Back to the Future with Tape Drives
Over the last 30 years tape technology has not changed as much compared with disk-based storage, but in most environments (especially enterprise environments) tape is still a requirement for reliability. Even if you have an off-site remote mirror, in most cases tapes are still used. Given tape's continued popularity, we'll cover many of the issues surrounding tape hardware.
It may seem a little quaint to discuss tape as we enter 2003. Tape technology has not kept pace with disk technology, and in fact significant improvement in tape performance has only recently been available with the release of the StorageTek T9940B drive. But there still is much to learn from an evaluation of tape technologies, which we'll cover in this article.
Most enterprise environments use tapes that write in a linear format. However, another tape type exists that generally has a higher density: helical tape. Helical tape has more head contact with head on the tape drive. With linear tape the data is written lengthwise down the drive. With helical tape the data is written horizontally across the tape, hence the reason there is more contact with the heads. Here are some general comparisons between these two tape types:
- A very small defect on a helical tape can corrupt the data if the error correction buffer is full. Error correction space is often left on the tape and if that space fills up the tape becomes unreadable.
- Helical tape heads wear out long before linear tape heads given that the tape heads make more intimate contact with the tape.
- Reliability is generally higher for linear tapes over helical for both the media and head life of the drive because more contact means more wear.
- Because of media wear high-end linear tapes generally have a longer storage life than high-end helical tapes.
Linear tape vendors/types include IBM 3590B/E, STK 9840/9940, Quantum SuperDLT, older DLT 7000/8000, and LTO.
Helical tape vendors include Sony, which makes AIT-1 and AIT-2 as well as the DTF line of tapes. Other helical types include 8mm Mammoth and Mammoth-2 4mm(DAT).
Unlike disks and RAID, almost all tapes automatically compress the data input stream. This is an important consideration when determining drive types, as different drives have different compression algorithms. Enterprise tape drives from IBM and StorageTek have higher compression rates than lower-end drives like DLT and Mammoth. Drive vendors often provide estimated compression rates but these are averages and your mileage may vary. Compression is important given the cost of the media as a function of the drive cost. Take the following example:
Drive Cost: $35,000
Media Cost: $75
Compression: 5 to 1
Drive Size: 250 GB
Drive Cost: $5,000
Media Cost: $75
Compression: 2 to 1
Drive Size: 250 GB
Let's say you have 400TB of raw data that will need to be backed up, so you're shopping for a new tape system. Drive One will require 327 pieces of media at a cost of $24,525, for a total system cost of $59,525. Drive Two will require 820 pieces of media at a cost of $61,500, for a total system cost of $66,500.
Clearly compression must be a consideration in the total cost of ownership of for tape systems -- but your mileage for compression on each drive type with your data will vary. One quick way of looking to see if your data is compressible is to use the gzip program with the -9 option:
# gzip -9 filename
You will have to test each of the tape drive that are under consideration with a statistically significant sample of your data to determine how your data behaves with the drive.