Saturday, February 16, 2008

Mirror Write Consistency

Mirror Write Consistency

Mirror Write Consistency (MWC) ensures data consistency on logical volumes in case asystem crash occurs during mirrored writes. The active method achieves this by loggingwhen a write occurs. LVM makes an update to the MWC log that identifies what areas ofthe disk are being updated before performing the write of the data. Records of the last 62distinct logical transfer groups (LTG) written to disk are kept in memory and also written toa separate checkpoint area on disk (MWC log). This results in a performance degradationduring 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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