Buffer Busy Waits Ratio (vwaitratio.sql)

What This Script Does

This script calculates the buffer busy waits ratio by comparing buffer busy wait events against total logical reads (consistent gets + db block gets). This ratio is a critical performance metric that indicates contention for buffer cache resources. High ratios suggest that sessions are frequently waiting for buffers that are being read into memory or modified by other sessions, which can significantly impact system performance.

The Script

select
       (cg.value) consistent_gets,
       (bg.value) db_block_gets,
       (ev.total_waits) buffer_busy_waits,
       (ev.total_waits)
       /
       ( (cg.value) + (bg.value) ) ratio
from v$sysstat cg,
     v$sysstat bg,
     v$system_event ev
where cg.statistic# = 39
  and bg.statistic# = 38
  and ev.event = 'buffer busy waits'
/

Key Features

Buffer Contention Analysis: Measures buffer pool contention severity
Simple Ratio Calculation: Easy-to-interpret single metric
System-Wide View: Provides overall buffer contention assessment
Performance Baseline: Establishes baseline for buffer efficiency
Trend Analysis Support: Enables historical performance tracking
Quick Assessment: Rapid identification of buffer contention issues

Usage

@vwaitratio.sql

No parameters required - calculates current system-wide ratio.

Required Privileges

SELECT on V$SYSSTAT
SELECT on V$SYSTEM_EVENT

Sample Output

CONSISTENT_GETS  DB_BLOCK_GETS BUFFER_BUSY_WAITS      RATIO
--------------- -------------- ----------------- ----------
      987654321      123456789             45678 .000046234

Key Output Columns

CONSISTENT_GETS: Total consistent mode block reads (for queries)
DB_BLOCK_GETS: Total current mode block reads (for modifications)
BUFFER_BUSY_WAITS: Total number of buffer busy wait events
RATIO: Buffer busy waits per logical read (lower is better)

Understanding Buffer Busy Waits

What Causes Buffer Busy Waits

Block Contention Scenarios

-- Common causes of buffer busy waits:
-- Multiple sessions reading same hot blocks
-- Sessions waiting for blocks being read from disk
-- Contention on frequently modified blocks
-- High-concurrency access to small tables
-- Sequence cache exhaustion causing contention

Buffer States Causing Waits

-- Buffer states that cause waits:
-- Buffer being read from disk by another session
-- Buffer being written to disk
-- Buffer in transition state during modification
-- Buffer header locks during concurrent access

Ratio Interpretation

Performance Thresholds

-- Buffer busy waits ratio guidelines:
-- <0.01%: Excellent (ratio < 0.0001)
-- 0.01-0.1%: Good (ratio 0.0001-0.001)
-- 0.1-1%: Fair (ratio 0.001-0.01)
-- >1%: Poor (ratio > 0.01) - investigation needed

Impact Assessment

-- Performance impact correlation:
-- Low ratio: Minimal buffer contention
-- Medium ratio: Moderate performance impact
-- High ratio: Significant performance degradation
-- Very high ratio: System bottleneck requiring attention

Common Use Cases

Buffer Pool Performance Analysis
- Assess overall buffer cache efficiency
- Identify buffer contention issues
- Support buffer cache sizing decisions
- Monitor buffer pool health
Performance Troubleshooting
- Diagnose application slowdowns
- Identify hot block contention
- Support system tuning efforts
- Validate optimization effectiveness
Capacity Planning
- Establish performance baselines
- Monitor growth impact on buffer efficiency
- Plan infrastructure scaling
- Support hardware sizing decisions
System Monitoring
- Real-time performance health checks
- Alert threshold establishment
- Performance trend analysis
- SLA compliance monitoring

Advanced Analysis

Root Cause Investigation

High Ratio Investigation

-- When ratio is high, investigate:
-- Hot blocks causing contention
-- Poorly designed sequences
-- Small table full scans
-- Index contention on popular values
-- Inefficient SQL causing excessive logical reads

Temporal Analysis

-- Time-based analysis:
-- Peak usage period correlation
-- Application activity correlation
-- Batch processing impact
-- User load relationship

Buffer Contention Patterns

Hot Block Identification

-- Common hot block scenarios:
-- Right-hand index insertions (sequences, timestamps)
-- Popular lookup table blocks
-- Frequently updated master records
-- System catalog blocks under heavy DDL

Application-Specific Patterns

-- Application design issues:
-- Unoptimized sequence caching
-- Poor transaction design
-- Inadequate indexing strategies
-- Excessive small table scans

Optimization Strategies

Buffer Cache Optimization

Cache Sizing

-- Buffer cache optimization:
-- Increase DB_CACHE_SIZE for better hit ratios
-- Implement multiple buffer pools for different access patterns
-- Consider KEEP pool for frequently accessed small objects
-- Use RECYCLE pool for large scan operations

Buffer Pool Configuration

-- Advanced buffer configuration:
-- Multiple block size buffer pools
-- Automatic memory management (AMM)
-- Buffer cache advisory for optimal sizing
-- Platform-specific optimization (HugePages, etc.)

Application-Level Optimization

SQL Optimization

-- Reduce logical reads through:
-- Better indexing strategies
-- Query optimization and rewriting
-- Elimination of unnecessary full table scans
-- Improved join algorithms and ordering

Transaction Design

-- Transaction optimization:
-- Reduced transaction scope and duration
-- Batch processing optimization
-- Commit frequency optimization
-- Lock duration minimization

System-Level Optimization

Sequence Optimization

-- Sequence contention reduction:
-- Increase sequence cache size
-- Use multiple sequences for partitioned data
-- Consider UUID or application-generated keys
-- Implement sequence partitioning strategies

Object Design Optimization

-- Object structure optimization:
-- Table partitioning for hot tables
-- Index organization optimization
-- Freelists configuration for concurrent inserts
-- Block size optimization for workload

Integration with Other Metrics

Comprehensive Buffer Analysis

-- Correlate with other buffer statistics:
-- Buffer cache hit ratio
-- Physical reads vs. logical reads
-- Buffer pool advisory recommendations
-- Free buffer waits and write complete waits

Wait Event Analysis

-- Related wait events:
-- db file sequential read
-- db file scattered read
-- free buffer waits
-- write complete waits

Performance Context

System Resource Correlation

-- Correlate with system metrics:
-- CPU utilization patterns
-- I/O subsystem performance
-- Memory allocation and swapping
-- Network latency in RAC environments

Application Performance

-- Application impact assessment:
-- Response time correlation
-- Throughput impact measurement
-- User experience degradation
-- Service level agreement compliance

Monitoring and Alerting

Baseline Establishment

Performance Baselines

-- Establish normal operating ratios:
-- Business hours baseline
-- Peak load characteristics
-- Seasonal variation patterns
-- Growth trend analysis

Threshold Setting

-- Alert threshold recommendations:
-- Warning: Ratio > 0.001 (0.1%)
-- Critical: Ratio > 0.01 (1%)
-- Emergency: Ratio > 0.05 (5%)
-- Adjust based on application requirements

Automated Monitoring

Trend Analysis

-- Historical tracking:
-- Daily ratio averages
-- Peak period analysis
-- Monthly trend identification
-- Performance regression detection

Integration with Monitoring Tools

-- Monitoring tool integration:
-- Enterprise Manager integration
-- Third-party monitoring systems
-- Custom dashboard development
-- Alert notification systems

Troubleshooting Applications

High Ratio Resolution

Immediate Actions

-- Quick fixes for high ratios:
-- Identify and address hot blocks
-- Optimize high-frequency queries
-- Increase sequence cache sizes
-- Consider emergency buffer cache increase

Long-Term Solutions

-- Sustainable optimization:
-- Application design improvements
-- Database schema optimization
-- Infrastructure capacity planning
-- Systematic performance tuning

Performance Validation

Optimization Effectiveness

-- Measure improvement:
-- Before/after ratio comparison
-- Response time improvement
-- Throughput increase measurement
-- User satisfaction assessment

Continuous Monitoring

-- Ongoing validation:
-- Regular ratio monitoring
-- Performance trend tracking
-- Regression detection
-- Optimization maintenance

Best Practices

Regular Assessment

Monitoring Schedule

-- Assessment frequency:
-- Real-time during peak periods
-- Daily health checks
-- Weekly trend analysis
-- Monthly capacity reviews

Documentation

-- Performance documentation:
-- Baseline ratio recordings
-- Optimization history
-- Configuration changes
-- Performance impact notes

Optimization Methodology

Systematic Approach

-- Optimization process:
-- Baseline establishment
-- Root cause identification
-- Targeted optimization implementation
-- Results validation and documentation

Preventive Measures

-- Prevention strategies:
-- Proactive capacity planning
-- Regular performance reviews
-- Application design standards
-- System health monitoring

vbhstat.sql - Buffer header statistics
vdbbufratio.sql - Database buffer ratios
hotblocks.sql - Hot block identification
vbpcache.sql - Buffer pool cache analysis

Extended Analysis

Buffer Efficiency Calculation

Enhanced Ratio Analysis

-- Extended buffer efficiency metrics:
SELECT
  consistent_gets,
  db_block_gets,
  buffer_busy_waits,
  ROUND(buffer_busy_waits / (consistent_gets + db_block_gets) * 100, 4) as wait_pct,
  CASE
    WHEN buffer_busy_waits / (consistent_gets + db_block_gets) < 0.0001 THEN 'Excellent'
    WHEN buffer_busy_waits / (consistent_gets + db_block_gets) < 0.001 THEN 'Good'
    WHEN buffer_busy_waits / (consistent_gets + db_block_gets) < 0.01 THEN 'Fair'
    ELSE 'Poor'
  END as performance_rating
FROM (SELECT query results);

Summary

This script is essential for:

Buffer Performance Assessment - Quick evaluation of buffer cache contention levels
Performance Troubleshooting - Identifying buffer-related performance bottlenecks
Capacity Planning - Understanding buffer efficiency trends and planning capacity
System Monitoring - Establishing baselines and monitoring buffer pool health
Optimization Validation - Measuring effectiveness of buffer-related optimizations