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Figure 2: Disk Space requirements
Data stored with
Data stored normally
deduplication
1st daily full backup
500 GB
500 GB
1st daily incremental backup
50 GB
5 GB
2nd daily incremental backup
50 GB
5 GB
3rd daily incremental backup
50 GB
5 GB
4th daily incremental backup
50 GB
5 GB
5th daily incremental backup
50 GB
5 GB
2nd weekly full backup
500 GB
25 GB
3rd weekly full backup
500 GB
25 GB
25th weekly full backup
500 GB
25 GB
Total
12,750 GB
1,125 GB
This example uses a system containing 500 GB of backup data that equates to 500 GB of storage for the
first traditional full backup. If 10% of the files change between backups, then a traditional incremental
backup would send about 10% of the size of the full backup or about 50 GB to the backup device.
However, because data deduplication operates at the block level, instead of the file level, in actuality only
a 1% change in the data has occurred. This means only 5 GB of block level changes or 5 GB of data
stored with deduplication. Over time, the savings multiply. When the next full backup is stored, it will not
be 500 GB. With deduplication the equivalent full backup is only 25 GB. A backup system with data
deduplication enabled would use the same amount of storage in six months that would typically be
required to store only one week of traditional backup data. Over a 6 month period data deduplication
would provide an 11:1 effective savings in storage capacity. It also provides the ability to restore from
further back in time without having to go to physical tape for the data. The key thing to remember here is
that the deduplication ratio depends primarily on two things:
• What percentage of the data is changing between backups (percentage of data in percentage
of files)
• How long is the retention period of the backups stored on disk
For example, a 0.5% daily change in the data in 10% of the files would yield a 50:1 deduplication ratio
over one year of daily full backups. Obviously, the percentage daily change rate is quite difficult to predict
for complex systems, especially for applications like Exchange, SQL, and Oracle so benchmarking is
strongly advised.
As already indicated, backup data retention period and backup data change rate matters to find out what
the approximate deduplication ratio will be. Figure 3 shows the approximate space saving based on the
given daily change rate and backup policy.
5
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