redis Persistence is aof

Concept

Log every write operation in the form of a log , take Redis All written instructions executed are recorded ( Reading operation does not record ), Only files can be added but not rewritten ,redis At the beginning of startup, it will read the file and rebuild the data , In other words ,redis In case of restart, execute the write instruction from the front to the back according to the contents of the log file to complete the data recovery

Aof What is kept is appendonly.aof file

Profile location

AOF start-up / Repair / recovery

Normal recovery

1 Modify the configuration file

2 start-up redis, Write 2 Time , then flushall. see aof file , Recovery found no data , stay aof Delete... From the file flushall command , Start again redis, Data found to exist .

Abnormal recovery

1 Backup aof file , modify aof file , Joining is not in line with redis Content of grammar
2 Try to start redis, Can't start
3 Use Redis-check-aof --fix Make repairs
4 restart redis, normal

Rewrite

 Concept ：AOF By means of document addition , The documents will become larger and larger to avoid this , New rewrite mechanism , When AOF When the file size exceeds the set threshold ,Redis Will start AOF The content of the file is compressed , Keep only the smallest instruction set that can recover data . You can use commands bgrewriteaof
 principle ：AOF When files continue to grow and become too large , Meeting fork A new process to rewrite the file ( Also write temporary documents first and then rename), Traverse the data in memory of the new process , Each record has one Set sentence . rewrite aof Operation of file , Not reading the old aof file , Instead, the database contents in the whole memory are rewritten with a new command aof file , It's a bit like a snapshot .
 Trigger mechanism ：Redis It will record the last time it was rewritten AOF size , The default configuration is when AOF The file size is last rewrite Double the size and the file is larger than 64M Trigger when 

Advantages and disadvantages

advantage ：
1 Sync per change ：appendfsync always Synchronous persistence Every time there is a data change, it will be recorded to the disk immediately Poor performance but good data integrity
2 A second synchronous ：appendfsync everysec Asynchronous operations , Record every second If it goes down in a second , There is data loss
3 Out of sync ：appendfsync no No synchronization
shortcoming ：
1 Data from the same data set aof Documents are much larger than rdb file , Recovery is slower than rdb
2Aof Operating efficiency is slower than rdb, The efficiency of synchronization strategy per second is better , Out of sync efficiency and rdb identical

summary