10.2.7 Sequential Prefetch Threshold

The sequential prefetch threshold (SPTH) is set by DB2 at 90% of each virtual buffer pool. This threshold is checked at two different times:

•Before scheduling a prefetch operation. If the threshold has been exceeded, the prefetch is not scheduled.

•During buffer allocation for an already-scheduled prefetch operation. If the threshold has been exceeded, the prefetch is canceled.

When the sequential prefetch threshold is reached, sequential prefetch is disabled until more buffers become available. When this occurs, the performance of operations that use sequential prefetch is adversely affected.

10.3 Data Write Operations

When an application updates data, the updated pages are kept in the virtual buffer pool. Eventually, the updated data pages in the virtual bufferpool have to be written to disk. Write operations can be either asynchronous or synchronous, with the execution of the unit of work.

10.3.1 Asynchronous Writes

Most DB2 writes are done asynchronously from the application program and chained whenever possible. This helps performance and implies that the application may have long since finished, by the time its data updates are written to disk. Updated pages are kept in the virtual bufferpool for possible reuse. The reuse ratio can be obtained from the DB2PM statistics report. See J in Figure 35 on page 110, for an example.

Updated pages are written asynchronously, when:

•A checkpoint is taken, which happens whenever:

•The DB2 parameter LOGLOAD limit is reached.

•An active log is switched.

•The DB2 subsystem stops executing normally.

•The percentage of updated pages in a virtual buffer pool for a single data set exceeds a preset limit called the vertical deferred write threshold (VDWQT).

•The percentage of unavailable pages in a virtual buffer pool exceeds a preset limit called the deferred write threshold (DWQT).

Because these operations are independent from the application program, the DB2 accounting trace cannot show these writes. The DB2 PM statistics report is required to see the asynchronous writes. This is shown in H in Figure 35 on page 110.

DB2 I/O Operations 107

Page 128 Page 130
Page 129
Image 129
IBM 5695-DF1, 5655-DB2 manual Data Write Operations, Sequential Prefetch Threshold, Asynchronous Writes
Page 128 Page 130

5695-DF1, 5655-DB2 specifications

IBM 5655-DB2 and 5695-DF1 are significant components within the IBM software ecosystem, predominantly focusing on data management and integration solutions. These offerings cater primarily to enterprise environments that require robust database management systems and associated frameworks to maintain and manipulate data efficiently.

IBM 5655-DB2 is a well-known relational database management system (RDBMS) that excels in managing large volumes of structured data. Its architecture is designed to support high availability, scalability, and performance, crucial for businesses operating in today’s data-driven world. Some of its main features include advanced indexing capabilities, support for complex queries, and dynamic workload management. Additionally, it provides strong concurrency controls, which enable multiple users to access and manipulate data simultaneously without compromising data integrity.

One of the key characteristics of DB2 is its support for various data types, including JSON and XML, making it versatile for modern applications that generate data in diverse formats. It also features robust security mechanisms to protect sensitive data, aligning with compliance standards across industries. Integration with analytics tools further allows businesses to derive insights from their data, enhancing decision-making processes.

On the other hand, IBM 5695-DF1, also known as the InfoSphere DataStage, is a powerful data integration tool that facilitates the extraction, transformation, and loading (ETL) of data from various sources to target systems. It empowers organizations to streamline their data flows, ensuring that clean, consistent information is available for analysis and operational use. Key features of 5695-DF1 include a user-friendly graphical interface that enhances developer productivity and a rich set of connectors for numerous data sources, enabling seamless data integration.

DataStage also supports real-time data integration, allowing businesses to keep their data synchronized across multiple platforms. Its parallel processing capabilities dedicatedly optimize performance, enabling organizations to handle vast datasets efficiently. It incorporates data quality tools that help in validating and cleansing data before it is used for decision-making processes.

Both IBM 5655-DB2 and 5695-DF1 are part of a broader strategy to accommodate the evolving landscape of data management. Businesses leverage these technologies to enhance their data architectures, fostering agility and competitive advantage in their respective markets. Their integration capabilities, along with a focus on security and scalability, position them as vital assets in modern enterprise environments. Whether managing critical data within a database or ensuring seamless data flow across systems, these IBM offerings provide a comprehensive approach to handling complex data challenges.
Top Page Image Contents