1 Overview of High Availability

This chapter contains the following sections:

1.1 Introduction to High Availability

Databases and the Internet have enabled worldwide collaboration and information sharing by extending the reach of database applications throughout organizations and communities. This reach emphasizes the importance of high availability in data management solutions. Both small businesses and global enterprises have users all over the world who require access to data 24 hours a day. Without this data access, operations can stop, and revenue is lost. Users, who have become more dependent upon their solutions, now demand service-level agreements from their Information Technology (IT) departments and solution providers. Increasingly, availability is measured in dollars, euros, and yen, not just in time and convenience.

Enterprises have used their IT infrastructure to provide a competitive advantage, increase productivity, and empower users to make faster and more informed decisions. However, with these benefits has come an increasing dependence on that infrastructure. If a critical application becomes unavailable, then the business can be in jeopardy. Revenue and customers can be lost, penalties can be owed, and bad publicity can have a lasting effect on customers and a company's stock price. It is important to examine the factors that determine how your data is protected and maximize availability to your users.

1.2 What is Availability?

Availability is the degree to which an application, service, or function is accessible on demand. Availability is measured by the perception of an application's end user. End users experience frustration when their data is unavailable or the computing system is not performing within certain expectations, and they do not understand or care to differentiate between the complex components of an overall solution. Performance failures due to higher than expected usage create the same havoc as the failure of critical components in the solution.

Reliability, recoverability, timely error detection, and continuous operations are primary characteristics of a highly available solution:

  • Reliability: Reliable hardware is one component of a high availability solution. Reliable software—including the database, Web servers, and applications—is just as critical to implementing a highly available solution. A related characteristic is resilience. For example, low-cost commodity hardware, combined with software such as Oracle RAC, can be used to implement a very reliable system, because the resilience of an Oracle RAC database allows processing to continue even though individual servers may fail.

  • Recoverability: Because there may be many choices for recovering from a failure, it is important to determine what types of failures may occur in your high availability environment and how to recover from those failures in a timely manner that meets your business requirements. For example, if a critical table is accidentally deleted from the database, what action should you take to recover it? Does your architecture provide the ability to recover in the time specified in a service level agreement (SLA)?

  • Timely error detection: If a component in your architecture fails, then fast detection is essential to recover from the unexpected failure. While you may be able to recover quickly from an outage, if it takes an additional 90 minutes to discover the problem, then you may not meet your SLA. Monitoring the health of your environment requires reliable software to view it quickly and the ability to notify the database administrator of a problem.

  • Continuous operation: Providing the ability for continuous access to your data is essential when very little or no downtime is acceptable to perform maintenance activities. Activities, such as moving a table to another location in the database or even adding CPUs to your hardware, should be transparent to the end user in a high availability architecture.

More specifically, a high availability architecture should have the following traits:

  • Tolerate failures such that processing continues with minimal or no interruption

  • Be transparent to—or tolerant of—system, data, or application changes

  • Provide built-in preventative measures

  • Provide proactive monitoring and fast detection of failures

  • Provide fast recoverability

  • Automate detection and recovery operations

  • Protect the data so that there is minimal or no data loss

  • Implement the operational best practices to manage your environment

  • Achieve the goals set in SLAs (for example, recovery time (RTO) and recovery point (RPO)) for the lowest possible total cost of ownership.

1.3 Importance of Availability

The importance of high availability varies among applications. However, the need to deliver increasing levels of availability continues to accelerate as enterprises re-engineer their solutions to gain competitive advantage. Most often, these new solutions rely on immediate access to critical business data. When data is not available, the operation can cease to function. Downtime can lead to lost productivity, lost revenue, damaged customer relationships, bad publicity, and lawsuits.

It is not always easy to place a direct cost on downtime. Angry customers, idle employees, and bad publicity are all costly, but not directly measured in currency. On the other hand, lost revenue and legal penalties incurred because SLA objectives are not met can easily be quantified. The cost of downtime can quickly grow in industries that are dependent on their solutions to provide service.

Other factors to consider in the cost of downtime are the maximum tolerable length of a single unplanned outage, and the maximum frequency of allowable incidents. If the event lasts less than 30 seconds, then it may cause very little impact and may be barely perceptible to end users. As the length of the outage grows, the effect may grow exponentially and result in a negative impact on the business. Alternatively, frequent outages, even if short in duration, may similarly disrupt business operations. When designing a solution, it is important to understand the true cost of downtime to understand how the business can benefit by availability improvements.

Oracle provides a range of high availability solutions that fit every organization regardless of size. Small workgroups and global enterprises alike are able to extend the reach of their critical business applications. With Oracle and the Internet, applications and data are reliably accessible everywhere, at any time.

1.4 Causes of Downtime

One of the challenges in designing a high availability solution is examining and addressing all of the possible causes of downtime. It is important to consider causes of both unplanned and planned downtime when designing a fault tolerant and resilient IT infrastructure. Planned downtime can be just as disruptive to operations, especially in global enterprises that support users in multiple time zones.

Table 1-1 describes unplanned outage types and provides examples of each type.

Table 1-1 Causes of Unplanned Downtime

Type Description Examples

Site failure

A site failure outage occurs when an event causes all or a significant portion of an application to stop processing or slow to an unusable service level. A site failure may affect all processing at a data center, or a subset of applications supported by a data center.

  • Extended site-wide power failure

  • Site-wide network failure

  • Natural disaster making a data center inoperable

  • Terrorist or malicious attack on operations or the site

Computer failure

A computer failure outage occurs when the system running the database becomes unavailable because it has crashed or is no longer accessible.

  • Database system hardware failure

  • Operating system failure

  • Oracle instance failure

  • Network interface failure

Storage failure

A storage failure outage occurs when the storage holding some or all of the database contents becomes unavailable because it has shut down or is no longer accessible.

  • Disk drive failure

  • Disk controller failure

  • Storage array failure

Human error

A human error outage occurs when unintentional or malicious actions are committed that cause data in the database to become logically corrupt or unusable. The service level impact of a human error outage can vary significantly depending on the amount and critical nature of the affected data.

  • File deletion (at the file system level).

  • Dropped database object

  • Inadvertent data changes

  • Malicious data changes

Data corruption

A corrupt block is a block that has been changed so that it differs from what Oracle Database expects to find. Block corruptions fall under two categories: physical and logical block corruptions:

  • In a physical corruption, which is also called a media corruption, the database does not recognize the block at all: the checksum is invalid, the block contains all zeros, or the header and footer of the block do not match.

  • In a logical corruption, the contents of the block are logically inconsistent. Examples of logical corruption include corruption of a row piece or index entry.

Block corruptions can also be divided into interblock corruption and intrablock corruption:

  • In intrablock corruption, the corruption occurs in the block itself and can be either a physical or a logical corruption.

  • In an interblock corruption, the corruption occurs between blocks and can only be logical corruption.

A data corruption outage occurs when a hardware, software or network component causes corrupt data to be read or written. The service level impact of a data corruption outage may vary, from a small portion of the database (down to a single database block) to a large portion of the database (making it essentially unusable).

  • Operating system or storage device driver failure

  • Faulty host bus adapter

  • Disk controller failure

  • Volume manager error causing bad disk read or writes

  • Software defects

Lost Writes

A lost write is another form of data corruption, but it is much more evasive to detect and repair quickly. A data block stray or lost write occurs when:

  • For a lost write, an I/O subsystem acknowledges the completion of the block write even though the write I/O did not occur in the persistent storage. On a subsequent block read on the primary database, the I/O subsystem returns the stale version of the data block, which might be used to update other blocks of the database, thereby corrupting it.

  • For a stray write, the write I/O completed but it was written somewhere else, and a subsequent read operation returns the stale value.

  • For an Oracle RAC system, a read I/O from one cluster node returns stale data after a write I/O is completed from another node (lost write). For example, this occurs if an NFS file system is mounted in Oracle RAC without disabling attribute caching (for example, without using the noac option). In this case, the write I/O from one node is not immediately visible to another node because it is cached.

  • Operating system or storage device driver failure

  • Faulty host bus adapter

  • Disk controller failure

  • Volume manager error

  • Other application software

  • Lack of Network File Systems (NFS) write visibility across a cluster

Hang or slow down

Hang or slow down occurs when the database or the application is unable to process transactions because of a resource or lock contention. Perceived hang can be caused by lack of system resources.

  • Database or application deadlocks

  • Runaway processes that consume system resources

  • Log on or system faults

  • Combination of application peaks with lack of system or database resources

  • Archived redo log destination or flash recovery area destination become full

Table 1-2 describes planned outage types and provides examples of each types.

Table 1-2 Causes of Planned Downtime

Type Description Examples

System and database changes

Planned system changes occur when performing routine and periodic maintenance operations and new deployments.

Planned system changes include any scheduled changes to the operating environment that occur outside the organizational data structure in the database.

The service level impact of a planned system change varies significantly depending on the nature and scope of the planned outage, the testing and validation efforts made before implementing the change, and the technologies and features in place to minimize the impact.

  • Adding or removing processors to or from an SMP server

  • Adding or removing nodes to or from a cluster

  • Adding or removing disks drives or storage arrays

  • Changing configuration parameters

  • Upgrading or patching system hardware and software

  • Upgrading or patching Oracle software

  • Upgrading or patching application software

  • System platform migration

  • Database relocation

  • Moving from 32 bits to 64 bits

  • Migrating to cluster architecture

  • Migrating to new storage

Data changes

Planned data changes occur when there are changes to the logical structure or physical organization of Oracle Database objects. The primary objective of these changes is to improve performance or manageability.

  • Table definition changes

  • Adding table partitioning

  • Creating and rebuilding indexes

Application Changes

Planned application changes may include data changes and schema and programmatic changes. The primary objective of these changes is to improve performance, manageability, and functionality.

  • Application upgrades

Oracle offers high availability solutions to help avoid both unplanned and planned downtime, and recover from failures. Chapter 2 discusses each of these high availability solutions in detail.

1.5 What Does This Book Contain?

Choosing and implementing the architecture that best fits your availability requirements can be a daunting task. This architecture must:

  • Encompass redundancy across all components

  • Provide protection and tolerance from computer failures, storage failures, human errors, data corruption, lost writes, system hangs or slowdown, and site disasters

  • Recover from outages as quickly and transparently as possible

  • Provide solutions to eliminate or reduce planned downtime

  • Provide consistent high performance

  • Be easy to deploy, manage, and scale

  • Achieve SLA's at the lowest possible total cost of ownership

To help you select the most suitable architecture for your organization, this book describes several high availability architectures and provides guidelines for choosing the one that best meets your requirements. Knowledge of the Oracle Database server, Oracle RAC, and Oracle Data Guard terminology is required to understand the configuration and implementation details.

Chief technology officers and information technology architects can benefit from reading the following chapters:

Database administrators and network administrators can find useful information in the following chapters:

See Also:

Oracle High Availability Best Practice recommendations in the: