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Similarly, in case of three machines, the ratio becomes 1.5:1; for four machines, it is 1.6:1. It
approaches 2:1 as more such machines are backed up to the same vault. This means that you can
buy, say, a 10-TB storage device instead of a 20-TB one.
The actual amount of capacity reduction is influenced by numerous factors such as the type of data
that is being backed up, the frequency of the backup, and the backups' retention period.
2.14.6.6
Deduplication restrictions
Block-level deduplication restrictions
During a disk backup to an archive in a deduplicating vault, deduplication of a volume's disk blocks is
not performed in the following cases:
If the volume is a compressed volume
If the volume's allocation unit size—also known as cluster size or block size—is not divisible by
4 KB
Tip:
The allocation unit size on most NTFS and ext3 volumes is 4 KB and so allows for block-level
deduplication. Other examples of allocation unit sizes allowing for block-level deduplication include 8 KB,
16 KB, and 64 KB.
If you protected the archive with a password
Tip:
If you want to protect the data in the archive while still allowing it to be deduplicated, leave the archive
non-password-protected and encrypt the deduplicating vault itself with a password, which you can do
when creating the vault.
Disk blocks that were not deduplicated are stored in the archive as they would be in a non-
deduplicating vault.
File-level deduplication restrictions
During a file backup to an archive in a deduplicating vault, deduplication of a file is not performed in
the following cases:
If the file is encrypted and the
In archives, store encrypted files in decrypted state
check box in
the backup options is cleared (it is cleared by default)
If the file is less than 4 KB in size
If you protected the archive with a password
Files that were not deduplicated are stored in the archive as they would be in a non-deduplicating
vault.
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.
