43 lines
2.4 KiB
ReStructuredText
43 lines
2.4 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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============= ================ =============== ===============================
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Offset Type Name Description
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============= ================ =============== ===============================
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0x0 Array of Orphan inode Each __le32 entry is either
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__le32 entries entries empty (0) or it contains
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inode number of an orphan
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inode.
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blocksize-8 __le32 ob_magic Magic value stored in orphan
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block tail (0x0b10ca04)
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blocksize-4 __le32 ob_checksum Checksum of the orphan block.
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============= ================ =============== ===============================
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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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