Unlike the fracture log, the WIL is active at all times and not just when the mirror is fractured.  During normal operation the WIL tracks every write to both the primary and secondary images and records this in a bitmap.  This bitmap is updated when a write I/O arrives for the source LUN and this entry is cleared after the data is written to both the Primary and Secondary LUNs (although this write will initially only be to write cache on both CLARiiON arrays if write cache is enabled).  Therefore, the performance demands on the WIL are proportional to the write I/O for synchronously mirrored LUNs on that SP.

The Write Intent Log best practice is as follows:

• Fibre Channel or Enterprise Flash drives can be used for the WIL LUNs.  While it is possible to use SATA2 drives, this is not recommended.
• Avoid using the Vault drives for the WIL (drives 0_0_0 to 0_0_4).  The WIL will be under heavy load at times of heavy write I/O to the mirrored LUNs and if the Vault drives were used, this can increase SP utilization and response times (article emc207795).
• Virtual Provisioning LUNs are not supported for the WIL; RAID group-based LUNs or metaLUNs should be used.
• If FAST Cache is enabled for the CLARiiON, it should be disabled for both WIL LUNs.  However, EFD/SSD can be used for the WIL, if those particular drives are in a RAID group (that is, not in a Pool or used as FAST Cache).
• The WIL is small write intensive and so RAID 1/0 is the recommended RAID level.
• For CX and CX3 systems, Release 26 improves the performance of the WIL, particularly if there is forced flushing (article emc186107), so this is the recommended level of FLARE.