Sun Microsystems Virtual Tape Library - Manual (Page 19)

Advantages of VTL tape virtualization
96267 • G Chapter 1 Introduction: VTL appliances and enterprise data-protection 5
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
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
virtual
library
physical
library
primary storage
A backup to physical tape can fail when a tape
drive jams or media breaks, because a retry may
not be possible before the backup window closes.
physical
library
backup window
FAILURE!
SUCCESS!
A backup to disk-based virtual media always
succeeds, so all data is copied to secondary
media before the backup window closes.
Virtual volumes are
subsequently copied to
physical media outside
the backup window. If
problems arise, copy jobs
can be retried later.