In-Depth

Choosing the Right Backup Tape

With the drop in hard drive prices, many system administrators are faced with more data to back up than before.

• By Douglas Mechaber
• 11/01/2001

With the drop in hard drive prices, many system administrators are faced with more data to back up than before. It may well be time to think about wholesale upgrades to your tape storage systems. The table lists some popular backup tape formats. Other formats (such as Ecrix’s VXA1, Onstream’s ADR, and Tandberg’s SLR) haven’t caught on, probably because other OEMs haven’t adapted these formats. Mammoth’s (Exabyte) helical scan 8mm format has faded in popularity and seems to be losing the battle with AIT (Advanced Intelligent Tape). The members of the AIT Forum (www.aittape.com/) argue that its design makes AIT drives more reliable than Mammoth-type drives, but Exabyte obviously disagrees with this. Exabyte provides a white paper (www.exabyte.com/support/online/documentation/ whitepapers/costown.cfm) that has a good discussion of costs of tapes, drives and doing backup, though it’s slanted toward Exabyte. Exabyte has announced plans to “merge” with Ecrix, as the VXA-1 and Mammoth markets are largely separate.

Older formats (remember QIC tapes?) aren’t even large enough for small home computers these days. Travan tapes were supposed to solve that problem; but they, too, have been outgrown. The Travan format that does have 20GB capacity, Travan NS, is difficult to find. Cost and convenience lead most small system owners to utilize Jaz; Zip; and, increasingly, CD-RW for backup rather than tape.

As a system administrator, how should you choose? Obviously, capacity is paramount, but backward compatibility may also be a factor. If so, then Super DLT may be a logical choice, as SDLT drives will read (and write) to older DLT tapes. DDS drives are also backward-compatible with older DDS formats. Tape cost may also be a factor: Recent online prices are about $140 each for SDLT tapes, Ultrium tapes are around $110, AIT tapes are approximately $70, DLT tapes cost from $50 to $80, and DDS tapes range from about $12 to $40 (depending on capacity). DDS drives are still popular in the OEM market, though the DDS developers (Sony, HP and Seagate) have announced they wouldn’t develop a DDS-5 format. They will, however, continue to suppport existing units. Both DLT (as SDLT, by Quantum) and Linear Tape Open in the Ultrium format (developed by Seagate, HP and IBM) have clear upgrade paths for increased capacity and speed. The Ultrium consortium (www.ultrium.com) plans an eight-fold increase when it releases its fourth-generation product. That will provide an 800GB/1.6TB capacity tape, with an uncompressed transfer speed of 80 to160 MB/s—and double that for compressed files. Quantum has similar plans for SDLT’s migration to super capacity tapes. An 11-magazine Seagate autoloader built on Ultrium technology presently has a 2.2 TB capacity, and future generations will continue to increase that capacity.

The factor that many sysadmins miss when choosing a tape drive is reliability. Error rates for data reliability are comparable for most formats above (one error in 1017 bits), with the exception of DDS (one error in 1015 bits). However, it’s been my experience that, within a two-year window, an inordinate number of DLT drives fail at the hardware level. AIT drives have one of the best MTBF rates on the market, but the tape is about the same price as DLT.

Another important point to consider is restore speed; when you need a server restored, you need it restored as quickly as possible. AIT tapes have a built-in memory chip, making searching for files on AIT tapes considerably faster than searching on DLT tapes. DDS drives seem more reliable than DLT drives, but the DAT tapes they use are subject to rapid failure. This phenomena seems quite batch dependent; but, on average, over the past few years, I’ve seen one in 10 to one in 20 DAT tapes fail prematurely. On several occasions, I have seen multiple DAT tapes within a batch fail quickly, but haven’t noted this problem with DLT tapes.

Ultrium has the clear performance advantage over SDLT, while SDLT has backward compatibility. DDS drives range from approximately $400 to $800, but are at the end of their product life. AIT drives are more than $3,000, Mammoth drives are about $4,000, and DLTs vary in price (depending on capacity) but are in the $5,000 range. Ultrium drives and SDLT drives may also be found in the $5,000 range. But remember: Drive cost is minimal compared to cost of tape, personnel, and potential lost productivity.