User`s guide

What ratio to expect

Although, in some situations, the deduplication ratio may be very high (in the previous example,

increasing the number of machines would lead to ratios of 3:1, 4:1, etc.), a reasonable expectation for

a typical environment is a ratio between 1.2:1 and 1.6:1.

As a more realistic example, suppose that you are performing a file-level or disk-level backup of two

machines with similar disks. On each machine, the files common to all the machines occupy 50% of

disk space (say, 1 GB); the files that are specific to each machine occupy the other 50% (another

1 GB).

In a deduplicating vault, the size of the first machine's backup in this case will be 2 GB, and that of the

second machine will be 1 GB. In a non-deduplicating vault, the backups would occupy 4 GB in total. As

a result, the deduplication ratio is 4:3, or about 1.33:1.

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