How to calculate the size of your NetBackup image database
An important factor when you design your backup system is to calculate how much disk space is needed to store your NetBackup image database. Your image database keeps track of all the files that have been backed up.
The image database size depends on the following variables, for both full backups and incremental backups:
The number of files being backed up
The frequency and the retention period of the backups
You can use either of two methods to calculate the size of the NetBackup image database. In both cases, since data volumes grow over time, you should factor in expected growth when calculating total disk space used.
NetBackup can be configured to automatically compress the image database to reduce the amount of disk space required. When a restore is requested, NetBackup automatically decompresses the image database, only for the time period needed to accomplish the restore. You can also use archiving to reduce the space requirements for the image database. More information is available on catalog compression and archiving.
See the NetBackup Administrator's Guide, Volume I.
Note:
If you select NetBackup's true image restore option, your image database becomes larger than an image database without this option selected. True image restore collects the information that is required to restore directories to their contents at the time of any selected full or incremental backup. The additional information that NetBackup collects for incremental backups is the same as the information that is collected for full backups. As a result, incremental backups take much more disk space when you collect true image restore information.
First method: You can use this method to calculate image database size precisely. It requires certain details: the number of files that are held online and the number of backups (full and incremental) that are retained at any time.
To calculate the size in gigabytes for a particular backup, use the following formula:
Image database size = (132 * number of files in all backups)/ 1GB
To use this method, you must determine the approximate number of copies of each file that is held in backups and the typical file size. The number of copies can usually be estimated as follows:
Number of copies of each file that is held in backups = number of full backups + 10% of the number of incremental backups held
The following is an example of how to calculate the size of your NetBackup image database with the first method.
This example makes the following assumptions:
Number of full backups per month: 4
Retention period for full backups: 6 months
Total number of full backups retained: 24
Number of incremental backups per month: 25
Retention period for incremental backups per month: 1 month
Total number of files that are held online (total number of files in a full backup): 17,500,000
Solution:
Number of copies of each file retained:
24 + (25 * 10%) = 26.5
NetBackup image database size for each file retained:
(132 * 26.5 copies) = 3498 bytes
Total image database space required:
(3498 * 17,500,000 files) /1 GB = 61.2 GB
Second method: Multiply by a small percentage (such as 2%) the total amount of data in the production environment (not the total size of all backups). Note that 2% is an example; this section helps you calculate a percentage that is appropriate for your environment.
Note:
You can calculate image database size by means of a small percentage only for environments in which it is easy to determine the following: the typical file size, typical retention policies, and typical incremental change rates. In some cases, the image database size that is obtained using this method may vary significantly from the eventual size.
To use this method, you must determine the approximate number of copies of each file that are held in backups and the typical file size. The number of copies can usually be estimated as follows:
Number of copies of each file that is held in backups = number of full backups + 10% of the number of incremental backups held
The multiplying percentage can be calculated as follows:
Multiplying percentage = (132 * number of files that are held in backups / average file size) * 100%
Then, the size of the image database can be estimated as:
Size of the image database = total disk space used * multiplying percentage
The following is an example of how to calculate the size of your NetBackup image database with the second method.
This example makes the following assumptions:
Number of full backups per month: 4
Retention period for full backups: 6 months
Total number of full backups retained: 24
Number of incremental backups per month: 25
Retention period for incremental backups per month: 1 month
Average file size: 70 KB
Total disk space that is used on all servers in the domain: 1.4 TB
Solution:
Number of copies of each file retained:
24 + (25 * 10%) = 26.5
NetBackup image database size for each file retained:
(132 * 26.5 copies) = 3498 bytes
Multiplying percentage:
(3498/70000) * 100% = 5%
Total image database space required:
(1,400 GB * 5%) = 70 GB