Tape Subsystems and Disk Subsystems for Backup

Another important application for subsystem technology is the automation of storage for backup and recovery. Backup and recovery is one of the primary ongoing storage applications in most Information Technology (IT) organizations. As the amount of data has increased, many organizations are finding it necessary to automate backup tasks to reduce errors and improve administrator efficiency. The application of backup is discussed in Chapter 13, "Network Backup: The Foundation of Storage Management."

Backup and recovery in storage networks uses three basic types of storage subsystems, each with its own variations:

• Autoloaders

• Libraries

• Disk subsystems

Autoloaders

The basic idea of a tape autoloader is to provide automated tape changes for one or two tape drives. Autoloaders are relatively small subsystems with small footprints and the ability to store up to 20 tapes. A robotic mechanism changes the tapes as needed under the control of backup software. Autoloaders are intended to be dedicated to large servers or shared among small groups of smaller servers.

Autoloaders typically have parallel SCSI interfaces and do not have integrated SAN interfaces, which means they need to be connected to some form of SAN/SCSI router to use them on the SAN. As SCSI routers add significant cost relative to the cost of an autoloader, autoloaders have not been as widely deployed in SANs as tape libraries.

Libraries

Tape libraries pick up where autoloaders leave off and grow to be fairly large in some cases. Tape libraries for storage networks can hold up to 16 tape drives and store thousands of tapes. Tape libraries are commonly used in SANs to centralize backup operations. They offer significant management advantages over manual tape administration methods and can contribute significant savings to backup administration.

Unlike autoloaders, libraries are available with SAN interfaceseither through the use of embedded HBAs or by implementing SAN/SCSI storage routers. In addition, libraries usually have embedded bar code readers to identify tapes quickly as opposed to having to load tapes into tape drives so that the software can identify them.

Libraries might also have the ability to initiate data transfers by functioning as data movers for serverless backup products.

Disk Subsystems

Disk subsystems are becoming much more commonly used for backup to increase backup performance. Disk storage can be used the way disk drives normally are or can be made to operate as virtual tape libraries using device and media emulation techniques. After copies of backup data are written to disk storage, tape equipment is often used to make redundant and historical copies of the data. Some backup subsystems integrate disk and tape in a single backup subsystem. This approach can significantly simplify the restore process compared to using nonintegrated disk and tape subsystems.

The entire topic of backup and recovery is covered more extensively in Chapter 13.