Installation Guide

2.4. Understanding Data Redundancy
S3 and Acronis Backup Gateway storage access points, that is, for the S3 (public) and Acronis Backup
Gateway roles.
2.4 Understanding Data Redundancy
Acronis Storage protects every piece of data by making it redundant. It means that copies of each piece of
data are stored across different storage nodes to ensure that the data is available even if some of the storage
nodes are inaccessible.
Acronis Storage automatically maintains the required number of copies within the cluster and ensures that all
the copies are up-to-date. If a storage node becomes inaccessible, the copies from it are replaced by new ones
that are distributed among healthy storage nodes. If a storage node becomes accessible again after downtime,
the copies on it which are out-of-date are updated.
The redundancy is achieved by one of two methods: replication or erasure coding (explained in more detail in
the next section). The chosen method affects the size of one piece of data and the number of its copies that
will be maintained in the cluster. In general, replication offers better performance while erasure coding leaves
more storage space available for data (see table).
Acronis Storage supports a number of modes for each redundancy method. The following table illustrates data
overhead of various redundancy modes. The first two lines are replication and the rest are erasure coding.
Redundancy
mode
Minimum num-
ber of nodes re-
quired
How many nodes can
fail without data loss
Storage overhead,
%
Raw space required
to store 100GB of data
2 replicas 2 1 100 200GB
3 replicas 3 2 200 300GB
Encoding 3+2 5 2 67 167GB
Encoding 5+2 7 2 40 140GB
Encoding 7+2 9 2 29 129GB
Encoding 17+3 20 3 18 118GB
You choose a data redundancy mode when configuring storage access points and their volumes. In particular,
when:
• creating LUNs for iSCSI storage access points,
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