Crystal Ball Looks at Future Uses of Storage Virtualization

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Storage virtualization came about as a set of services to enable true sharing and pool of storage devices within a SAN. With the emergence of storage over IP, storage networks need no longer to be confined to fiber channel SANs, but will cross over IP networks, by connecting multiple SANs together. The services that storage virtualization provide in the future will need to take into account the convergence of SANs and storage over IP. This means, first of all, the ability to administer and monitor multiple SANs centrally from a single administrative console. Administration is no longer limited to elements within a SAN, but must also include the ability to monitor elements connecting SANs through the IP networks, such as SAN gateways and routers.

Disaster recovery applications stand to gain the most benefit from storage virtualization services across SANs. Extending mirroring and copying capabilities to remote sites enables automated and more reliable disaster recovery options than backing up to tape and manually transporting tape cartridges to remote sites. With remote mirroring and remote copying over long distance IP networks, multiple copies of the data can exist in multiple remote sites. The latency of long-distance IP networks requires other mirroring options besides pure synchronous, where a write isnt complete until all copies of the data are written.

The types of remote mirroring include asynchronous (buffered) and semi-synchronous mirroring. In asynchronous mirroring, the write completes as soon as one of the mirrors is written. Semi-synchronous mirroring allows a limited number of writes ahead. In other words, if a backlog of previous writes havent been completed at the remote mirror, then the system wont accept any more asynchronous writes until some of the outstanding writes to the mirror have been completed. To support backup to a remote tape library, remote snapshot and point-in-time copy capabilities also are necessary. A number of high-end RAID applications and SAN applications offer some of these remote replication options.

To cross the SAN boundary, storage virtualization must also support the bridging of fibre channel traffic to an IP network. Currently, there are two main approaches to bridging the two types of networks: fibre channel over IP and iSCSI. Fibre channel over IP encapsulates fibre channel frames in IP packets to transport fibre channel commands through an IP networks. Once the fibre channel frame arrives at the target SAN, then the IP packet is stripped of the IP headers down to the fibre channel frame content. In this approach, only the gateways at the end of the SAN need to know about fibre channel over IP in order to assemble and disassemble the packet.

The advantage of fibre channel over IP is that it doesnt require any changes to the existing fibre channel or IP infrastructure. It doesnt require hosts to be equipped with new drivers or host bus adapters that recognize the new protocol. Only edge fibre channel over IP gateways need to be introduced into the networks.

iSCSI is a mapping of SCSI-3 to TCP. Native iSCSI support requires drivers. Host bus adapterss and network interface cards need to be installed at the host side to recognize this protocol. At the storage side, it requires a new iSCSI array attachment. In addition, new SAN gateways, switches or SAN routers are required to bridge fibre channel elements to a gigabit Ethernet.

The advantages of iSCSI are that it allows direct connection of iSCSI- enabled hosts and storage devices to an IP network, while still accommodating fibre channel storage and hosts to participate as well. However, it does require reconfiguration of the host and storage to provide direct connection. The performance of TCP as a storage transport protocol is also an issue. However, a number of host bus adapters, and network interface cards offer TCP processing in hardware.

Regardless of the protocol of choice, storage virtualization services should make the existence of different protocols transparent. In fact, the platforms where storage virtualization is implemented as SANs and IP converge may very well be within the SAN gateway, or the SAN routers or the switches that connect SANs to IP networks, as well as iSCSI hosts and storage. Some vendors have also started to implemented SAN appliances that server as server gateways between SANs and data networks. These SAN appliances currently use standard servers and operation systems. In the future, special-purpose appliances and operating systems may be developed to extend storage virtualization over IP.

Backup support is another key advance for storage virtualization in the future. If you can virtualize storage, the next step is to virtualize tape. Tape virtualization means first of all to bring the tape devices into the SAN so that they can be shared among hosts and servers. Then storage virtualization appliances can make true serverless backup a reality by providing extended copy functions. Extended copying essentially moves the burden of doing backup from the backup server to the storage virtualization appliance. This procedure will remove the need for a separate backup server and allow virtualization of both tape and storage, whether they coexist in the same SAN or in different SANs, located in different areas connected over a long-distance network.

Elizabeth M. Ferrarini She is a free-lance writer from Boston, MA.

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