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Several operations are available to manipulate replication sessions 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 at a particular point in time.
Pause and resume are used to stop and start replication between the resources for a particular replication session. In Unisphere, the pause operation can only be issued from the source system. If the session is paused while a sync is in progress, all incremental changes on the destination are discarded and the destination is reverted back to the common base. All I/O is tracked when the replication session is paused. When the session is resumed, replication will resume and synchronizations to the destination storage resource will be based on the current RPO if asynchronous replication is being used. When a manual replication session is resumed, no replication updates occur until a manual sync is issued. While an asynchronous and manual replication session is paused, no sync updates can be performed on the session.
The resume option is also available after a failover operation. If a resume operation is initiated, replication begins running in the reverse direction. After a failover, it is possible for changes to be made to the destination resource. When running the resume operation, a checkbox is available to resync and overwrite any data written to the remote system. This checkbox may be necessary in situations where there are different changes on each system. The administrator must acknowledge they want replication to overwrite any changes made to the other system before the resume operation can complete successfully, as shown in Figure 29.
Pausing a replication session may be done for several reasons to stop updates from occurring to the destination. 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. Depending on the network speeds between sites, this may save time and avoid lengthy initial synchronizations.
With asynchronous replication, updates to a destination storage resource happen at a set interval based on the defined RPO. At any point in time when replication is active, and an update is not already occurring, a sync operation can be issued to synchronize the latest changes to the destination resource. The sync operation is also used to update a remote image when manual replication is configured. After the sync operation is selected, all data changed since the last update will be copied to the destination storage resource. Issuing a manual sync operation also updates a destination image’s size when the source image size has been changed.
When using asynchronous replication, the failover with sync option allows you to failover the session after first completing a last synchronization between the images. The failover with sync option is used in planned failover events. The failover with sync option is only available on the source storage resource when the replication session is operating normally and will result in a short period of data being unavailable during the failover operation. Before issuing the failover with sync operation, we need to ensure that the RPO has not surpassed the one-hour mark since last sync. In such scenario, we need to issue a manual sync first, wait until it completes to then run a failover with sync. When issuing the failover with sync, it is highly suggested to quiesce I/O to the source image first. After the synchronization completes, the destination storage resource is available for production I/O and the original source will no longer allow Read/Write I/O. If host access is configured on the destination resource, hosts can access the data at this time. Replication will not be resumed in either direction when the failover with sync operation is used.
The failover option is only available on the destination of the replication session. When issuing a failover operation on a replication session from the destination system, an unplanned failover is initiated, and a final synchronization of the data from the source storage resource is not completed. An unplanned failover assumes a disaster has occurred on the production system, and the destination image is made Read/Write available. When failover is selected on a destination resource of a replication session, Read/Write access is removed from the original source if the source is available to receive management commands. 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. If I/O occurs to the original destination resource while in this state, the data must be replicated to the original source when the source becomes available.
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 failover with sync or failover option is used, the failback option becomes available. Failback restores the replication session to the state before failover with sync or failover was issued. After a failover, it is possible for changes to be made to the destination resource. When running a failback operation, the following options are available:
These options may be necessary in situations where there are different changes on each system. The administrator must acknowledge which changes they want to keep before the failback operation can complete successfully.
Alternatively, running a resume, failover with sync, and another resume from the other system results in the same behavior as the first option. Once the failover with sync operation completes, the original configuration and replication direction is applied. Note that once a resume operation is initiated, the failback option is no longer available since replication is already running in the reverse direction. The failback options are shown in Figure 30.
When the original source resource becomes available, the user has the option of issuing the Resume operation to replicate data to the original source. Clicking resume will reverse the direction of replication and resume updates based on the user RPO from the original destination resource to the original source. A manual sync must be issued when using manual replication. To return replication to the original replication configuration, the failover with sync operation can be issued on the original destination. A final synchronization of the data is performed, and the replication will be failed over to the original source. It is suggested to quiesce all I/O before issuing the failover with sync operation. The original source will become Read/Write available and host access will be removed from the destination resource. You must issue Resume on the source to resume replication from the source to destination.
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 may need to occur if replication is reconfigured. In Dell UnityOS 5.1 and later, a replicated snapshot may be used to avoid a full synchronization under certain circumstances. This feature is discussed in the Common Base Snapshots section of this paper.
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.
Dell UnityOS version 4.2 introduces the feature group operations at a NAS Server level. Group operations allow for the replication operation to be propagated from the NAS Server to the file systems automatically and works with local and remote asynchronous replication. The group operations are available for the following operations: Failover, Failover with Sync, Failback, Pause, and Resume. These operations remain available at the file system level. All other operations (Create, Sync, Delete, and Modify) remain individual commands. Before issuing a failover with sync operation, it is suggested to issue a manual sync first, to reduce the amount of data to copy during the failover with sync.
Do not perform a group operation at both sides of a replication session simultaneously. This action is not prohibited by the storage system, however, a group operation performed simultaneously at both sides of a replication session can cause the group replication session to enter an unhealthy state. Although a group asynchronous replication session looks like one operation, each file system is replicated individually. If any of the individual file system replication sessions fail, you can resolve the issue and then select the individual file system to replicate. Group operations skips file system replication sessions that are in a paused, error, or non-replicated state. Figure 69 in the Viewing the Replication Sessions section provides an example for the group Failover operation.
Dell UnityOS 5.2 expands the system level replication actions by including the pause and resume actions at the replication connection level. As shown in Figure 31, the system level actions can be found from the Protection & Mobility > Replication > Connections page. After selecting one of the connections, select the More Actions dropdown and see the system level actions. The system level resume and pause actions will impact the replication sessions for both file and block resources. When a replication connection is selected and pause or resume is issued, all replication sessions replicating to and from the selected system are either paused or resumed.
By default, selecting to pause or resume at the system level, would impact the replication sessions of the same type. However, the user can select which replication session type (Asynchronous, Synchronous, or Both) is intended to be paused or resumed, as shown in Figure 32.
If the remote Dell Unity system is running UnityOS version prior to 5.2, only sessions with source role on local Unity system would be paused or resumed. If a system reboots during a pause or resume action, the interconnection between the systems needs to be re-established before the action can succeed. While there is no interconnection between the system, the pause and resume jobs would fail.
As seen in Figure 31, the failover can be run at the replication connection level. This action is only applicable for the replication sessions for NAS Servers and its file systems and is equivalent to running a failover operation on the destination resource. Figure 33 shows the confirmation window when a user selects to failover action at the system level.