Disk mirroring performance implications

From a performance standpoint, mirroring is costly, mirroring with Write Verify is costlier still (extra disk rotation per write), and mirroring with both Write Verify and Mirror Write Consistency is costliest of all (disk rotation plus a seek to Cylinder 0).

If mirroring is being used and Mirror Write Consistency is on (as it is by default), consider locating the copies in the outer region of the disk, because the Mirror Write Consistency information is always written in Cylinder 0. From a fiscal standpoint, only mirroring with writes is expensive. Although an lslv command will usually show Mirror Write Consistency to be on for non-mirrored logical volumes, no actual processing is incurred unless the COPIES value is greater than one. Write Verify defaults to off, because it does have meaning (and cost) for non-mirrored logical volumes.

A mirror write consistency option called Passive Mirror Write Consistency (MWC) 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).