diff options
author | Darrick J. Wong <darrick.wong@oracle.com> | 2018-07-29 15:42:00 -0400 |
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committer | Theodore Ts'o <tytso@mit.edu> | 2018-07-29 15:42:00 -0400 |
commit | 18ba5a45ce1f590b3a2460f8f4ad52daf88290c4 (patch) | |
tree | f14a3e03f8dda2b793a2265cd30ea73be73c4d10 /Documentation/filesystems | |
parent | 33dfadc747a88243d616d8cb5c59ec3031dc60dd (diff) |
ext4: import multi-mount protection chapter from wiki page
Import the chapter about multi-mount protection from the on-disk format
wiki page into the kernel documentation.
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Diffstat (limited to 'Documentation/filesystems')
-rw-r--r-- | Documentation/filesystems/ext4/ondisk/globals.rst | 1 | ||||
-rw-r--r-- | Documentation/filesystems/ext4/ondisk/mmp.rst | 77 |
2 files changed, 78 insertions, 0 deletions
diff --git a/Documentation/filesystems/ext4/ondisk/globals.rst b/Documentation/filesystems/ext4/ondisk/globals.rst index 5891ee137749..fe6e107bf515 100644 --- a/Documentation/filesystems/ext4/ondisk/globals.rst +++ b/Documentation/filesystems/ext4/ondisk/globals.rst @@ -9,3 +9,4 @@ have static metadata at fixed locations. .. include:: super.rst .. include:: group_descr.rst .. include:: bitmaps.rst +.. include:: mmp.rst diff --git a/Documentation/filesystems/ext4/ondisk/mmp.rst b/Documentation/filesystems/ext4/ondisk/mmp.rst new file mode 100644 index 000000000000..b7d7a3137f80 --- /dev/null +++ b/Documentation/filesystems/ext4/ondisk/mmp.rst @@ -0,0 +1,77 @@ +.. SPDX-License-Identifier: GPL-2.0 + +Multiple Mount Protection +------------------------- + +Multiple mount protection (MMP) is a feature that protects the +filesystem against multiple hosts trying to use the filesystem +simultaneously. When a filesystem is opened (for mounting, or fsck, +etc.), the MMP code running on the node (call it node A) checks a +sequence number. If the sequence number is EXT4\_MMP\_SEQ\_CLEAN, the +open continues. If the sequence number is EXT4\_MMP\_SEQ\_FSCK, then +fsck is (hopefully) running, and open fails immediately. Otherwise, the +open code will wait for twice the specified MMP check interval and check +the sequence number again. If the sequence number has changed, then the +filesystem is active on another machine and the open fails. If the MMP +code passes all of those checks, a new MMP sequence number is generated +and written to the MMP block, and the mount proceeds. + +While the filesystem is live, the kernel sets up a timer to re-check the +MMP block at the specified MMP check interval. To perform the re-check, +the MMP sequence number is re-read; if it does not match the in-memory +MMP sequence number, then another node (node B) has mounted the +filesystem, and node A remounts the filesystem read-only. If the +sequence numbers match, the sequence number is incremented both in +memory and on disk, and the re-check is complete. + +The hostname and device filename are written into the MMP block whenever +an open operation succeeds. The MMP code does not use these values; they +are provided purely for informational purposes. + +The checksum is calculated against the FS UUID and the MMP structure. +The MMP structure (``struct mmp_struct``) is as follows: + +.. list-table:: + :widths: 1 1 1 77 + :header-rows: 1 + + * - Offset + - Type + - Name + - Description + * - 0x0 + - \_\_le32 + - mmp\_magic + - Magic number for MMP, 0x004D4D50 (“MMP”). + * - 0x4 + - \_\_le32 + - mmp\_seq + - Sequence number, updated periodically. + * - 0x8 + - \_\_le64 + - mmp\_time + - Time that the MMP block was last updated. + * - 0x10 + - char[64] + - mmp\_nodename + - Hostname of the node that opened the filesystem. + * - 0x50 + - char[32] + - mmp\_bdevname + - Block device name of the filesystem. + * - 0x70 + - \_\_le16 + - mmp\_check\_interval + - The MMP re-check interval, in seconds. + * - 0x72 + - \_\_le16 + - mmp\_pad1 + - Zero. + * - 0x74 + - \_\_le32[226] + - mmp\_pad2 + - Zero. + * - 0x3FC + - \_\_le32 + - mmp\_checksum + - Checksum of the MMP block. |