These sit between the hosts and storage and in the case of in-band (symmetric) appliances can provide all of the benefits and services discussed in this article. I/O requests are targeted at the appliance itself, which performs the meta-data mapping before redirecting the I/O by sending its own I/O request to the underlying storage. The in-band appliance can also provide caching of data, and most implementations provide some form of clustering of individual appliances to maintain an atomic view of the metadata as well as cache data.