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Introduction to the Teradata® RDBMS for UNIX® Version 2 Release 2.1 - NCR

NCR Introduction to the Teradata® RDBMS for UNIX® Version 2 Release 2.1 - NCR, 1998. - 315 p.
Download (direct link): inntroduktionteradata1998.pdf
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About This Chapter

This topic provides an overview of the fault tolerance capabilities of the Teradata RDBMS.

The Teradata RDBMS addresses the critical requirements of reliability, availability, serviceability, usability, and installability (RASUI) by combining multiple microprocessors (in so-called symmetric multiprocessing, or SMP), parity or mirrored disk storage, and protection of the database from operating anomalies of the client platform.

Some fault tolerance is provided by hardware and some by software. Some is mandatory and some is optional. The high availability of the Teradata RDBMS is one of its more remarkable features.

The Teradata RDBMS provides the following software fault tolerance capabilities:

• vproc migration

• Fallback tables and AMP clusters

• Journaling

• Archive and Restore utility

• System maintenance facilities

The Teradata RDBMS provides the following hardware fault tolerance capabilities:

• Dual BYNETs (5100M systems only)

• RAID disk units

• Multiple channel and LAN connections

• Isolation from client hardware defects

• Battery backup for all cabinets

• Redundant power supplies and fans

• Hot swap capability for RAID disks, fans, and power supplies

• Cliques

• Separate diagnostic processor

• A single system view provided by the AWS controlling all cabinets in a multinode system.

Introduction to the Teradata RDBMS for UNIX

9-1
Fault Tolerance

Software Fault Tolerance

Introduction

vproc Migration

Software Fault Tolerance

The Teradata RDBMS provides many facilities for software fault tolerance. These are:

• vproc migration

• Fallback tables and AMP clusters

• Journaling

• Archive and restore utility

• System maintenance facilities

Parsing Engine (PE) and Access Module Process (AMP) software replaces the hardware devices that existed on the old Teradata dBc/ 1012 and System 3600 configurations.

Because these “processors” are now software, they can migrate from their home node to another node within the same hardware clique if the home node fails for any reason.

Preferred migratory destinations are user-configurable, but in general, you should allow the system to determine which vprocs migrate to which nodes.

vproc migration permits complete operation of the affected “processors” during any node failure. vproc migration is illustrated in Figure 9-1, where a failed node is indicated by a large X and migration is indicated by arrows pointing to nodes that are still running.

9-2

Introduction to the Teradata RDBMS for UNIX
Fault Tolerance

Software Fault Tolerance

Figure 9-1 vproc Migration

Note that PEs for channel-attached connections cannot migrate during a node failure because they are dependent on the channel hardware physically attached to their node.

Fallback tables are a component of the fault tolerance provided by Fallback Tables 9 the Teradata RDBMS.

A fallback table is a duplicate copy of a primary table. Each row in a fallback table is stored on a different AMP from the one to which the primary row hashes. In this way, the likelihood of loss of data due to simultaneous losses of the two AMPs, or their associated disk storage, is extremely reduced.

The disadvantage of this method is that it requires twice the storage space and twice the I/O (on inserts, update, and deletes only) of tables maintained without fallback. The advantage is that data is almost never lost because of a down AMP. Data is fully available

Introduction to the Teradata RDBMS for UNIX

9-3
Fault Tolerance

Software Fault Tolerance

during an AMP or disk outage, and recovery is automatic after repairs have been made.

The Teradata RDBMS for UNIX permits the definition of fallback for individual tables. As a general rule, it is wise to run all tables critical to your enterprise in fallback mode. Other tables can be run in nonfallback mode in order to maximize resource usage.

The vproc migration feature offered by systems using disk array technology can safely recover this storage capacity by running without fallback. Running in this mode does not provide availability of data during an AMP outage, however.

You specify whether a table is fallback or not using the CREATE TABLE (or ALTER TABLE) statement. The default is not to create tables with fallback.

AMP Clusters

Clustering is a means of logically grouping AMPs to minimize (or eliminate) data loss that might occur from losing an AMP. Note that AMP clusters are used only for fallback data.

AMP clustering is best explained with pictures. The first picture illustrates a situation in which there is fallback but no AMP clustering.

Figure 9-2

Unclustered AMPs With Fallback

AMP1

AMP2

AMP3

AMP4

Primary copy area 1,9,17 2,10,18 3,11,19 4,12,20
Fallback copy area 21,22,15 1,23,8 9,2,16 17,10,3
AMP5 AMP6 AMP7 AMP8
Primary copy area 5,13,21 6,14,22 7,15,23 8,16,24
Fallback copy area 18,11,4 19,12,24 20,5,6 13,14,7
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