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- .. 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:
- ============= ================ =============== ===============================
- Offset Type Name Description
- ============= ================ =============== ===============================
- 0x0 Array of Orphan inode Each __le32 entry is either
- __le32 entries entries 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.
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