Database Reference
In-Depth Information
1.6.2 Virtual Tape Libraries (VTLs)
Virtual tape libraries are a somewhat new concept in storage and primarily
came about due to the cheap cost and prevalence of disk. The concept of a VTL
is to use disk to mimic tape such that the application using the VTL does not
know that it is manipulating disk rather than tape. These are primarily used
in backup applications to eliminate dependence on streaming to tape for good
performance and to increase the speed of recovery. Testing with at least one
VTL in an archive application that writes small files or blocks of data to tape
showed only a 2x improvement over a high-capacity tape drive. The thought
was that the VTL would do much better given that the tape drive used did not
handle small files with many tape marks well. In testing, the site discovered
that disk is not that much faster or more ecient than tape at processing tape
marks. One major problem with VTLs is that they have a cost somewhere
between tape and high-end disk arrays. For mass storage systems, there seems
to be little need or use for VTLs when most are already hierarchical storage
systems with both disk and tape being used for both performance and cost
reasons.
There have been other attempts at placing magnetic disk drives in tape
cartridges so that they could fit in tape robotic libraries and mimic tape
cartridges. However, there are few applications that can make use of removable
disk, and the disk drives must be ruggedized for extra wear and tear. None of
these solutions has come to market.
1.6.3 Redundant Arrays of Independent Tape (RAIT)
Many high-performance computing sites could take advantage of redundant
tape systems or redundant arrays of inexpensive tape (RAIT). There are sev-
eral hardware or software solutions on the market, but none that satisfy the
needs of providing extreme bandwidths and parity protection. The hardware
solution from Ultera provides mirroring or parity protection with an SCSI
hardware controller card that connects SCSI tape drives together to per-
form RAIT. However, the solution is limited in bandwidth for what high-
performance storage would require. The software solutions available, primar-
ily HPSS, Veritas, or Legato, generally meet high-performance bandwidth
requirements but do not provide parity protection to reconstruct data in
the event that a tape within the stripe is lost. At the time of this writing,
HPSS is currently working to design and develop a RAIT solution for its
customers.
One of the most promising RAIT systems, developed by StorageTek, made
it to the prototype stage demonstrating the loss of two tape drives during
a read while continuing to stream data to the application.
9
During the final
phase of development of turning the prototype into a production system, the
company halted the project because the market demand for the product was
believed to be too limited.
