An example of estimating backup space requirements
A user estimates usage and retention patterns
- Retention: number of daily backups, number of weekly backups.
- Usage: size of each daily and weekly incremental backup
The space required to backup storage is estimated
Sb = So * [ 1 + Rd*(#d-1) + Rw*(#w-1) ]
where:
- Sb is the amount of backup space required
- So is online space being backed up
- #d is number of daily copies
- #w is number of weekly copies
- Rd is the size ratio of a daily backup to a full backup
- Rw is the size ratio of a weekly backup to a full backup
And allocated
- You want to back up about 5 GB of online storage.
(So is estimated at 5GB)
- You want to keep 12 daily backups and 9 weekly backups.
(#d is 12, #w is 9)
- A daily backup is estimated to involve 10% of the online files.
(Rd is estimated at .1)
- A weeky backup is estimated to involve 20% of online files.
(Rw is estimated at .2)
So the backup space evaluates as
Sb = 5 GB * [1+.1*(11) + .2*(8)] = 5 GB * 3.7 = 18.5 GB
So the recommended purchase of backup space would be 20 GB, costing $32 per year.
Initial estimates need not be perfect, because changes based upon experience and usage can be made quarterly.
Last Updated: April 5, 2006
