How Many Cartridges Will I Need to Backup My Data?
Insights December 17, 2008

Based on a recent study and suggestions I plan to come up with the post describing approximately how many tapes will we need to backup the data.

Ideal Data Transfer Rate = Amount of data to backup / Backup Window

Suppose you have 100 GB of data to backup and an 8 hour window then ideal data transfer rate would be 12.5 GB/hr . You would choose the Drive based on that requirement. After selecting the appropriate drive technology that provides the performance and selecting the cartridge capacity you need, we will focus on the how many cartridges you will need to have available.The number of cartridges depends upon the amount of data that you are backing up, the frequency of your backup, your retention period and the capacity of media used to store your backups.

A simple formula that can be used is

Number of Tapes= (total data to backup * Frequency of backup * Retention Period)/Tape Capacity

Following is an example:-

Total amount of data to backup                       =          100GB

Full Backups per month                                   =          4

Retention Period for full backup                     =          6 months

Incremental backups per month                       =          30

Retention Period for Incremental backups       =          1 month

Preliminary Calculation is:-

Size of full backup = 100GB * 4 per month * 6 months =2.4 TB

Size of Incremental backup = (20 percent of 100 GB) * 30 * 1month = 600GB

Total Data stored = 2.4 TB + 600 GB =3TB


Tape Drive = DLT 7000

Tape capacity without compression = 31.5 GB

Total Tapes needed for full backup = 2.4 TB /31.5 GB =76.2=77

Total Tapes needed for Incremental backup = 600 GB /31.5 GB=19.1=20

Total Tapes needed = 77 +20 =97.

Thus we would expect to have a minimum of 97 active cartridges at any given time. This also assumes that the cartridges will be filled to capacity and there will be no unused tape. These calculations are based on no compression. This does not give you idea of the steps necessary to plan for an appropriately sized tape library. It is never recommended implementing an enterprise backup strategy that does not include a robotic tape library with a barcode reader.