The EMC Clariion storage array treats the first 5 disks in the first tray of disk special.  This is what is known as the "vault", and it is used as a dumping ground for cache in the event of certain failures that threaten the safety of cache.  EMC Best Practices suggest that you do not use these drives for anything other than the vault if your array is large enough to reasonably do this.  They also suggest not mixing these drives with drives from other raid groups.  The reason for this is that the vault drives appear smaller than a non-vault drive of the same size.  This happens because some of these drives are reserved for use by the Clariions.  Mixing them with other drives makes all the drives in the mixed raid group appear smaller, thus wasting space from every drive in the shared raid group.

Based on these best practices, I will generally get the smallest fibre channel drives availble to use for the vault, and then put those drives alone in a raid group.  I typically then bind a LUN to fill up this entire RAID group so that the space doesn't look available.

Assuming you have NAVI CLI installed, here is a quick script you can use to setup your vault drives like this.

navicli -h 10.80.200.35 createrg 0 0_0_0 0_0_1 0_0_2 0_0_3 0_0_4 -rm no -pri high
navicli -h 10.80.200.35 bind r5 0 -rg 0