ECS 3.5.0.1 Administration Guide

TSO behavior with the ADO bucket setting turned on

Turning the ADO setting on for a bucket marks the bucket, and all of the objects in the bucket, as available during an outage. You can turn the ADO setting on for a bucket so that the primary copies of the objects in that bucket are available, even when the site that owns the bucket fails. If the ADO setting is turned off for a bucket, the read/write requests for the objects in the bucket that is owned by a failed site cannot be made from the other sites.

When you turn the ADO setting on, the following occurs during a TSO:

• Object data is accessible for both read and write operations during the outage.
• File systems within file system-enabled (HDFS/NFS) buckets that are owned by the unavailable site are read-only during an outage.

You can turn the ADO setting on when you create a bucket, and you can change this setting after the bucket is created (as long as all sites are online.) You can turn the ADO setting on when creating a bucket from the following interfaces:

• ECS Portal (see Create a bucket)
• ECS Management REST API
• ECS CLI
• Object API REST interfaces such as S3, Swift, and Atmos

With the ADO setting turned on for a bucket and upon detecting a temporary outage, read/write requests from applications that are connected to a nonowner site are accepted and honored, as shown in the following illustration.

The ECS system operates under the eventual consistency model during a TSO with ADO turned on for buckets. When a change is made to an object at one site, it will be eventually consistent across all copies of that object at other sites. Until enough time elapses to replicate the change to other sites, the value might be inconsistent across multiple copies of the data at a particular point in time.

An important factor to consider is that turning the ADO setting on for buckets has performance consequences; ADO-enabled buckets have slower read/write performance than buckets with the ADO turned off. The performance difference is because when ADO is turned on for a bucket, ECS must first resolve object ownership to provide strong consistency when all sites become available after a TSO. When ADO is turned off for a bucket, ECS does not have to resolve object ownership because the bucket does not enable change of object ownership during a TSO.

The benefit of the ADO setting is that it enables you to access data during temporary site outages. The disadvantage is that the data returned may be outdated and read/write performance on ADO buckets will be slower.

By default, the ADO setting is turned off because there is a risk that object data retrieved during a TSO is not the most recent.

TSO behavior with the ADO bucket setting that is turned on is described for the following ECS system configurations:

• Two-site geo-federated deployment with ADO-enabled buckets
• Three-site active federated deployment with ADO-enabled buckets
• Three-site passive federated deployment with ADO-enabled buckets