1 .. SPDX-License-Identifier: GPL-2.0
2
3 Multiple Mount Protection
4 -------------------------
5
6 Multiple mount protection (MMP) is a feature t
7 filesystem against multiple hosts trying to us
8 simultaneously. When a filesystem is opened (f
9 etc.), the MMP code running on the node (call
10 sequence number. If the sequence number is EXT
11 open continues. If the sequence number is EXT4
12 fsck is (hopefully) running, and open fails im
13 open code will wait for twice the specified MM
14 the sequence number again. If the sequence num
15 filesystem is active on another machine and th
16 code passes all of those checks, a new MMP seq
17 and written to the MMP block, and the mount pr
18
19 While the filesystem is live, the kernel sets
20 MMP block at the specified MMP check interval.
21 the MMP sequence number is re-read; if it does
22 MMP sequence number, then another node (node B
23 filesystem, and node A remounts the filesystem
24 sequence numbers match, the sequence number is
25 memory and on disk, and the re-check is comple
26
27 The hostname and device filename are written i
28 an open operation succeeds. The MMP code does
29 are provided purely for informational purposes
30
31 The checksum is calculated against the FS UUID
32 The MMP structure (``struct mmp_struct``) is a
33
34 .. list-table::
35 :widths: 8 12 20 40
36 :header-rows: 1
37
38 * - Offset
39 - Type
40 - Name
41 - Description
42 * - 0x0
43 - __le32
44 - mmp_magic
45 - Magic number for MMP, 0x004D4D50 (“MM
46 * - 0x4
47 - __le32
48 - mmp_seq
49 - Sequence number, updated periodically.
50 * - 0x8
51 - __le64
52 - mmp_time
53 - Time that the MMP block was last update
54 * - 0x10
55 - char[64]
56 - mmp_nodename
57 - Hostname of the node that opened the fi
58 * - 0x50
59 - char[32]
60 - mmp_bdevname
61 - Block device name of the filesystem.
62 * - 0x70
63 - __le16
64 - mmp_check_interval
65 - The MMP re-check interval, in seconds.
66 * - 0x72
67 - __le16
68 - mmp_pad1
69 - Zero.
70 * - 0x74
71 - __le32[226]
72 - mmp_pad2
73 - Zero.
74 * - 0x3FC
75 - __le32
76 - mmp_checksum
77 - Checksum of the MMP block.
Linux® is a registered trademark of Linus Torvalds in the United States and other countries.
TOMOYO® is a registered trademark of NTT DATA CORPORATION.