Round Robin (Default) 

Attempts to balance the load across the submirrors. All reads are made in a round-robin order (one after another) from all submirrors in a mirror. 

Geometric 

Enables reads to be divided among submirrors on the basis of a logical disk block address. For instance, with a two-way submirror, the disk space on the mirror is divided into two equally-sized logical address ranges. Reads from one submirror are restricted to one half of the logical range, and reads from the other submirror are restricted to the other half. The geometric read policy effectively reduces the seek time necessary for reads. The performance gained by this mode depends on the system I/O load and the access patterns of the applications. 

First 

Directs all reads to the first submirror. This should be used only when the device(s) comprising the first submirror are substantially faster than those of the second submirror. 

Parallel (Default) 

A write to a mirror is replicated and dispatched to all of the submirrors simultaneously. 

Serial 

Performs writes to submirrors serially (that is, the first submirror write completes before the second is started). The serial option specifies that writes to one submirror must complete before the next submirror write is initiated. The serial option is provided in case a submirror becomes unreadable, for example, due to a power failure.