Mirror Write Consistency Check (MWCC) is a method of tracking the last 62 writes to a mirrored logical volume. If the AIXsystem crashes, upon reboot the last 62 Logical Track Group (LTG) writes to mirrors are examined and one of the mirrorsis used as a “source” to synchronize the mirrors (based on the last 62 disk locations that were written). The LTG is alwaysof size 128 KBytes. This “source” is of importance to parallel mirrored systems. In sequentially mirrored systems, the“source” is always picked to be the Primary disk. If that disk fails to respond, the next disk in the sequential ordering will bepicked as the “source” copy. There is a chance that the mirror picked as “source” to correct the other mirrors was not theone that received the latest write before the system crashed. Thus, the write that may have completed on one copy andincomplete on another mirror would be lost. AIX does not guarantee that the absolute, latest write request completedbefore a crash will be there after the system reboots. But, AIX will guarantee that the parallel mirrors will be consistent witheach other. If the mirrors are consistent with each other, then the user will be able to realize which writes were consideredsuccessful before the system crashed and which writes will be retried. The point here is not data accuracy, but dataconsistency. The use of the Primary mirror copy as the source disk is the basic reason that sequential mirroring is offered.Not only is data consistency guaranteed with MWCC, but the use of the Primary mirror as the source disk increases thechance that all the copies have the latest write that occured before the mirrored system crashed. Some usrs turn offMWCC for increased write performance or the software product they use requires that MWCC be turned off. In this case,where MWCC can’t guarantee mirror consistency the user should run a forced volume group sync (syncvg -f <vgname>)immediately after coming up from a system crash.
Mirror Write Consistency Check
Mirror Write Consistency Check (MWCC) is a method of tracking the last 62 writes to a mirrored logical volume. If the AIXsystem crashes, upon reboot the last 62 Logical Track Group (LTG) writes to mirrors are examined and one of the mirrorsis used as a “source” to synchronize the mirrors (based on the last 62 disk locations that were written). The LTG is alwaysof size 128 KBytes. This “source” is of importance to parallel mirrored systems. In sequentially mirrored systems, the“source” is always picked to be the Primary disk. If that disk fails to respond, the next disk in the sequential ordering will bepicked as the “source” copy. There is a chance that the mirror picked as “source” to correct the other mirrors was not theone that received the latest write before the system crashed. Thus, the write that may have completed on one copy andincomplete on another mirror would be lost. AIX does not guarantee that the absolute, latest write request completedbefore a crash will be there after the system reboots. But, AIX will guarantee that the parallel mirrors will be consistent witheach other. If the mirrors are consistent with each other, then the user will be able to realize which writes were consideredsuccessful before the system crashed and which writes will be retried. The point here is not data accuracy, but dataconsistency. The use of the Primary mirror copy as the source disk is the basic reason that sequential mirroring is offered.Not only is data consistency guaranteed with MWCC, but the use of the Primary mirror as the source disk increases thechance that all the copies have the latest write that occured before the mirrored system crashed. Some usrs turn offMWCC for increased write performance or the software product they use requires that MWCC be turned off. In this case,where MWCC can’t guarantee mirror consistency the user should run a forced volume group sync (syncvg -f <vgname>)immediately after coming up from a system crash.