EMC CX700 Planning Manual page 78

Data Replication and Data Mobility Software
4-16 EMC CLARiiON CX300, CX500, CX500i, and CX700 Storage Systems Configuration Planning Guide
If you decide to use the write intent log feature for even one remote
mirror, then you must bind two LUNs of at least 128 Mbytes for the
write intent log.
Optional Write Intent Log Worksheet
LUN Name or ID
First LUN
Second LUN
LUN Size
(128 Mbytes minimum)