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