1.. SPDX-License-Identifier: GPL-2.0
 2
 3Orphan file
 4-----------
 5
 6In unix there can inodes that are unlinked from directory hierarchy but that
 7are still alive because they are open. In case of crash the filesystem has to
 8clean up these inodes as otherwise they (and the blocks referenced from them)
 9would leak. Similarly if we truncate or extend the file, we need not be able
10to perform the operation in a single journalling transaction. In such case we
11track the inode as orphan so that in case of crash extra blocks allocated to
12the file get truncated.
13
14Traditionally ext4 tracks orphan inodes in a form of single linked list where
15superblock contains the inode number of the last orphan inode (s_last_orphan
16field) and then each inode contains inode number of the previously orphaned
17inode (we overload i_dtime inode field for this). However this filesystem
18global single linked list is a scalability bottleneck for workloads that result
19in heavy creation of orphan inodes. When orphan file feature
20(COMPAT_ORPHAN_FILE) is enabled, the filesystem has a special inode
21(referenced from the superblock through s_orphan_file_inum) with several
22blocks. Each of these blocks has a structure:
23
24============= ================ =============== ===============================
25Offset        Type             Name            Description
26============= ================ =============== ===============================
270x0           Array of         Orphan inode    Each __le32 entry is either
28              __le32 entries   entries         empty (0) or it contains
29	                                       inode number of an orphan
30					       inode.
31blocksize-8   __le32           ob_magic        Magic value stored in orphan
32                                               block tail (0x0b10ca04)
33blocksize-4   __le32           ob_checksum     Checksum of the orphan block.
34============= ================ =============== ===============================
35
36When a filesystem with orphan file feature is writeably mounted, we set
37RO_COMPAT_ORPHAN_PRESENT feature in the superblock to indicate there may
38be valid orphan entries. In case we see this feature when mounting the
39filesystem, we read the whole orphan file and process all orphan inodes found
40there as usual. When cleanly unmounting the filesystem we remove the
41RO_COMPAT_ORPHAN_PRESENT feature to avoid unnecessary scanning of the orphan
42file and also make the filesystem fully compatible with older kernels.

 1.. SPDX-License-Identifier: GPL-2.0
 2
 3Orphan file
 4-----------
 5
 6In unix there can inodes that are unlinked from directory hierarchy but that
 7are still alive because they are open. In case of crash the filesystem has to
 8clean up these inodes as otherwise they (and the blocks referenced from them)
 9would leak. Similarly if we truncate or extend the file, we need not be able
10to perform the operation in a single journalling transaction. In such case we
11track the inode as orphan so that in case of crash extra blocks allocated to
12the file get truncated.
13
14Traditionally ext4 tracks orphan inodes in a form of single linked list where
15superblock contains the inode number of the last orphan inode (s_last_orphan
16field) and then each inode contains inode number of the previously orphaned
17inode (we overload i_dtime inode field for this). However this filesystem
18global single linked list is a scalability bottleneck for workloads that result
19in heavy creation of orphan inodes. When orphan file feature
20(COMPAT_ORPHAN_FILE) is enabled, the filesystem has a special inode
21(referenced from the superblock through s_orphan_file_inum) with several
22blocks. Each of these blocks has a structure:
23
24============= ================ =============== ===============================
25Offset        Type             Name            Description
26============= ================ =============== ===============================
270x0           Array of         Orphan inode    Each __le32 entry is either
28              __le32 entries   entries         empty (0) or it contains
29	                                       inode number of an orphan
30					       inode.
31blocksize-8   __le32           ob_magic        Magic value stored in orphan
32                                               block tail (0x0b10ca04)
33blocksize-4   __le32           ob_checksum     Checksum of the orphan block.
34============= ================ =============== ===============================
35
36When a filesystem with orphan file feature is writeably mounted, we set
37RO_COMPAT_ORPHAN_PRESENT feature in the superblock to indicate there may
38be valid orphan entries. In case we see this feature when mounting the
39filesystem, we read the whole orphan file and process all orphan inodes found
40there as usual. When cleanly unmounting the filesystem we remove the
41RO_COMPAT_ORPHAN_PRESENT feature to avoid unnecessary scanning of the orphan
42file and also make the filesystem fully compatible with older kernels.