IBM has implemented a new feature implemented for JFS2 filesystems to prevent simultaneous mounting within PowerHA clusters.

While PowerHA can give concurrent access of volume groups to multiple systems, mounting a JFS2 filesystem on multiple nodes simultaneously will cause filesystem corruption. These simultaneous mount events can also cause a system crash, when the system detects a conflict between data or metadata in the filesystem and the in-memory state of the filesystem. The only exception to this is mounting the filesystem read-only, where files or directories can't be changed.

In AIX 7100-01 and 6100-07 a new feature called "Mount Guard" has been added to prevent simultaneous or concurrent mounts. If a filesystem appears to be mounted on another server, and the feature is enabled, AIX will prevent mounting on any other server. Mount Guard is not enabled by default, but is configurable by the system administrator. The option is not allowed to be set on base OS filesystems such as /, /usr, /var etc.

To turn on Mount Guard on a filesystem you can permanently enable it via /usr/sbin/chfs:

# chfs -a mountguard=yes /mountpoint
/mountpoint is now guarded against concurrent mounts.

The same option is used with crfs when creating a filesystem.

To turn off mount guard:

# chfs -a mountguard=no /mountpoint
/mountpoint is no longer guarded against concurrent mounts.

To determine the mount guard state of a filesystem:

# lsfs -q /mountpoint
Name      Nodename Mount Pt    VFS  Size    Options Auto Accounting
/dev/fslv --       /mountpoint jfs2 4194304 rw      no   no
  (lv size: 4194304, fs size: 4194304, block size: 4096, sparse files: yes,
  inline log: no, inline log size: 0, EAformat: v1, Quota: no, DMAPI: 
  no, VIX: yes, EFS: no, ISNAPSHOT: no, MAXEXT: 0, MountGuard: yes)

The /usr/sbin/mount command will not show the mount guard state.

When a filesystem is protected against concurrent mounting, and a second mount attempt is made you will see this error:

# mount /mountpoint
mount: /dev/fslv on /mountpoint:
Cannot mount guarded filesystem.
The filesystem is potentially mounted on another node

After a system crash the filesystem may still have mount flags enabled and refuse to be mounted. In this case the guard state can be temporarily overridden by the "noguard" option to the mount command:

# mount -o noguard /mountpoint
mount: /dev/fslv on /mountpoint:
Mount guard override for filesystem.
The filesystem is potentially mounted on another node.

Reference: http://www-01.ibm.com/support/docview.wss?uid=isg3T1018853



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