AIX:Mirror Write Consistency

The LVM always ensures data consistency among mirrored copies of a logical volume during normal I/O processing.

For every write to a logical volume, the LVM generates a write request for every mirror copy. A problem arises if the system crashes in the middle of processing a mirrored write (before all copies are written). If mirror write consistency recovery is requested for a logical volume, the LVM keeps additional information to allow recovery of these inconsistent mirrors. Mirror write consistency recovery should be performed for most mirrored logical volumes. Logical volumes, such as the page space that do not use the existing data when the volume group is re-varied on, do not need this protection. The Mirror Write Consistency (MWC) record consists of one sector. It identifies which logical partitions may be inconsistent if the system is not shut down correctly. When the volume group is varied back on-line, this information is used to make the logical partitions consistent again. Note: With Mirror Write Consistency LVs, because the MWC control sector is on the edge of the disk, performance may be improved if the mirrored logical volume is also on the edge.

Beginning in AIX® 5, a mirror write consistency option called Passive Mirror Write Consistency is available. The default mechanism for ensuring mirror write consistency is Active MWC. Active MWC provides fast recovery at reboot time after a crash has occurred. However, this benefit comes at the expense of write performance degradation, particularly in the case of random writes. Disabling Active MWC eliminates this write-performance penalty, but upon reboot after a crash you must use the syncvg -f command to manually synchronize the entire volume group before users can access the volume group. To achieve this, automatic vary-on of volume groups must be disabled.

Enabling Passive MWC not only eliminates the write-performance penalty associated with Active MWC, but logical volumes will be automatically resynchronized as the partitions are being accessed. This means that the administrator does not have to synchronize logical volumes manually or disable automatic vary-on. The disadvantage of Passive MWC is that slower read operations may occur until all the partitions have been resynchronized.

You can select either mirror write consistency option within SMIT when creating or changing a logical volume. The selection option takes effect only when the logical volume is mirrored (copies > 1).