Data deduplication ratios are related to the number of changes occurring to the data. Each percentage increase in data change drops the ratio; the commonly cited 20:1 ratio is based on average data change rates of approximately 5%.
Vendors assume that compression will reduce deduplicated data by a factor of 2:1. If the deduplication ratio were 15:1, for example, compression could increase that ratio to 30:1. But users with large amounts of data stored in compressed formats, such as jpeg, mpeg or zip, aren't likely to realize the extra bump compression provides.
The length of time data is retained will affect reduction rates. To achieve a ratio of 10:1 or 30:1, you may need to retain and deduplicate a single data set over a 20-week period. If you don't have the capacity to store the data for that long, the data-reduction rate will be lower.
Lastly, full backups give deduplication software a more granular view into the backup, so more frequent full backups will achieve higher ratios.
--Jerome M. Wendt
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This was first published in September 2007