What is Redis aof-use-rdb-preamble and How Does It Work?

What is aof-use-rdb-preamble?

aof-use-rdb-preamble is a key configuration option introduced in Redis 4.0 (enabled by default since 7.0+) that implements hybrid persistence by combining RDB snapshots and AOF logs.

# Redis 7.0+ default configuration (recommended for production)
aof-use-rdb-preamble yes

Why Use Hybrid Persistence?

Traditional Persistence Limitations

Three Core Advantages

1. Lightning-Fast Recovery

• RDB Binary Loading: 5-10x faster than pure AOF text parsing
• Benchmark Data: 1GB data recovers in 2-5s (vs 30-60s with pure AOF)

2. Dual Data Protection

• Point-in-Time Snapshots: RDB ensures base data integrity
• Operation Log Tracking: AOF records every data change

3. Intelligent Storage Management

# Compression configurations
rdbcompression yes                      # RDB compression (default)
aof-rewrite-incremental-fsync yes       # AOF incremental fsync

How Hybrid Persistence Works

Core Workflow

1. Write Handling Phase

• Client Requests: Process SET/INCR write operations
• Memory Execution: Update in-memory data immediately
• Dual Logging:
  • Write to AOF buffer (text format)
  • Periodically generate RDB snapshots (binary format)

2. Persistence Trigger Phase

• Initial Trigger:
  • Manual BGSAVE command
  • Reaching save thresholds (e.g., 1 write in 900s)
• Subsequent Triggers:
  • Append AOF after each write
  • AOF file reaches size threshold (default 4GB)

3. File Construction Phase

• RDB Header:

  File Header Structure:
  REDIS0009        # RDB version identifier
  FE 00           # Database selector
  FB              # Hash table size indicator
  0000000000000000 # Key-value data
  

• AOF Log Appending:

  *3\r\n$3\r\nSET\r\n$5\r\nkey01\r\n$7\r\nvalue01\r\n
  *2\r\n$4\r\nINCR\r\n$7\r\ncounter\r\n
  

4. Data Recovery Phase

• Priority Order:
  1. Load latest RDB snapshot
  2. Replay AOF logs sequentially
• Optimizations:
  • Multi-threaded RDB loading
  • Batch AOF command execution

File Structure Example

Hybrid File Structure:
┌───────────────────┐
│   RDB Header      │ ← Binary snapshot
├───────────────────┤
│   AOF Log Entries │ ← Text-based operations
│   SET key1 value1 │
│   INCR counter    │
│   ...             │
└───────────────────┘

Production Configuration

Recommended Settings

# Enable AOF
appendonly yes

# Enable hybrid mode
aof-use-rdb-preamble yes

# Optimize rewrite triggers
auto-aof-rewrite-percentage 100  
auto-aof-rewrite-min-size 4gb    

# Performance tuning
aof-rewrite-incremental-fsync yes

Common Issues & Solutions

1. Configuration Validation

# Method 1: Check file header
head -c 5 appendonly.aof
# Should output "REDIS"

# Method 2: CLI verification
127.0.0.1:6379> CONFIG GET aof-use-rdb-preamble

2. File Corruption Recovery

# Step-by-step recovery
redis-check-rdb --fix appendonly.aof  # Fix RDB section
redis-check-aof --fix appendonly.aof  # Fix AOF section
redis-server --appendonly yes --appendfilename appendonly.aof

Performance Optimization

1. Capacity Planning:

   • Reserve 2x memory as disk space
   • Keep individual AOF files under 16GB

2. Key Metrics Monitoring:

   # Persistence latency
   redis-cli info persistence | grep aof_delayed_fsync
   
   # File size monitoring
   watch -n 1 "ls -lh appendonly.aof"
   

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