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SNIA defines Single Instance Store (SIS) as the replacement of duplicate files or objects with a reference to a shared copy1. While Single Instance Storage is not defined as a deduplication technology by SNIA, many people confuse SIS with deduplication.

The key difference between SIS and deduplication is that SIS evaluates the data stream at the file level by looking for duplicate files while deduplication evaluates the data stream at the block or byte level. With SIS, even a small change to a file will cause it to be seen as a new and different file and be stored again. This means, that if you have a document and a user copies or renames the file, it will be seen as an entirely new file and will be stored again. With deduplication, the entire file contents will be seen as duplicate. As a result SIS delivers less space savings versus deduplication.