linux/Documentation/filesystems/ext4/orphan.rst
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   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.
  43