Backup Tape Standards, Capacity, Speed: DDS/DAT, DLT, LTO Ultrium

In 2010 a 1-terabyte hard disk costs about $100. An 800 gigabyte LTO 4 tape cartridge costs about $40. This makes hard disks a practical alternative to tape, for data backup. However, tapes do still have a place in corporate IT departments. Magnetic tape drives are surprisingly fast. High-end systems can exceed 100 megabytes/second.

Tape advantages, compared to hard disks

• Simpler and more robust mechanical construction. The tape is physically separate from the tape drive (unlike a hard disk). There are no moving read/write heads that can crash.
• Smaller and lighter. This is useful if large quantities of data are stored or sent by courier to an off-site disaster recovery center.
• Long term archival reliability. Some tape manufacturers claim 30 years. Hard disk warranties rarely exceed 5 years.

Tape disadvantages, compared to hard disks

• No random access to the files, slower to restore a specific file.
• Lower read/write cycle lifespan, typically a few hundred per tape.

Introduction to Tape Drive Basics

Tape drives are built a lot like hard disks drives. Internal tape drives are caseless drives designed to be mounted inside a standard hard disk bay. Many have SATA, SAS or SCSI interfaces. External tape drives are standalone units, typically with a SAS, SCSI or USB interface. Rackmount units are also available for data centers.

A 2:1 compression ratio is assumed for most tape formats. A tape with a "real", native or uncompressed capacity of 160 gigabytes will also be stated as having a compressed (effective) capacity of 320 gigabytes.

Read/write speeds are commonly given in megabytes/second. Compressed and uncompressed data speeds are also used. A drive that can transfer 10 megabytes/second will be specified with a compressed transfer speed of 20 megabytes/second.
For a quick estimate of transfer times:

• 1 megabyte/second = 3.6 gigabytes/hour
• 10 megabytes/second = 36 gigabytes/hour
• 100 megabytes/second = 360 gigabytes/hour
  

In this article, only native or uncompressed figures are given for speeds and capacities.
The prices of tape drives given below are for single-tape drives. More expensive autoloader (jukebox or autochanger) tape drives are not considered. Transfer speeds are for a specific model, and can be higher or lower for different brands and models.
Sony discontinued the sale of their AIT tape drives in March 2010, therefore this format will not be covered. Travan (maximum 20 gigabytes) and VXA formats are also not covered.

Ultrium LTO Storage Tapes

LTO (Linear Tape Open) is an open standard from Hewlett-Packard, IBM and Quantum. These are high performance systems, with the higher capacity drives costing a few thousand dollars and tapes reaching $40. The different LTO formats, capacities and speeds are:

• LTO 1 tape, 100 gigabytes, 20 megabytes/second
• LTO 2 tape, 200 gigabytes, 40 megabytes/second
• LTO 3 tape, 400 gigabytes, 80 megabytes/second
• LTO 4 tape, 800 gigabytes, 120 megabytes/second
• LTO 5 tape, 1.5 terabytes (not released yet)
• LTO 6 tape, 3.2 terabytes (not released yet)

How to Choose the Best Backup Tape Format Type

DDS/DAT tapes provide affordable storage for small enterprises, or can be used as dedicated backup for individual servers. VXA backup tapes (not covered in this article) should also be considered. Data centers with large amounts of data and small backup windows will need to look at the larger, faster and more expensive LTO or DLT drives. NAS and RAID disks can replace or complement tape systems.

The LTO website has an overview of LTO Ultrium tape generations. The DAT website has a DAT/DDS roadmap. The Amazon website lists a wide variety of tapes and drives from different manufacturers. The list can be narrowed down by searching for specific tape formats.

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