ECS 3.5.0.1 Administration Guide

Configurations for availability, durability, and resilience

Depending on the number of sites in the ECS system, different data protection schemes can increase availability and balance the data protection requirements against performance. ECS uses the replication group to configure the data protection schemes (see Introduction to storage pools, VDCs, and replication groups). The following table shows the data protection schemes that are available.

Local Protection

Data is protected locally by using triple mirroring and erasure coding which provides resilience against disk and node failures, but not against site failure.

Full Copy Protection

When the Replicate to All Sites setting is turned on for a replication group, the replication group makes a full readable copy of all objects to all sites within the replication group. Having full readable copies of objects on all VDCs in the replication group provides data durability and improves local performance at all sites at the cost of storage efficiency.

Active

Active is the default ECS configuration. When a replication group is configured as Active, data is replicated to federated sites and can be accessed from all sites with strong consistency. If you have two sites, full copies of data chunks are copied to the other site. If you have three or more sites, the replicated chunks are combined (XOR'ed) to provide increased storage efficiency.

When data is accessed from a site that is not the owner of the data, until that data is cached at the non-owner site, the access time increases. Similarly, if the owner site that contains the primary copy of the data fails, and if you have a global load balancer that directs requests to a non-owner site, the non-owner site must recreate the data from XOR'ed chunks, and the access time increases.

Passive

The Passive configuration includes two, three, or four active sites with an additional passive site that is a replication target (backup site). The minimum number of sites for a Passive configuration is three (two active, one passive) and the maximum number of sites is five (four active, one passive). Passive configurations have the same storage efficiency as Active configurations. For example, the Passive three-site configuration has the same storage efficiency as the Active three-site configuration (2.0 times storage overhead).

In the Passive configuration, all replication data chunks are sent to the passive site and XOR operations occur only at the passive site. In the Active configuration, the XOR operations occur at all sites.

If all sites are on-premise, you can designate any of the sites as the replication target.

If there is a backup site hosted off-premise by a third-party data center, ECS automatically selects it as the replication target when you create a Passive geo replication group (see Create a replication group). If you want to change the replication target from a hosted site to an on-premise site, you can do so using the ECS Management REST API.