Creating Consistent Landscape Images during Online Operation

August 4, 2003 by admin

Business processes from mySAP Business Suite (which spans financials, Customer Relationship Management, Supply Chain Management, and Business Intellicence, among others) increasingly cover multiple components and systems. Consider a process like business partner management in mySAP Customer Relationship Management (mySAP CRM), which exchanges data between two or more systems running on different servers. When implementing such landscapes, customers often face the requirement of creating a consistent image of the entire landscape – what I will refer to as a consistent landscape image – for use as a backup or for building a consistent test environment. However, constant cross-system exchange of information makes preserving data consistency a challenge when trying to create a landscape image, particularly if 24×7 availability is required. Achieving data consistency among different systems is no problem if all the systems can be brought down for backup, but creating consistent landscape images during online operation may require a different approach than the single-system online backups that had been sufficient in the past.

Split-mirror technology

A practical solution has emerged by looking beyond the application level for data synchronization and, instead, considering a consistent landscape image based on split-mirror technology. A split-mirror copy traditionally refers to a single-system backup that temporarily halts WRITE operations in a controlled manner (SUSPEND/RESUME) and logs the status of transactions – as “complete” or “open” – in the database log. While the database is in SUSPEND mode, all volumes containing the database are copied on the storage level. No matter what systems – or how many – a customers processes use, they all ultimately store their data in a storage system, i.e. a dedicated computer with special-purpose disk storage management functions, specifically designed to consolidate the storage demands of multiple hosts. And more important, all the information needed for data exchange (Remote Function Call (RFC) tables) is also stored there.
So if consistency of data is the prime consideration, and availability is critical, the storage system can serve as a point of synchronization for cross-system data. Combined with the fault-tolerant RFC protocol, this approach allows preserving data consistency for an image taken of all systems simultaneously.
In fact, this technology has already been a success in tests specifically for SAP environments. SAP and its hardware partners have recently tested methodologies for creating a consistent image of a federated system landscape using the “consistency technology” currently available from select storage systems (see “Federated Backup Project” below). The result was a consistent image of data for the entire test landscape, making this an important consideration for any customer evaluating backup and recovery needs.

Uses for a consistent landscape image

A consistent landscape image can be used to set up a test landscape, for example, for a quality assurance (QA) system or for upgrade testing. It can serve as a backup as well, to allow users to restore the complete production landscape to a point of consistency in the past (for example, in case of a disaster).
A consistent landscape image could also be used to restore individual systems if necessary due to a system or hardware error (including both complete or incomplete recovery) – albeit with some important limitations. This will allow Backup Consolidation, which means replacing multiple, single-system backup strategies with a single process (i.e., the creation of a consistent landscape image as a backup). But for this particular use, note that – depending on the methods used to create the image – not all types of single-system recovery are possible from a consistent landscape image. So before users completely replace existing backup strategies, they have to be sure to determine whether all restore and recovery options are supported by the respective technology!
Backup Consolidation can reduce cost and administrative work, while providing the additional option of restoring the complete landscape to these consistency points.

How is a consistent landscape image created?

Consistency technology creates a synchronization point by momentarily holding I/Os on either the database level or the storage level, simultaneously, for all systems of the landscape. In this case, we’ll focus on two approaches tested in the Federated Backup Project:

  • Consistent Split is an operation available from some third-party storage systems to create a consistent landscape image at the storage level – and requires only a single command. Consistent Split instantly holds I/Os for all relevant storage volumes while a split-mirror copy is generated. All storage volumes reflect exactly the same state, thus yielding a consistent image. This applies to volumes on a single storage system, but also can be extended for volumes located on multiple storage systems either by defining Consistency Groups – groups of storage volumes that can span multiple storage systems – or by coordinating Consistent Split operations from a dedicated host running consistency software.
  • Coordinated Suspend works by momentarily suspending I/O operations on all involved database systems (database suspend) while a split-mirror copy is taken for all related volumes. Coordinating these suspend operations and enclosing comprehensive error-handling procedures is the main challenge for implementing this solution.
The Two Consistency Technologies At-a-Glance

The Two Consistency Technologies At-a-Glance

Each method has strengths and drawbacks as shown in the side-by-side comparison beside.
Coordinated Suspend is an extension of the split-mirror backup procedure for single systems, and thus allows all types of single-system recoveries. This makes it an option for replacing all existing backups with a consistent landscape backup approach. In contrast, note that this is currently not true for images created with Consistent Split. With Consistent Split, the database system has no knowledge of the backup, so with most systems, individual system recovery is not generally possible from such an image. Starting with an image created with Consistent Split on Oracle, for example, it is possible to do a complete single-system recovery, but it is not possible to do an incomplete database recovery – unless the database was in “hot backup mode”.

Sample Setup for Using Consistency Technology

Sample Setup for Using Consistency Technology

The figure beside gives an overview of a setup using consistency technology. The basic setup is the same for either method: The production landscape runs application processes that are constantly exchanging data (1). These processes write data to one or more storage systems. To create a consistent landscape image, an additional set of disks is available to hold the copy (2). Depending on the approach, either the production volumes of the storage system(s) must be grouped together so they can be consistently split in a single operation (3a) or, with the Coordinated Suspend approach, users will create scripts and methods for suspending database write operations temporarily as the images are created by split-mirror (3b). This image can now be used to refresh a QA landscape (4a) or to restore the complete production landscape to a point of consistency (4b). When using this image for landscape copy, any pending RFCs – that is, those RFCs that were waiting to be processed when the split operation was executed – need to be processed to ensure data consistency in the copied landscape (5). To avoid having these RFCs go into the production landscape, careful precautions must be taken before starting the SAP systems on the landscape image!

The Federated Backup Project: testing consistency technologies at work

One goal of the Federated Backup Project was to show, in cooperation with our hardware partners, that consistency technologies work in an SAP environment. We developed a methodology to simulate business processes exchanging data between multiple systems of our test landscape, and to verify this test data for consistency. With a test tool designed by SAP for this purpose, we ran a high RFC communication workload while creating an image of the landscape with and without consistency technology. We started a second landscape on this image, and found that only with the use of consistency technology were we able to create a landscape image that was in a consistent state. (The complete test setup and results can be found at http://service.sap.com/atg ->Backup and Restore ->Federated Backup.)

For more information

For more information on system landscape copy, see the corresponding Best Practices documents at http://service.sap.com/solutionmanagerbp.
Additional information on backup and restore concepts and strategies is available from the Best Practice “Backup and Restore for mySAP Business Suite” at http://service.sap.com/solutionmanagerbp and at http://service.sap.com/atg-> Backup and Restore

Source: SAP Insider

Jochen Wolter

Jochen Wolter

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