Sun Microsystems Virtual Tape Library manual Shorter run times for non‐sequential backup jobs

Advantages of VTL tape virtualization

Shorter run times for non‐sequential backup jobs

Disk‐based VTL systems reduce run time when storage operations are poorly matched to the operational characteristics of tape backup systems. Properly configured, streaming tape backups achieve transfer rates that are as high as or higher than those attainable by disk technology. But many common jobs—such as incrementals and full backups of workstations—produce semi‐random I/O. Non‐ sequential I/O keeps tape drives busy mounting, unmounting, and positioning media, greatly reducing throughput. Disk‐based secondary storage is much better suited to these semi‐random backup jobs.

Improved reliability

Disk‐based VTL systems can significantly increase the reliability of the backup process. Backup jobs are more likely to succeed the first time, because the critical step—the creation of a copy of the data—is a simple, fast write to a RAID subsystem. Jammed tapes, lack of ready media, and off‐line drives no longer ruin jobs. See the figure below:

Backup is more reliable with virtual tape libraries

	backup window
	physical	A backup to physical tape can fail when a tape
primary storage	library	drive jams or media breaks, because a retry may
		not be possible before the backup window closes.
	FAILURE!	A backup to disk-based virtual media always
		succeeds, so all data is copied to secondary
		media before the backup window closes.
	SUCCESS!		Virtual volumes are
			subsequently copied to
			physical media outside
	virtual	physical	the backup window. If
			problems arise, copy jobs
	library	library	can be retried later.

Better utilization of tape subsystems

Disk‐based VTL systems can improve utilization, performance, and reliability of tape‐storage subsystems. When non‐sequential I/O is backed up to disk, tape can be reserved for sequential jobs that can stream a physical tape drive. Large‐scale full backups can, for instance, go directly to tape, insuring maximum performance. Jobs

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