Friday, November 23, 2007

Mirror Write Consistency

Mirror Write Consistency (MWC) ensures data consistency on logical volumes in case a
system crash occurs during mirrored writes. The active method achieves this by logging
when a write occurs. LVM makes an update to the MWC log that identifies what areas of
the disk are being updated before performing the write of the data. Records of the last 62
distinct logical transfer groups (LTG) written to disk are kept in memory and also written to
a separate checkpoint area on disk (MWC log). This results in a performance degradation
during random writes.
With AIX V5.1 and later, there are now two ways of handling MWC:
• Active, the existing method
• Passive, the new method

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