Understanding Redis Persistence: RDB and AOF Overview

1. RDB (Redis Database) Overview

RDB persistence creates snapshots of the dataset at specified intervals (N seconds or after M changes), storing a full backup of the data.

2. RDB Features

2.1 Advantages

Single file, easy to transfer, suitable for disaster recovery.

Faster recovery of large datasets compared to AOF.

Backup performed by a forked child process, leaving the parent process free of additional I/O, resulting in better performance than AOF.

2.2 Disadvantages

Unexpected crashes between intervals can cause significant data loss.

Forking a child process for very large datasets can be time‑consuming and may affect normal request latency.

3. RDB Strategies

RDB backup can be triggered automatically or manually.

3.1 Automatic Triggers

Based on the save rules configured in the Redis config.

Executing FLUSHALL command.

Graceful shutdown of Redis.

3.2 Manual Triggers

SAVE – synchronous execution.

BGSAVE – asynchronous execution.

3.3 Operation Process

1. Fork a child process; the fork operation does not duplicate data, improving speed and reducing memory usage (kernel‑level fork() ).

2. The child process receives pointers to all data.

3. If data changes during the fork, copy‑on‑write (COW) creates separate copies for the parent and child.

4. The child writes the pointed‑to data to the backup file.

5. Backup completes.

4. RDB Configuration

Configuration Item

Description

save

Save after N seconds or M changes (triggers BGSAVE).

stop-writes-on-bgsave-error

Disallow writes if snapshot fails.

rdbcompression

Enable/disable RDB file compression.

rdbchecksum

Enable/disable RC64 checksum.

dbfilenameRDB

Name of the RDB file.

dirRDB

Directory where the RDB file is stored.

5. Additional RDB Notes

5.1 Point‑in‑time Consistency

After a snapshot, any subsequent modifications are retained at their original timestamps, preventing temporal inconsistencies.

5.2 Size Recommendations

It is advisable to keep Redis memory usage under 10‑15 GB to avoid slowing down RDB writes.

5.3 Repairing Corrupted RDB Files

Use the redis-check-rdb tool provided in the Redis installation directory to repair damaged snapshots.

6. AOF (Append Only File) Overview

AOF persistence logs every write operation; on restart, Redis replays the logged commands to reconstruct the dataset. AOF files can be rewritten and compressed to control size.

6.1 AOF File Protocol Example

# Assuming the client executed SET KEY VALUE, the AOF content would be:
*3
$3
SET
$3
KEY
$5
VALUE

The asterisk (*) indicates the number of subsequent lines to read, while the dollar sign ($) specifies the length of the following line.

7. AOF Features

7.1 Advantages

Minimal data loss on crashes (typically ≤2 seconds).

7.2 Disadvantages

Slower data recovery compared to RDB.

AOF files are larger than RDB files.

Performance depends on the chosen appendfsync strategy; it can be slower than RDB.

8. AOF Strategies

8.1 Automatic Triggers (appendfsync)

no – never fsync; writes are flushed when the OS buffer fills (fast).

everysec – default; fsync every second.

always – fsync after every write (slow but safest).

8.2 Manual Trigger

BGREWRITEAOF – asynchronous rewrite of the AOF file.

9. AOF Configuration

Configuration Item

Description

appendonly

Enable or disable AOF.

AOFappendfilename

Name of the AOF file.

appendfsync

Persistence strategy for AOF.

no-appendfsync-on-rewrite

Delay fsync during AOF rewrite.

auto-aof-rewrite-percentage

Growth percentage that triggers AOF rewrite.

auto-aof-rewrite-min-size

Minimum size to trigger AOF rewrite.

aof-load-truncated

Load truncated AOF files.

aof-use-rdb-preamble

Enable mixed RDB‑AOF persistence (available from Redis 4.0).

10. Additional AOF Information

10.1 AOF Rewrite Version Differences

Pre‑4.0: Remove canceling commands and merge duplicates.

Post‑4.0: Write a full RDB snapshot first, then continue logging new writes as AOF.

10.2 Repairing Corrupted AOF Files

Because AOF is an incremental backup, unexpected crashes can corrupt the file, preventing Redis from loading it on restart.

Remediation steps:

Regularly back up AOF files.

Use the redis-check-aof tool in the Redis installation directory to repair the file, then compare differences with diff -u .