Understanding Replication

Volumes are stored as pieces called containers that contain files, directories, and other data. By default, the maximum container size is 32 GB. The maximum container size is set by the cldb.container.sizemb parameter (see the config commands). Containers are replicated to protect data. Normally, each container has three copies stored on separate nodes to provide uninterrupted access to all data, even if a node fails.

For each volume, you can specify a desired and minimum data replication factor and a desired and minimum namespace (name container) replication factor.

When enabled, the CLDB manages the namespace container replication separate from the data container replication. This capability is used when you have low volume replication but want to have higher namespace replication.

Note: The namespace container parameters, nsreplication or nsminreplication must be the same or larger than the equivalent data replication parameter, replication or minreplication.
  • The replication factor is the number of replicated copies you want for normal operation and data protection. When the number of copies falls below the desired replication factor, but remains equal to or above the minimum replication factor, the CLDB actively creates additional copies of the container while trying to minimize the impact of making an additional copy of the container. Re-replication occurs after the timeout specified in the cldb.fs.mark.rereplicate.sec parameter (configurable using the config API). The minimum replication factor is 1 and the maximum is 6 (default: 3).
  • The minimum replication factor is the smallest number of copies you want in order to adequately protect against data loss. When the replication factor falls below this minimum, re-replication occurs aggressively if data is being actively written to the container. If enforceminreplicationforio property is set to true, writes succeed only when the minimum replication factor requirements are met. If property is set to true and minimum number of copies are not available, the client is asked to retry. In the case of a:
    • Hard mount, the client might try for up to 10 mins and then return an error
    • Soft mount, the client might return an error
    The minimum replication factor is 1 and the maximum is 6 (default: 2). In all cases, the minimum replication factor cannot be greater than the replication factor. When you increase the minimum replication factor, if enforceminreplicationforio property (configurable at the volume level) is set to true, the minimum number of copies requirement will not be enforced during writes until new copies of all containers associated with the volume are created.
  • The namespace replication factor is the number of namespace container replicated copies you want for normal operation and data protection. When the number of copies falls below the desired replication factor, but remains equal to or above the minimum replication factor, the CLDB actively creates additional copies of the container while trying to minimize the impact of making an additional copy of the container. Re-replication occurs after the timeout specified in the cldb.fs.mark.rereplicate.sec parameter (configurable using the config API). The minimum replication factor is 1 and the maximum is 6 (default: 3).
  • The minimum namespace replication factor is the minimum number of namespace container replicated copies you want in order to adequately protect against data loss. When the replication factor falls below this minimum, re-replication occurs aggressively if data is being actively written to the container. If enforcemineplicationforio property (configurable at the volume level) is set to true, writes succeed only when the minimum replication factor requirements are met. If property is set to true and minimum number of copies are not available, the client is asked to retry. In the case of a:
    • Hard mount, the client might try for up to 10 mins and then return an error
    • Soft mount, the client might return an error
    The system will not wait for lost replicas to become available again. The minimum replication factor is 1 and the maximum is 6 (default: 2). In all cases, the minimum replication factor cannot be greater than the replication factor. When you increase the minimum replication factor, if enforceminreplicationforio property is set to true, the presence of the minimum number of copies will not be enforced during writes until new copies of all containers associated with the volume are created.

If any containers in the CLDB volume fall below the minimum replication factor, cluster is inaccessible until aggressive re-replication restores the minimum level of replication. If a disk failure is detected, any data stored on the failed disk is re-replicated without regard to the timeout specified in the cldb.fs.mark.rereplicate.sec parameter.

If all copies of a container, which is neither under nor over replicated, are on the same rack, MapR automatically detects and distributes the copies, such that they are all not on the same rack, after 12 hours. If a container is under replicated and MapR is unable to find a different rack for the new copy, the creation of the copy is deferred. If another rack is unavailable for the new copy after 3 hours, MapR creates a copy of the container on the same rack and if this results in all copies of the container being on the same rack, MapR distributes the copies after 12 hours. Also, during replication, MapR tries to defer the scenarios where all copies end up on the same rack. As per deferring policy:

  • If a container has copies less than the "minimum replication" but greater than 2 and if both copies end up on the same rack, then MapR tries to create the third copy on a different rack for up to 3 hours.
  • If a container has copies more than the minimum but less than the desired and if all copies are on the same rack, then MapR tries to create the next copy on a different rack for up to 3 hours.

If namespace (NS) replication and minimum namespace replication are not set explicitly, they assume the same values as (data) replication and minimum replication respectively. This means that all changes to (data) replication and minreplication parameters are also reflected in nsreplication and nsminreplication. If nsreplication or nsminreplication is modified or specified during creation, nsreplication and nsminreplication start assuming values different from replication and minreplication.