When setting write policy to “write back” and read policy either to “read ahead” or “disable”, both write and read will access the cache.
If write policy was set to ”write through” or “bad BBU” and BBU is bad or not available, only read action will access cache.
Please also refer to the MegaRAID user manual page 56 and 57 for more details.
http://www.supermicro.com/about/policies/disclaimer.cfm?url=/manuals/other/MegaRAID_SAS_Software_Rev_I_UG.pdf