Acronis Backup for Windows Server Essentials - User Guide - Manual (Page 227)

227 Copyright © Acronis International GmbH, 2002-2014
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. 231).
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.
Deduplication at target
After a backup to a deduplicating vault is completed, the storage node runs the indexing activity. This
activity deduplicates the data in the vault as follows:
1. It moves the data blocks from the temporary file to a special file within the vault, storing
duplicate items there only once. This file is called the deduplication data store.
2. It saves the hash values and the links that are necessary to "assemble" the deduplicated data to
the deduplication database.
3. After all the data blocks have been moved, it deletes the temporary file.
As a result, the data store contains a number of unique data blocks. Each block has one or more
references from the backups. The references are contained in the deduplication database. The
backups remained untouched. They contain hash values and the data that cannot be deduplicated.