Virtual SAN and virtual tape

New Data Center Strategies Newsletter By Andreas M. Antonopoulos, Network World, 09/12/06

Nemertes Research recently published its third volume of data center research on the topic of storage. Our research shows that just under a third of companies are using storage virtualization technologies, with more companies planning to evaluate or deploy storage virtualization in the near future. There are broadly two different storage virtualization technologies in use: virtual storage-area network and virtual tape.

Storage virtualization is an emerging technology that allows organizations to decouple the physical location and implementation of storage from the applications that are consuming storage.

With a virtual SAN, many different storage subsystems can be “pooled” into one larger logical (virtual) storage system. The virtual storage pool can then be logically partitioned and allocated to specific applications and servers. Because the logical partitions do not necessarily reside on a specific drive or storage array, they can be reallocated, moved, reduced or expanded.

With storage virtualization, for example, an engineer can migrate blocks of storage from one storage array to another without any disruption or reconfiguration to the application using that storage.

Storage virtualization is implemented either in-band or out-of-band. With out-of-band virtualization, the software managing the translation between a virtual pool of storage and the physical drives is outside the data path - connected to the storage network switch or the device drivers on the individual servers. With in-band virtualization, all storage requests go through a device sitting in the data path that translates from virtual to physical storage. All the main storage vendors offer some sort of storage virtualization, either in-band or out-of-band.

One challenge that arises with increased storage capacity is the increasing backup window (the time from the start to completion of a backup). Transfer speeds for tape have not increased as fast as hard drive density. As a result, as storage capacity increases, backup windows have increased.

Virtual tape is a technology that uses hard drives that appear as tape drives. Most of the major storage vendors make storage systems for use as virtual tape drives (e.g., IBM VTS, EMC Clariion Disk Library, etc.). Because of the higher write speed of disks, virtual tape drives can complete a backup much faster, freeing up the application or database, which was locked for consistency.

Once the data is on the virtual tape, it can be transferred to actual tape drives without any concern about the length of the backup window. Thus, virtual tape acts like a buffer for real tape, speeding up backups.

Also, since backed up data is often retrieved in the first 24 to 48 hours, the virtual tape offers a compromise between offline tape and online disk, allowing the retrieval of recently backed-up data directly from the disk.

Fifty-five percent of participants in our research are currently using virtual tape in their production environment. We expect the adoption of virtual tape to increase as more companies evaluate and deploy virtual tape solutions. Many participants in our research indicated they were evaluating virtual tape for deployment in the next six to 12 months.