\_FILE) is enabled, the filesystem has a special inode +(referenced from the superblock through s\_orphan_file_inum) with several +blocks. Each of these blocks has a structure: + +.. list-table:: + :widths: 8 8 24 40 + :header-rows: 1 + + * - Offset + - Type + - Name + - Description + * - 0x0 + - Array of \_\_le32 entries + - Orphan inode entries + - Each \_\_le32 entry is either empty (0) or it contains inode number of + an orphan inode. + * - blocksize - 8 + - \_\_le32 + - ob\_magic + - Magic value stored in orphan block tail (0x0b10ca04) + * - blocksize - 4 + - \_\_le32 + - ob\_checksum + - Checksum of the orphan block. + +When a filesystem with orphan file feature is writeably mounted, we set +RO\_COMPAT\_ORPHAN\_PRESENT feature in the superblock to indicate there may +be valid orphan entries. In case we see this feature when mounting the +filesystem, we read the whole orphan file and process all orphan inodes found +there as usual. When cleanly unmounting the filesystem we remove the +RO\_COMPAT\_ORPHAN\_PRESENT feature to avoid unnecessary scanning of the orphan +file and also make the filesystem fully compatible with older kernels. |