1 .. SPDX-License-Identifier: GPL-2.0
2
3 Checksums
4 ---------
5
6 Starting in early 2012, metadata checksums wer
7 and jbd2 data structures. The associated featu
8 The desired checksum algorithm is indicated in
9 of October 2012 the only supported algorithm i
10 structures did not have space to fit a full 32
11 lower 16 bits are stored. Enabling the 64bit f
12 structure size so that full 32-bit checksums c
13 structures. However, existing 32-bit filesyste
14 enable 64bit mode, at least not without the ex
15 patches to do so.
16
17 Existing filesystems can have checksumming add
18 ``tune2fs -O metadata_csum`` against the under
19 encounters directory blocks that lack sufficie
20 checksum, it will request that you run ``e2fsc
21 directories rebuilt with checksums. This has t
22 removing slack space from the directory files
23 indexes. If you _ignore_ this step, your direc
24 protected by a checksum!
25
26 The following table describes the data element
27 of checksum. The checksum function is whatever
28 (crc32c as of October 2013) unless noted other
29
30 .. list-table::
31 :widths: 20 8 50
32 :header-rows: 1
33
34 * - Metadata
35 - Length
36 - Ingredients
37 * - Superblock
38 - __le32
39 - The entire superblock up to the checksu
40 the superblock.
41 * - MMP
42 - __le32
43 - UUID + the entire MMP block up to the c
44 * - Extended Attributes
45 - __le32
46 - UUID + the entire extended attribute bl
47 zero.
48 * - Directory Entries
49 - __le32
50 - UUID + inode number + inode generation
51 fake entry enclosing the checksum field
52 * - HTREE Nodes
53 - __le32
54 - UUID + inode number + inode generation
55 The checksum field is set to zero.
56 * - Extents
57 - __le32
58 - UUID + inode number + inode generation
59 the checksum field.
60 * - Bitmaps
61 - __le32 or __le16
62 - UUID + the entire bitmap. Checksums are
63 and truncated if the group descriptor s
64 * - Inodes
65 - __le32
66 - UUID + inode number + inode generation
67 field is set to zero. Each inode has it
68 * - Group Descriptors
69 - __le16
70 - If metadata_csum, then UUID + group num
71 else if gdt_csum, then crc16(UUID + gro
72 descriptor). In all cases, only the low
73
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