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This is part nine of the series that began with
What is System-Managed Storage?
What is Remote Copy?
Remote copy is a storage-based disaster recovery and workload migration solution that provides the ability to copy data in real time to a remote location. It is an extended function on the IBM 3990 Model 6 Storage Control, along with the appropriate levels of MVS/DFP and DFSMS/MVS.
Remote copy offers two options: extended remote copy (XRC) and peer-to-peer remote copy (PPRC). Both address the problem of unrecoverable data that occurs between the last, safe backup of an application system to a recovery system and the time when the application system fails.
Using Remote Copy for Disaster Recovery
Disasters occur in many forms. Some happen suddenly and stop all processing at a single point in time; others interrupt operations in stages, occurring over several seconds or minutes. IBM's remote copy function addresses the problems caused by intermittent and gradual system failures.
- With PPRC, no DASD data is lost between the last update at the application system and the recovery at the recovery system site.
- With XRC, the only data lost is data "in transit" between the time when the application system fails and the recovery system recovers.
Remote copy is based upon two systems: a primary application system and a recovery system, either in the same building, or at remote locations. Each system has specific DASD that handles data that you have identified as remote copy-managed. In case of a disaster or failure at the application location, data is recovered on the recovery system.
Once established, remote copy:
- Provides a real-time copy of both SMS and non-SMS-managed data.
- Makes changes to your data remotely as you make changes at your application location.
- Is application independent.
- Supports all DASD data needed for application recovery.
- Runs virtually unattended.
Using Remote Copy for Workload Migration
Remote copy can also be used for workload migration from one location to another. It enhances the ability to move data from one set of DASD volumes to another with minimal impact to continuous availability applications.
Remote copy can copy data from one volume to another if the number of bytes per track and number of tracks per cylinder are the same for the two devices, and the target device has an equal or greater number of cylinders. The only way to copy data between unlike devices or from larger devices to smaller devices is by using DFSMSdss (or DFDSS, or comparable product) logical dump, restore, or copy functions.
Selecting a Remote Copy Option
Depending upon your specific needs, you can choose extended remote copy (XRC) or peer-to-peer remote copy (PPRC). They differ in:
- Effect on primary application DASD I/O performance
- How current the data is
- Use of system resources
- Control of the remote copy operating facilities
- Distance between sites
Extended Remote Copy
XRC, available with DFSMS/MVS 1.1.0 and above, is a combined hardware and software solution. XRC is designed for those sites that:
- Need to maintain the highest levels of performance on their application host system.
- Can accept a gap of a few seconds between writes on the application host system and writes on the recovery system.
XRC is cooperatively implemented between IBM 3990 storage subsystems and DFSMS/MVS. It provides an automatic, asynchronous copy operation, over distance, with minimal I/O performance impact. XRC supports both ESCON and parallel channel operation. It requires ESA/390 hardware and an ES/9000 environment. In a multi-system environment, it also requires a sysplex timer.
To maintain high performance, XRC allows the primary application's I/O operation to signal completion before receiving confirmation of the write at the recovery system. Because the copy may be only seconds behind the primary write, there may be little or no data loss if a system failure occurs when data is "in transit" between the two locations.
Peer-to-Peer Remote Copy
PPRC, available on MVS/DFP 3.1.1 and above, is a hardware solution. PPRC is designed for those sites that:
- Need the recovery system to always be fully current with the primary application system.
- Can also accept some performance impact to application write I/O operations.
A PPRC data copy to the recovery storage subsystem is synchronous with the primary volume's I/O. PPRC does not consider the application system DASD write complete until the recovery system write is complete. Each application system write to the recovery subsystem causes an increase to the application system response time.
PPRC requires ESCON paths between application and recovery storage controls. You can use existing ESCON paths or add dedicated paths. PPRC also requires 3990 Model 6s as both application and recovery site storage controls and remote copy-capable Licensed Internal Code.
Remote Copy Requirements
Before installing and using remote copy you need to plan for it. Understanding the software and hardware requirements will help you prepare to implement remote copy in your installation.
Two major XRC functions require software support:
- Data Mover function -- DFSMS/MVS 1.1.0 and above, and MVS/ESA 4.3.0 and above.
- Host application functions -- Host application systems must have a common time reference for data consistency.
In a single system environment or an environment with multiple logically partitioned modes (LPARs), the system clock provides a common time reference. The minimum software level is MVS/SP 3.1.0 with MVS/DFP 3.2.0.
In a multi-system environment, the sysplex timer, or equivalent, provides the common time reference. The minimum software level is MVS/ESA 4.1.0 with MVS/DFP 3.2.0.
Two PPRC functions require software support:
- TSO command functions -- MVS/DFP 3.1.1 and above.
- Optional error recovery procedure (ERP) functions -- MVS/DFP 3.2.0 and above.
Remote copy supports DASD channel extenders for instances where a direct ESCON connection is not possible. They allow you to configure remote copy DASD beyond the maximum ESCON channel distance or where ESCON fiber optic cables cannot physically be placed.
In Summary
Using the IBM 3990 Model 6 Storage Control along with MVS/DFP and DFSMS/MVS allows you to take advantage of remote copy capability. The two options offered by remote copy provide you with flexibility, and enhance your options for disaster recovery and work load migration.
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