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DB2 Performance Best Practices September 13, 2007 Matt Emmerton DB2 Performance and Solutions Development IBM Toronto Laboratory [email protected] Best Practices Overview. Many areas of focus System Hardware System Software Database Application All require proper design and tuning.

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DB2 PerformanceBest PracticesSeptember 13, 2007Matt EmmertonDB2 Performance and Solutions Development IBM Toronto [email protected]


Best practices overview
Best Practices Overview

  • Many areas of focus

    • System Hardware

    • System Software

    • Database

    • Application

    • All require proper design and tuning


System software
System Software

  • Operating System

    • Ensure that latest required maintenance level and all required APARs have been applied

    • Only use certified/supported device drivers

  • Database Engine

    • Ensure that latest required maintenance level and all required APARs have been applied

    • Latest fixpacks provide bug-fixes and performance improvements

  • DB2 Viper (v9)

    • AIX 5.2 + TL08 SP2 + 64-bit kernel

    • AIX 5.3 + TL04 SP2 + APARs + 64-bit kernel

    • Recommended: AIX 5.3 + TL05 SP3 + 64-bit kernel

  • DB2 Viper 2 (v9.5)

    • AIX 5.3 + TL06 + 64-bit kernel (tentative)


Database design
Database Design

  • Good design involves many areas:

    • Disk hardware

    • Database

    • Tablespaces

    • Tables

    • Indexes

    • Bufferpools


Database design disk hardware
Database Design – Disk Hardware

  • Number and size of disks matter

    • Few large disks will create a performance bottleneck

    • Many small disks will pose an administrative nightmare

    • Both OLTP and DSS applications benefit from more disks

  • Separation of logs and data

    • Logs should be on separate disk subsystem with separate controller(s), RAID-5 at a minimum

    • Data should be on separate disk subsystem with separate controller(s), RAID-1 at a minimum

  • Be sure to take advantage of the caching technologies of your RAID subsystem

    • Enabling read cache (on data disks) can provide performance benefits on DSS workloads (repeated scans can be cached)

    • Enabling write cache (on log disks) can provide performance benefits on OLTP workloads.

      • Must be battery-backed and mirrored to be durable

      • Battery-backed write cache has improved OLTP performance up to 3% in some workloads


Database design database
Database Design – Database

  • Database encoding

    • Unicode databases should only be created in the following circumstances:

      • Need to store non-ASCII data

      • Need to store XML data

    • Use native code pages when possible

      • Very important when migrating from System i/p environments

  • Object page size

    • Using few page sizes will simplify memory management

    • Choice of page size can impact performance

      • OLTP benefits most from 4K and 8K page sizes

      • DSS benefits most from 16K and 32K page sizes


Database design tablespace type
Database Design – Tablespace Type

  • DMS – Database Managed Storage

    • Database manages allocation of tablespace extents to database objects

    • Raw (device) access gives top performance

    • File (filesystem) access gives moderate to high performance

      • Space is pre-allocated at creation time

      • When DIO and CIO features are enabled, performance is comparable to raw

  • SMS – System Managed Storage

    • OS manages allocation of tablespace extents to database objects

    • Filesystem access gives moderate performance

      • Space is allocated at runtime

      • Fragmentation and flush delays can degrade performance


Database design tablespace types
Database Design – Tablespace Types

  • Automatic Storage

    • Resizes tablespaces as necessary at runtime

    • Threshold for resize and amount of resize can be controlled by DBA

    • Created as DMS File for regular or large tablespaces

    • Created as SMS for temporary tablespaces


Database design filesystem features
Database Design – Filesystem Features

  • Direct I/O (DIO)

    • DIO enables filesystem cache bypass

      • DB2 already caches pages, so why have the OS cache again?

      • Reduces filesystem cache growth and thus limits potential paging activity and related VMM problems

    • Enabled automatically if the underlying filesystem supports DIO

      • Can be over-ridden via ALTER TABLESPACE

    • Improves performance on DMS File to within 5-10% of DMS raw

  • Concurrent I/O (CIO)

    • Enables concurrent readers and writers to separate regions of a single file

    • Enabled automatically if the underlying filesystem supports CIO


Database design tablespaces
Database Design – Tablespaces

  • Tablespaces should be spread over many filesystems or disk devices

    • Using more than one container is a good thing!

    • This will improve performance via parallelism at the device level

  • Extent size should be a multiple of the RAID stripe size

    • This will improve disk performance via parallellism at the disk level


Database design bufferpools
Database Design - Bufferpools

  • On a dedicated database server, all available memory should be assigned to bufferpools

  • 32-bit DB2 has memory limitations:

    • Only supported on Windows

    • Limited to 2-4 GB depending on system config

  • 64-bit DB2 has no memory limitations on any platform


Database design tablespace bufferpools
Database Design – Tablespace/Bufferpools

  • Bufferpools are assigned to tablespaces

    • Minimum 2 bufferpools and tablespaces per page size

      • One bufferpool/tablespace for temporary objects using SMS

      • One bufferpool/tablespace for permanent objects using DMS

  • Separate bufferpools for additional tablespaces may improve performance

    • Separating objects by type (tables, indexes)

    • Separating objects by access pattern (sequential, random, head, tail)


Database design tables
Database Design – Tables

  • Use primitive types for primary keys

    • Integer types are very efficient

      • Primary key access via index scans

      • Referential integrity checking and enforcement

    • Character types are expensive and inefficient

  • Consider setting table properties via ALTER TABLE

    • APPEND: Indicates that rows are seldom deleted

    • LOCKSIZE: Can choose from row or table locking

      • Table-based locking is beneficial for read-only table


Database design tables1
Database Design – Tables

  • Row compression

    • Beneficial in situations where I/O is the dominant factor

      • reading fewer pages from disk improves response time

      • extra CPU cost for decompression can be absorbed

      • most useful for read-only queries

    • Varying benefit in CPU-bound and/or read-write situations

      • CPU-bound environments can’t absorb the cost of decompression

      • Read-Write situations require decompression and compression which increases CPU cost

      • Update log records are 2-3x the size

      • Only beneficial when the I/O rates are reduced enough to absorb the extra CPU used by compression


Database design indexes
Database Design – Indexes

  • Avoid primary indexes on CHAR/VARCHAR fields

    • Very expensive to scan and enforce RI

    • Fine to use for secondary index access

  • Tables with fewer indexes are better for performance

    • Fewer index updates during UPDATE/INSERT/DELETE

  • Consider setting INDEX properties via ALTER INDEX

    • PAGE SPLIT LOW/HIGH: Indicates to DB2 how to populate pages during index reorganization

    • CLUSTER: Creates a clustering index

    • INCLUDE: Includes table columns within index structure for quick retrieval


Database design advisor
Database Design – Advisor

  • DB2 Design Advisor (db2advis)

    • Analyses queries and suggests indexes, MDCs, MQTs and statistics that would be beneficial

    • Can read queries from:

      • Command-line

      • User-specified input file

      • Dynamic SQL cache

    • Will suggest schema changes:

      • Indexes to create/delete

      • Multi-Dimension Clustered tables (MDCs)

      • Materialized Query Tables (MQTs)

    • Will suggest maintenance operations:

      • RUNSTATS (with or without sampling and/or distribution stats)

      • REORG


Database tuning
Database Tuning

  • Tuning requires lots of data collection and analysis

    • OS tools

      • Vmstat, iostat

    • DB2 tools

      • Snapshots, event monitors

  • Tuning can happen at all areas of the solution stack

    • OS level

      • Filesystem tuning, VMM tuning

    • Database level

      • Db2set, dbm cfg, db cfg parameters

      • Schema changes

      • Consider using our autoconfigure tool


Tuning operating system
Tuning – Operating System

  • Paging Space

    • Should be on multiple disks

    • Not shared with any other data (/home, DB data or logs)

    • Paging should be the exception, not the rule on a database server

  • System Tools

    • Vmstat gives a continuous picture of system behaviour

      • Run queue (process/thread scheduling)

      • Kernel events (context switches, interrupts, system calls)

      • CPU usage (system, user, idle, I/O pending)

    • Iostat gives a continuous picture of disk behaviour

      • Per-controller and per-array breakdowns

      • Can be used to isolate I/O problems

      • Can expose DB2 tuning or application problems


Tuning filesystems aix
Tuning – Filesystems (AIX)

  • Filesystem cache size can be reduced from AIX defaults

    • JFS Filesystem Cache

      • Minperm/Maxperm are the min/max number of pages allocated to JFS filesystem cache

      • Strict_maxperm determines whether this is a soft (0) or hard (1) limit

    • JFS2/NFS Filesystem Cache

      • Maxclient% is the number of pages allocated to JFS2 filesystem cache

      • Strict_maxclient determines whether this is a soft (0) or hard (1) limit

    • Must take into consideration the I/O characteristics of the system

      • Smaller filesystem cache can reduce system paging

      • Larger filesystem cache can benefit SMS tablespace performance (especially for temporary tables)

    • All of this is especially beneficial when using DIO/CIO for the majority of tablespaces in a database


Tuning aio aix
Tuning – AIO (AIX)

  • Asynchronous I/O (AIO)

    • Allows DB2 to perform useful work while I/Os are being processed

    • AIO on filesystem-based (DMS File and SMS) tablespaces use AIO kprocs

      • The number of aioserver kprocs can be configured via the ‘maxservers’ tunable

      • Not uncommon to have a large number of AIO kprocs, as one is created for each concurrent AIO request

      • Newer versions of AIX will not use kprocs but instead will use internal structures to manage filesystem AIO


Tuning database autonomics
Tuning – Database – Autonomics

  • Many sizing, tuning and administrative tasks have been automated in v9 and v9.5

    • Most manual controls are still available

  • Configuration Advisor

    • Will set various database parameters based on system characteristics (#CPU, memory, etc)

  • Automatic Runstats

  • Automatic Backup

  • Self-Tuning Memory Manager

    • Will manage the amount of memory needed for LOCKLIST, Package Cache, Application Heap, Bufferpools, Sort Heap



Tuning database number of agents
Tuning – Database – Number of Agents

  • NUM_INITAGENTS

    • Number of agents to initialize at startup

    • Set to the average number of connections

      • Can minimize the amount of time it takes to start up the set of active agents required at runtime

  • NUM_POOLAGENTS

    • Number of agents that are maintained during runtime

    • Set to the average number of connections

    • Can be configured automatically by DB2 (starting in v9.5)


Tuning database application memory
Tuning – Database – Application Memory

  • PCKCACHESZ

    • “Package Cache Size”

    • Represents the amount of memory used to cache compiled statements in the database engine

    • If this is too small, compiled statements will be purged from the cache and thus will need to be recompiled (under the covers) before they can be executed again

    • Take database snapshots, and look for “pkg_cache_num_overflows”. If this is high, then it is a good indication that the cache is too small

    • Can be configured automatically by DB2 (starting in v9.5)

    • Can be tuned automatically tuned via STMM (starting in v9)

  • APPLHEAPSZ

    • “Application Heap Size”

    • Represents the amount of memory used as a “working set” for each database connection

    • May get SQL0954C errors if your application heap is too small

    • Can be configured automatically by DB2 (starting in v9.5)

    • Can be tuned automatically tuned via STMM (starting in v9)


Tuning database logger
Tuning – Database – Logger

  • LOGFILSIZ

    • “Log File Size”

    • Increase from the default; 5000 pages is a good starting point

  • LOGBUFSIZ

    • “Log Buffer Size”

    • Increase when “log pages read” counter is high in the database snapshots

    • A large LOGBUFSIZ ensures that when a transaction rolls back, it does not have to read log pages from disk (for past transactions) in order to complete the rollback operation

  • SOFTMAX

    • “Soft Checkpoint – Maximum Log Files”

    • The checkpoint interval expressed as the number of log files (in percent – 100 = 1 log file)

    • Modified data pages are written to disk after the transactions they are associated with are outside of the SOFTMAX interval

    • Large values will incur more I/O at recovery time (as more log must be read to recover)

    • Small values will incur more I/O at runtime (as data pages are written to disk sooner)


Tuning database bufferpools
Tuning – Database – Bufferpools

  • NUM_IOCLEANERS

    • “Number of Page Cleaners”

    • Can be configured automatically by DB2 (starting in v9)

  • NUM_IOSERVERS

    • “Number of Prefetchers”

    • Can be configured automatically by DB2 (starting in v9)

  • SOFTMAX

    • Primarily a log tunable, but also controls the rate at which dirty pages are written to disk

  • CHNGPGS_THRESH

    • “Changed Pages Threshold”

    • Indicates the threshold of (dirty pages / total pages) at which to start writing dirty pages to disk

    • Lower values will provide a more constant I/O behaviour

  • DB2_USE_ALTERNATE_PAGE_CLEANING (APC)

    • Enables a different page cleaning algorithm that is more proactive and responsive to changing system dynamics

    • Known to be beneficial for most OLTP environments

    • Not the best on DSS workloads that use block-based bufferpools or large TEMPs

      • V9 FP3a and FP4 will have changes to fix the TEMP issue for DSS workloads


Tuning database sorting
Tuning – Database – Sorting

  • SHEAPTHRES/SHEAPTHRES_SHR

    • Instance-wide soft limit on the number of pages to use for private and shared sorts

    • A limit set by the DBA

    • Can be tuned automatically by DB2 (starting in v9.5)

  • SORTHEAP

    • Per-sort limit on the number of pages to use for each private or shared sort

    • Can be tuned automatically by DB2 (starting in v9)

  • Tuning

    • Look for “sort overflows” in Database Snapshots

      • These indicate when sorts could not be contained in memory and had to “spill” to disk (as temporary pages which are placed in temporary tablespaces)

      • Sorts that spill are very inefficient (at least 2 additional I/Os per page that spills)

    • DSS workloads typically benefit from large SORTHEAP

    • OLTP workloads typically do not benefit from large SORTHEAP


Application design and tuning
Application Design and Tuning

  • Static SQL / PSM

    • Access plans “set in stone” when application is compiled

    • Make sure that code is bound against a database tuned for production use

  • Dynamic SQL (CLI)

    • Take the time to use prepared statements with parameter markers

    • The extra cost of using prepared statements is negligible when compared to the package cache churn when statements are continually purged and then recompiled

  • Stored Procedures / SQL Procedures (PSM)

    • Stored procedures can be used to execute multiple statements on the server

    • This can be used to minimize client/server network traffic and associated processing

  • SQL Functions

    • Can be used to simplify application logic by reducing repetitive code


Application design and tuning1
Application Design and Tuning

  • Database Connections

    • Minimize the number of connections you application uses

    • Connections are not cheap – each connection uses ~100KB on the server

    • Consider using connection concentrator if you need a large number of connections

  • Analyse Query Plans

    • Use the db2exfmt and db2expln tools to produce query execution plans for your SQL statements

    • This will allow you to see what methods DB2 is using to execute the query

    • Changing the query or the schema can improve bad query plans

  • Always Close Cursors

    • Always close cursors once all data has been read

    • This frees up system resources used to maintain cursor state

    • In some cases, this will release locks

  • Always Commit

    • Always commit transactions, even read-only transactions

    • This releases locks and will reduce the amount of work required to traverse the lock lists


Questions
Questions?

  • Any questions?


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