The REC (Remote Equivalent Copy) function creates (additional) copies of the application data of a local storage system in other, geographically distant (remote) storage systems. When it has been split from the original data, the copied data can then be edited separately. REC offers:
Synchronous and asynchronous replication to one or more remote storage systems. This makes sense in particular for high availability, disaster protection and migration scenarios.
Cascaded remote copy and concurrent remote copy
These functions are used particularly in high availability and disaster protection configurations when remote replication in the local area is to be combined with remote replication over a long distance.
Remote replication consists of an original volume as the source unit which is mirrored to a target unit in a remote storage system.
For replication purposes the local and remote storage systems are connected to each other over at least two remote links.
The two storage systems can be up to 10 km apart for synchronous replications. Longer distances between two storage systems are possible if extensions are used.
As REC is operated on the storage system itself independently of the application and operating system version, any application can use REC.
Remote copy pair (single REC configuration)
The remote copy pair consists of a source unit (original) and a target unit (data copy) which are both of the same size and of the same type.
The source unit is in the storage system that receives the writes in normal remote copy mode. It sends the updated data to the remote storage system, which saves the data on the target unit.
The remote copy pair is generated dynamically using the SHC-OSD command /START-REMOTE-COPY.
Cascaded remote copy pairs (cascaded REC configuration)
In the case of cascaded remote copy the source unit (original) has a target unit in synchronous copy mode. This target unit is at the same time the source unit of another target. It can be operated in synchronous or asynchronous copy mode for ETERNUS DX/AF.
The source unit (original) resides in the storage system that receives the reads and writes in normal remote copy mode. It sends the updated data synchronously to the first remote storage system, which saves the data on the first target unit. From this target unit, which is at the same time the source unit of another target unit, the updated data is transferred synchronously or asynchronously to the second target unit in another storage system and stored there.
Figure 19: Cascaded remote copy configuration
Cascaded remote copy can be configured using the START-REMOTE-COPY command.
Only cascaded remote copy configurations are supported in which the target units reside in different storage systems.
For source units which are used as the original in cascaded remote copy replication, the /SHOW-REMOTE-COPY command displays all mirror pairs in several lines. Thus in the figure above when UNIT=<Source Unit 2> is specified.
You can switch from cascaded remote copy to concurrent remote copy with the /SWAP-REMOTE-COPY TARGET-UNIT= command.
Concurrent remote copy pairs (concurrent REC configuration)
With concurrent remote copy, the source unit (original) has two to four target units. The source unit is in the storage system that receives the reads and writes in normal remote copy mode. It sends the updated data simultaneously to the remote storage systems, which save the data on the target units. Here the second storage system is operated in synchronous copy mode. For ETERNUS DX/AF the other storage systems can be operated in synchronous or asynchronous copy mode.
Figure 20: Concurrent remote copy configuration
Concurrent remote copy can be configured using the START-REMOTE-COPY command.
Only concurrent remote copy configurations are supported in which the target units reside in different storage systems.
For source units which are also used as target units in concurrent remote copy replication, the /SHOW-REMOTE-COPY command displays all mirror pairs in several lines.
You can switch from concurrent remote copy to cascaded remote copy with the /SWAP-REMOTE-COPY TARGET-UNIT= command.
Remote link
The remote link can be implemented via Fibre Channel or iSCSI. The remote link between the storage systems using Fibre Channel is a link between the FC ports of the two storage systems. The FC ports which are used for the remote link can also be operated in parallel as FC connections to the servers (CA/RA mode of the storage system). For redundancy reasons at least two remote links must be configured between two storage systems. The remote links can be operated directly or via FC switch.
The remote link for a remote copy pair is AVAILABLE when at least one of the remote links is available. If the remote link is not in the AVAILABLE status, the source and target units are not synchronized.
Device configuration
Special configuration of the devices for REC is not required. Only devices of the storage system which are connected to the server can be mapped to BS2000 mnemonic names.
Commands for REC (ETERNUS DX/AF)
Command | Function |
Interrupts remote copy mode | |
Resumes remote copy mode | |
Displays the status of remote copy pairs | |
Displays the configuration | |
Displays device configuration | |
Creates remote copy pair | |
Cancels a remote copy pair | |
Swaps the source and target property of a remote copy pair or makes a target unit accessible |
The functions for remote replication (REC) can be controlled via SHC-OSD for ETERNUS DX/AF. The SHOW commands display the status of the remote copy pairs and the configuration.