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Disk to disk to tape
With the advent of less-expensive ATA disk, Gartner Inc. is now advising users to use cheaper disk technologies as a cache buffer between primary disk and tape. Nick Allen, Gartner's vice president and research director, says: "With multiterabyte tape on the horizon, tape should be used primarily for archiving." (See "

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Backup and restore recommendations")

Cost comparison:
10TB of capacity
The relatively recent boost in tape capacities and vendors' road maps for their tape product families suggest tape will continue to cost less than enterprise disks by roughly a factor of 50 or more for the foreseeable future. Even ATA disk-based subsystems will remain roughly 10 times more expensive than modern tape. These estimates assume that the tape environment is completely automated, and these calculations assume 100% utilization of the media.


Source: Gartner Inc.

In other words, cheap disks aren't going to replace tape, just tape's role in the backup and restore process. Quantum Corp., whose DLTtape range of tapes now fits from 80GB to 320GB of compressed data on a cartridge, last year enumerated a storage road map, taking its first major step with the recent release of SDLT 600, which pushes per-cartridge capacity to 600GB with transfer rates of 64MB/s. By 2006, Quantum expects to extend its linear Super DLT technology to squeeze 2.4TB onto a tape.

For its part, the HP/IBM/-Seagate-backed LTO Ultrium technology anticipates an 18- to 24-month generational cycle, with the format's capacity due to expand from the 400GB of today's Ultrium-2 to approximately 800GB in the next generation, and 1.6TB after that.

The other major high-end tape solution--Sony's helical-scan Advanced Intelligent Tape (AIT)--is currently lagging its competitors at around 260GB capacity, but it is getting a facelift. In February, OEMs received the first shipments of first-generation SAIT-1 drives, which compress 1.3TB on a half-inch tape and promise up to 10.4TB before the format hits its limitations around 2010. Sony proudly proclaims that OEMs will build SAIT-1 libraries in configurations of up to 1,000 cartridges capable of storing 1.3 petabytes of data (see "Tape road maps").

The tape-SAN challenge
As tapes hold more data, customers are finding that tape presents a new set of problems when it's introduced into increasingly high-speed SAN environments. Underlying the problem is the fact that tape, with its roots as a one-to-one server-attached device, doesn't generally play well with devices in the SAN. It expects a continuous stream of data, and if that stream is interrupted, it will issue a SCSI interrupt, stop the tape, rewind and continue again.

This interruption can cause major problems for a Fibre Channel arbitrated loop (FC-AL) SAN--even when FC-attached tape libraries are being used--because the break will invoke a loop initialization primitive (LIP) call that suspends the entire SAN until attached devices are detected and port addresses reallocated. This could also be a problem if slow servers mean the fast-moving tapes get ahead of themselves and have to pause and rewind to compensate for the interruption in data flow. Although FC-AL SANs have become less common with the shift to SAN switching, tape-disk contention must still be addressed to eliminate potential hiccups.

Another potential source of problems is from the configuration of tape within the SAN: Sharing a host bus adapter (HBA) between tape and servers can throttle back performance when contention for the HBA becomes an issue. Spend the extra dollars to get a second HBA just for tape, then consider a third, redundant HBA to make sure the servers have enough bandwidth. Many customers use SAN zoning to link tape and server resources.

"Vendors will claim you can configure an HBA for both disk and tape, but usually you get conflicting parameters and more often than not, you'll have problems," says John Verdonik, director of professional services with storage integrator CNT, in Minneapolis. "Contention issues become apparent when you're connected to a SAN. People have expectations of throughput, but in reality we have to choose to get that throughput."

This was first published in September 2003

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