The issue is aggravated in highly integrated arrays like those from NetApp where NAS and SAN share a single data path. In NetApp filers, file- and block-based protocols are passed through the NetApp Write Anywhere File Layout (WAFL) file-system layer, resulting in a higher degree of dependency between NAS and SAN. WAFL was initially conceived for NAS and, despite a commendable degree of optimization, the WAFL translation overhead results in more CPU cycles for processing block-based protocols than a comparable single-protocol FC array.

Conversely, the gateway/dual-path approach in EMC Celerra or Pillar Axiom arrays don't have a WAFL-equivalent layer through which all data, regardless of protocol, must pass on the way to the disk subsystem. But the risk of NAS impacting SAN performance still exists for all multiprotocol arrays, as both NAS and SAN will eventually access the back-end storage.

To alleviate performance interdependencies, multiprotocol array vendors added quality of service to their arrays that enables users to set priorities and define the number of requests permitted for NAS and SAN. "A user can define a logical volume in our Axiom arrays that allocates 85% of resources to databases and 15% to file access," says a Pillar Data Systems spokesperson.

Practically, arrays that support both block- and file-based protocols aren't the arrays of choice for the performance required by high-end databases and transactional applications.

"If I am looking for the fastest block-based storage system, multiprotocol arrays like the ones from NetApp don't come to mind," says Greg Schulz, founder and senior analyst at StorageIO Group, a consulting firm in Stillwater, MN. "However, if I'm looking for the most feature-rich NFS/CIFS/block-based storage system, NetApp is at the top of the list."