Combining storage and networking technologies into an integral solution has revealed a number of inherent contradictions between the two. For example, traditional storage processes are based on a master-slave relationship between initiators (servers) and targets (storage devices). Storage devices such as disk arrays or tape subsystems wait passively for commands for data reads or writes. The target device requires little intelligence other than logic to perform RAID, tape loading, or other storage-specific functions. In contrast, traditional networking processes assume that all attached devices are active and intelligent. In conventional IP networking terminology, all attached devices in the network are "hosts," with each host responsible for making its presence known and for actively initiating communications with other networked devices.
The existence of "dumb" storage devices on a network has required new mechanisms to facilitate discovery of devices and communications on a Storage Area Network (SAN). Because a storage device is a passive participant in the SAN, the network must provide intelligence to make the storage device's presence and capabilities known to initiators. In Fibre Channel, the Simple Name Server (SNS) agent present in every Fibre Channel switch provides this intelligence. Passive participants (e.g., disk or tape subsystems) register their presence with the SNS of the Fibre Channel switch as they connect to switch ports. The Fibre Channel switch thus builds a database of available target resources in the SAN. When a server connects to the SAN, it queries the switch's SNS to discover what targets are available.
In addition, the Fibre Channel switch might enforce zoning policies that let only authorized servers discover and communicate with designated storage targets; this practice represents an additional layer of network intelligence to police conversations between servers and storage. Fibre Channel fabrics also provide Registered State Change Notification (RSCN) so that the network can notify initiators of the entry or exit of storage devices from the storage network.
Intelligence in the storage network can also reside in unique products that provide advanced services. Typically, SAN bridge products attached to the network house Third Party Copy (extended copy) agents, which enable server-free backup. When a tape backup launches, the Third Party Copy agent assumes the task of reading data from disk and writing data to tape without further server intervention.
IP storage networks are built on mainstream IP networking infrastructures that assume intelligence on the part of attached devices. Therefore, IP storage networking must supply additional intelligence in the network that's similar to the functions that Fibre Channel switches provide. The Internet Storage Name Service (iSNS) protocol provides for target discovery, zone policies management, and state change notification for IP storage. Similar to the DNS model, which associates Web sites with their IP addresses, the iSNS protocol combines IP discovery services with SNS storage target information, letting IP-attached servers find and establish authorized access to their intended IP-attached storage targets.
In the future, the pinnacle of intelligence for storage networks will be provided by storage virtualization. Storage virtualization hides the complexities of physical storage devices and gives the user a simplified view of available storage resources. Ideally, intelligent virtualization products will monitor the types of data being stored and, based on established policies, automatically determine the appropriate level of access speed, security, and backup that specific types of data require. Virtualization will also be able to automatically allocate additional storage capacity on the fly, without requiring manual configuration. Application-aware virtualization products might be third-party "black boxes" in the storage network or be embedded in IP storage switches or intelligent agents within storage targets. As the storage network becomes more intelligent, administering and allocating storage resources will require less human effort, which results in significant savings for customers and enables them to leverage SAN technology to its fullest.
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