Consistency Technologies Offer Alternatives
to Multiple Single-System Backups
from mySAP Business Suite (which spans financials,
CRM, SCM, and BI, among others) increasingly
cover multiple components and systems. Consider
a process like business partner management
in 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 image1 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 you require 24x7
availability. 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.
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.2
No matter what systems — or how many — your processes use,
they all ultimately store their data in a storage system.3
And more important, all the information needed for data exchange (RFC
tables4) is also stored there.
So if consistency of data is your prime
consideration, and availability is critical, your storage system can serve
as a point of synchronization for cross-system data. Combined with the
fault-tolerant RFC protocol, this approach allows you to preserve 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.
The Federated Backup
Project: Testing Consistency Technologies at
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.
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 system or for upgrade testing. It can
serve as a backup as well, to allow you 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.5
So before you go ahead and completely replace existing backup strategies,
be sure to determine whether all restore and recovery options are supported
by the respective technology!
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) 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?
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:6
- 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.
Each method has strengths and drawbacks
(see Figure 1 for a side-by-side comparison).
||The Two Consistency Technologies At-a-Glance
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.7
||Sample Setup for Using Consistency Technology
Figure 2 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 .
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 .
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
or, with the Coordinated Suspend approach,
you will create scripts and methods for suspending
database write operations temporarily as the
images are created by split-mirror .
This image can now be used to refresh
a QA landscape or
to restore the complete production landscape
to a point of consistency .
When using this image for landscape copy,
any pending RFCs8 need
to be processed to ensure data consistency
in the copied landscape .
To avoid having these RFCs go into the production
landscape, careful precautions must be taken
before starting the SAP systems on the landscape
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.
1 The data is, from an application point of view,
in a consistent state between the different systems.
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. See Siegfried Schmidt’s
article in the January 2003 SAP Insider (www.SAPinsider.com).
3 By storage system, we mean here a dedicated
computer with special-purpose disk storage management functions, specifically
designed to consolidate the storage demands of multiple hosts.
4 RFC (Remote Function Call) tables.
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.
6 For other options offered by consistency
technology, see the references at the end of the article.
the image was created with Consistent Split
and the database was not in "hot backup
8 That is, those RFCs that were waiting to be
processed when the split operation was executed
Wolter joined SAP in 1997 as
a member of the Advanced Technology Group.
There he was engaged in performance-tuning
for SAP R/3 systems and Informix databases.
Since 2000, his focus is on Backup & Restore
concepts for SAP system landscapes. In
addition to his work on the Federated
Backup Project, he advises on Backup & Restore
requirements with multi-system SAP landscapes.