232 Copyright © Acronis International GmbH, 2002-2015
The deduplication is performed on data blocks. The block size is 4 KB for disk-level backups and 1 B
to 256 KB for file-level backups. Each file that is less than 256 KB is considered a data block. Files
larger than 256 KB are split into 256-KB blocks.
Acronis Backup performs deduplication in two steps:
Deduplication at source
Performed on a managed machine during backup. The agent uses the storage node to determine
what data can be deduplicated and does not transfer the data blocks whose duplicates are
already present in the vault.
Deduplication at target
Performed in the vault after a backup is completed. The storage node analyses the vault's
contents and deduplicates data in the vault.
When creating a backup plan, you have the option to turn off deduplication at source for that plan.
This may lead to faster backups but a greater load on the network and storage node.
Deduplication database
Acronis Backup Storage Node managing a deduplicating vault, maintains the deduplication database,
which contains the hash values of all data blocks stored in the vault—except for those that cannot be
deduplicated, such as encrypted files.
The deduplication database is stored in the storage node local folder. You can specify the database
path when creating the vault.
The size of the deduplication database is about 1.5 percent of the total size of unique data stored in
the vault. In other words, each terabyte of new (non-duplicate) data adds about 15 GB to the
database.
If the database is corrupted or the storage node is lost, while the vault retains its contents, the new
storage node rescans the vault and re-creates the vault database and then the deduplication
database.
7.5.7.2 How deduplication works
Deduplication at source
When performing a backup to a deduplicating vault, Acronis Backup Agent calculates a fingerprint of
each data block. Such a fingerprint is often called a hash value.
Before sending the data block to the vault, the agent queries the deduplication database to
determine whether the block's hash value is the same as that of an already stored block. If so, the
agent sends only the hash value; otherwise, it sends the block itself. The storage node saves the
received data blocks in a temporary file.
Some data, such as encrypted files or disk blocks of a non-standard size, cannot be deduplicated. The
agent always transfers such data to the vault without calculating the hash values. For more
information about restrictions of deduplication, see Deduplication restrictions (p. 236).
Once the backup process is completed, the vault contains the resulting backup and the temporary
file with the unique data blocks. The temporary file will be processed on the next stage. The backup
(TIB file) contains hash values and the data that cannot be deduplicated. Further processing of this
backup is not needed. You can readily recover data from it.
