Redundancy modes

Virtuozzo Hybrid Infrastructure supports a number of modes for each redundancy method. Only predefined redundancy modes are available in the admin panel. The following table illustrates data overhead of various redundancy modes. The first three lines are replication and the rest are erasure coding.

The numbers of nodes listed in the table indicate only the requirements of each redundancy method but not the number of nodes needed for the Virtuozzo Hybrid Infrastructure cluster. The minimum and recommended cluster configurations are described in Quantity of servers.

Redundancy mode Nodes required to store data copies How many nodes can fail without data loss Storage overhead, percent Raw space needed to store 100 GB of data
1 replica (no redundancy) 1 0 0 100 GB
2 replicas 2 1 100 200 GB
3 replicas 3 2 200 300 GB
Encoding 1+0 (no redundancy) 1 0 0 100 GB
Encoding 1+1 2 1 100 200 GB
Encoding 1+2 3 1 200 300 GB
Encoding 3+1 4 1 33 133 GB
Encoding 3+2 5 2 67 167 GB
Encoding 5+2 7 2 40 140 GB
Encoding 7+2 9 2 29 129 GB
Encoding 17+3 20 3 18 118 GB

The 1+0, 1+1, 1+2, and 3+1 encoding modes are meant for small clusters that have insufficient nodes for other erasure coding modes but will grow in the future. As a redundancy type cannot be changed once chosen (from replication to erasure coding or vice versa), this mode allows you to choose erasure coding even if your cluster is smaller than recommended. Once the cluster has grown, more beneficial redundancy modes can be chosen.

You select a data redundancy mode when configuring storage services and creating storage volumes for virtual machines. No matter what redundancy mode you select, it is highly recommended to be protected against a simultaneous failure of two nodes, as that happens often in real-life scenarios.

By default, all encoding modes, except 1+0, allow write operations when one storage node or disk is inaccessible. When redundancy is 1, that is with the N+1 encoding mode, and two inaccessible storage nodes, the data may become unavailable even for reading and there is a high risk of data loss. Therefore, it is strongly recommended to use the encoding modes N+2 or N+3.