Mirror write consistency for hd8 and hd6


Env:  AIX 4.1.5

Problem:  The customer notices that mirror write consistency (MWC) for hd8
and hd6 is turned off.  He wants to turn this on.

Action taken: Mirror Write Consistency is a mechanism used for mirrored logical
volumes. It's intent is to insure that if the machine crashes during a write
to a mirrored LV, the mirrors will be consistent upon reboot.

This is accomplished by having the system write an intent to write to a LV on
a edge of a disk in a mirror write cache structure area. It then completes the
write to data section of the disk.

This has a performance impact in that 2 writes occur for each write operation as
well as a disk seek to each location.

Consistent mirrors are not needed on the system JFS log OR a paging space. Both
are overwritten on reboot, and previous data stored there is not important.

Further, these are high access LV's, and any performance problems related to disk
access would be compounded by having this enabled for them.

For this reason, MWC is disabled for these LV's.

Action Plan: CWCA

Testcase: none

Support Line: Mirror write consistency for hd8 and hd6 ITEM: BU5193L
Dated: February 1997 Category: N/A
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