60 Second Cram of tape drive capacities
DDS/DAT Drives
DDS1 2GB/4GB
DDS2 8GB/16GB
DDS3 12GB/24GB
DDS4 20GB/40GB
DDS5 36GB/72GB
DDS6 80/160GB
AIT Drives
AIT1 40Gb/100GB
AIT2 80GB/200GB
AIT3 100GB/260GB
LTO/Ultrium Drives
LTO1 100GB/200GB
LTO2 200GB/400GB
LTO3 400Gb/800GB
LTO4 800Gb/1600GB
Tape drive compression (confusion)
All tape drive manufacturers list 2 capacity figures in the specification of their tape drives, a lower capacity and a higher capacity. This is confusing and catches a lot of people out.
For example a DAT72 might imply it can backup 72GB of data to tape, but in the real world you could expect to see probably somewhere between 40-55GB of data being backed up.
Again a Ultrium 100GB/200GB might imply that you could backup 200GB of data onto tape, in the real world you might expect to see probably somewhere between 130GB to 160GB of data going to tape.
Why is this? It is important to remember that the reason 2 different capacity values are listed is due to the type of technology used in these modern tape drive’s hardware. The lower capacity value is generally the guaranteed capacity. The higher value can be misleading, but basically modern tape drives use compression. As data is transferred to tape, the tape drive hardware will compress data in order to use the tape media more efficiently.
Different types of data can compress at different rates. I.e. databases can be compressed more efficiently than say pictures or mp3s.
The higher capacity rating is generally the total capacity of the tape drive assuming — it can compress all your data at a 2:1 ratio. We all live in the real world where we can have a range of different applications and data stored on our computer systems, in our experience the higher capacity has never been attained and you should not rely on the higher compressed capacity to backup all of your data.