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27 Copyright 2015 EMC Corporation. All rights reserved. When the source site has been restored, or if maintenance is completed, you can return production to the source site. The symrdf failback command will set the R2s Write Disabled, the link Read Write and the R1s Read Write enabled. Merging of the device track tables between the source and target is done. The SRDF links are resumed. The accumulated invalid tracks are transferred to the source devices from the target devices. So all changes made to the data when in a failed over state will be preserved. As noted earlier, the Primary host can access the R1 devices and start production work as soon as the command completes. If a track that has not yet been sent over from the R2 is required on the R1, SRDF can preferentially read that track from across the links. As the R2s will be set to Write Disabled, it is important to shut down the applications using the R2 devices, and perform the appropriate host dependent steps to unmount filesystem/deactivate volume groups. If applications still actively access R2s when they are being set to Write Disabled, the reaction of the host accessing R2s will be unpredictable. In a true disaster, the failover process may not give an opportunity for a graceful shutdown. But a failback event should always be planned and done gracefully. SRDF/Synchronous Operations
28 Copyright 2015 EMC Corporation. All rights reserved. As can be seen in the output, the R1s are set to Read Write, R2s are set to Write Disabled, and the links are set to Read Write. The pair states go into SyncInProg. The accumulated invalid tracks have been transferred from the target array to the source array. Once all accumulated invalid tracks have been transferred, the pair state will go into Synchronized. Applications accessing the R2 devices must be stopped before a failback operation as the R2 devices are set to write-disabled. When a host suddenly loses RW access to a device while still actively accessing it, the results are unpredictable to say the least. SRDF/Synchronous Operations
29 Copyright 2015 EMC Corporation. All rights reserved. This lesson covered SRDF Disaster Recovery operations. Device and link states under different conditions were presented in detail. Host consideration when performing DR operations were also discussed. SRDF/Synchronous Operations
30 Copyright 2015 EMC Corporation. All rights reserved. This lesson covers SRDF Decision Support operations. Considerations for performing these operations are presented in detail. Concurrent SRDF where one R1 device is simultaneously paired with two R2 devices is also discussed. SRDF/Synchronous Operations
31 Copyright 2015 EMC Corporation. All rights reserved. The decision support operations for SRDF devices are: Split an SRDF pair which stops mirroring for the SRDF pairs in a device group.
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