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- .. SPDX-License-Identifier: GPL-2.0
- Checksums
- ---------
- Starting in early 2012, metadata checksums were added to all major ext4
- and jbd2 data structures. The associated feature flag is metadata\_csum.
- The desired checksum algorithm is indicated in the superblock, though as
- of October 2012 the only supported algorithm is crc32c. Some data
- structures did not have space to fit a full 32-bit checksum, so only the
- lower 16 bits are stored. Enabling the 64bit feature increases the data
- structure size so that full 32-bit checksums can be stored for many data
- structures. However, existing 32-bit filesystems cannot be extended to
- enable 64bit mode, at least not without the experimental resize2fs
- patches to do so.
- Existing filesystems can have checksumming added by running
- ``tune2fs -O metadata_csum`` against the underlying device. If tune2fs
- encounters directory blocks that lack sufficient empty space to add a
- checksum, it will request that you run ``e2fsck -D`` to have the
- directories rebuilt with checksums. This has the added benefit of
- removing slack space from the directory files and rebalancing the htree
- indexes. If you \_ignore\_ this step, your directories will not be
- protected by a checksum!
- The following table describes the data elements that go into each type
- of checksum. The checksum function is whatever the superblock describes
- (crc32c as of October 2013) unless noted otherwise.
- .. list-table::
- :widths: 20 8 50
- :header-rows: 1
- * - Metadata
- - Length
- - Ingredients
- * - Superblock
- - \_\_le32
- - The entire superblock up to the checksum field. The UUID lives inside
- the superblock.
- * - MMP
- - \_\_le32
- - UUID + the entire MMP block up to the checksum field.
- * - Extended Attributes
- - \_\_le32
- - UUID + the entire extended attribute block. The checksum field is set to
- zero.
- * - Directory Entries
- - \_\_le32
- - UUID + inode number + inode generation + the directory block up to the
- fake entry enclosing the checksum field.
- * - HTREE Nodes
- - \_\_le32
- - UUID + inode number + inode generation + all valid extents + HTREE tail.
- The checksum field is set to zero.
- * - Extents
- - \_\_le32
- - UUID + inode number + inode generation + the entire extent block up to
- the checksum field.
- * - Bitmaps
- - \_\_le32 or \_\_le16
- - UUID + the entire bitmap. Checksums are stored in the group descriptor,
- and truncated if the group descriptor size is 32 bytes (i.e. ^64bit)
- * - Inodes
- - \_\_le32
- - UUID + inode number + inode generation + the entire inode. The checksum
- field is set to zero. Each inode has its own checksum.
- * - Group Descriptors
- - \_\_le16
- - If metadata\_csum, then UUID + group number + the entire descriptor;
- else if gdt\_csum, then crc16(UUID + group number + the entire
- descriptor). In all cases, only the lower 16 bits are stored.
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