This video tutorial explains how reads and writes work on NetApp storage. It includes details on the interaction between NetApp’s Write Anywhere File Layout, NVRAM mirroring, the system memory cache, and disk. This is the first video in a series of three which also includes details about NetApp’s Virtual Storage Tier (VST) technologies.
http://www.flackbox.com/netapp-wafl-nvram-system-memory-cache/