Deduplication restrictions

Common restrictions

Encrypted backups cannot be deduplicated. If you want to use deduplication and encryption at the same time, leave the backups unencrypted and direct them to a location where both deduplication and encryption are enabled.

Disk-level backup

Deduplication of disk blocks is not performed if the volume's allocation unit size—also known as cluster size or block size—is not divisible by 4 KB.

The allocation unit size on most NTFS and ext3 volumes is 4 KB. This allows for block-level deduplication. Other examples of allocation unit sizes allowing for block-level deduplication include 8 KB, 16 KB, and 64 KB.

File-level backup

Deduplication of a file is not performed if the file is encrypted.

Deduplication and NTFS data streams

In the NTFS file system, a file may have one or more additional sets of data associated with it—often called alternate data streams.

When such file is backed up, so are all its alternate data streams. However, these streams are never deduplicated—even when the file itself is.