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Once a replication session has been created, several operations are available to manipulate the replication session as needed. Not all operations are available at all times, as some depend on the session being in a particular state. Also, certain operations will perform differently depending on which system they are issued on, source or destination. Only one replication operation can be issued and running per session at a particular point in time.
Pause and resume are used to stop and start replication between the systems for a particular replication session. In Unisphere, the pause operation can only be issued from the source system. While a session is paused, all write I/O to a storage resource is only saved on the source system before being acknowledged to the host. All I/O is tracked when the replication session is paused, and when the session is resumed, all blocks not already copied to the destination will be done at this time. During this time, the state of the replication session will be “Syncing”. Once all I/O is copied to the destination, the state will change to “In Sync”.
Pausing a replication session may be done for a number of reasons. Some reasons include the need to power off the source or destination system for planned maintenance, a configuration change on the network between systems, or interface changes on either system. Another reason may be to physically move a system from one data center or site to another. In certain circumstances, configuring replication and synchronizing the data between systems may be done within the same site, then the destination system is later moved to its final destination.
When issuing the failover operation, the current destination storage resource becomes the production resource. Before issuing the failover operation, it is highly suggested to quiesce I/O to the source image first. Once the failover operation is initiated the destination resource will be available for Read/Write operations and the original source resource will no longer be available for reads or writes. If host access is configured on the destination resource, hosts can access the data at this time. The effects of issuing the failover operation will depend on which system the failover was initiated from.
A failover issued from the production system is also referred to as a planned failover. The destination resource will become Read/Write available, and the direction for replicating data will switch. When this occurs, the original destination system will start replicating all new writes it receives for the storage resource to the original source system for the replication session. Issuing a failover from the source is suggested when testing a site failover to ensure the DR configuration is working properly. To return the session to its original state, first quiesce I/O to the original destination storage resource. Then, issue a failover from the original destination system. This will make the original storage resource the production image and start replication in the original direction.
If a failover operation is issued from a destination system, an unplanned failover or disaster situation is initiated. An unplanned failover assumes a disaster has occurred on the production system, and the destination image is made Read/Write available. The replication session will also pause and not automatically switch the direction for replication. The replication session is left in this state until the user issues another replication operation.
Starting with Dell UnityOS version 5.0, an unplanned failover operation can be initiated even when the replication is in a “Paused’ state. Previously, unplanned failover operations were not allowed while the session was in a “Paused” state. Any changes made on the source system while the session is paused are not replicated to the destination.
When the original production system becomes available, the user has the option of issuing the Resume operation on the session from the original destination to replicate data to the original source which will result in a full sync. Then, at a later time, they can issue the Failover operation from the destination system to return to the original state for replication, from the production system to the destination system. The user also has the option of issuing the Failback operation from the destination system when the source becomes available. The Failback operation will do a full synchronization of the storage resource on the source system with the data on the destination resource, block access to the destination storage resource, and make the original production resource Read/Write. Replication will also then be resumed from the production system to the destination system for the replication session. It is suggested to quiesce all I/O before issuing the Failback operation.
Deleting a replication session can be issued on the source system or destination system, but it is recommended that the operation be issued on the source when the source is available. When there are no issues in the configuration and a Delete operation is issued on the source system, the replication session will be deleted from both the source and destination system. The destination storage resource is not automatically deleted when the replication session is deleted. If the delete operation is issued while the destination system cannot be reached, the session will need to be deleted from the destination system manually. If the delete operation is issued from the destination system, the source session is left configured and must be deleted manually. Once a replication session is deleted, a full sync will need to occur if replication is reconfigured.
A delete operation can also be issued for a replication connection. A replication connection can only be deleted after all configured replication sessions using the connection have been deleted.