AIX:Mirror Write Consistency

Mirror write consistency

LVM ensures data consistency among mirrord copies. For every write to a volume, LVM generates a write reuqest for every mirror copy. A problem might arise if the system crashes before all copies are written.

When mirror write consistency is active, LVM keeps additional information for recovery purposes. Mirror write consistency shoudl be performed for most logical volumes. An exception is paging space, which does not use the existing date when the vlluemgroup is varied online.

Tme MWC record consists of one sector and describes which logical partitions may be inconsistent if the system is not shutdown correctly. When the volumegroup is varied back online, this record is read to make the logical partitions consistent again.

Since AIX5, a mirror write consistency option called Passive Mirror Write Consistency is available. Default is Active MWC. Active MWC offers fast recovery at reboot after a crash has occurred. This benefit comes at the expense of write performance degradation, particularly in the case of random writes. Disabling Active MWC elimiates this penalty, but upon reboot the syncvg -f command must be issued to manually synchronize the volume group before users can access the volumegroup. To achieve this, automatic vary-on of volumegroups must be disabled.

Eneabling PAssive MWC not only eliminates the write-performance penalty aassoicated with Acitve MWC, but logical volumes will be automatically resynced as the partitions are being accessed. This means that the administrator does not have to synchronize logica volumes maunally or disable automatic vary-on. Disavntage of Passive MWC is that slower read operations may occur until all partitions have been resynced.