Redis Persistence

redis Is a memory database , If you do not save the database state in memory to disk , Then once the server process exits , The status of the database in the server will also send messages , therefore redis Provides persistence .

RDB（redis database）

What is? rdb？

In master-slave replication ,rdb It's a spare .

Writes a snapshot of a data set in memory to disk within a specified time interval , That's what jargon says snapshot snapshot , When it recovers, it reads the snapshot file directly into memory .

Sometimes in a production environment , We will back up this file .

Redis Will create... Separately （fork） A subprocess to persist , The data will be written to a temporary file first , When the persistence process is over , Replace the last persistent file with this temporary file . The whole process , The main process is not going to do anything IO Operation of the . This ensures extremely high performance . If large-scale data recovery is needed , And it's not very sensitive to the integrity of data recovery , that RDB The way is better than AOF It's more efficient .RDB The disadvantage of is that the data after the last persistence process may be lost .

Our default is RDB, In general, there is no need to modify .

rdb The saved file is dump.rdb, Snapshots can be configured in the configuration file ！

dbfilename dump.rdb

rdb The trigger mechanism of

1.save When the rules are satisfied , Will automatically trigger rdb The rules 
2. perform flushall command , It will trigger our rdb The rules 
3. sign out redis, There will be rdb file 
 Also generated after backup dump.rdb file 

How to restore rdb file ？

1. Only need to rdb The documents are on our redis Just start the directory ,redis It will be checked automatically when it is started dump.rdb Recover the data

2. Check where you want to store it

127.0.01：6379> config get dir
1) "dir"
2) "usr/local/bin"  If it exists in this directory dump.rdb file , The data in it will be recovered automatically when it is started 

Theoretically , The default configuration is enough for us .

advantage ：

​ 1. Suitable for large-scale data recovery

​ 2. The requirement of data integrity is not high

shortcoming ：

​ 1. It takes a certain time interval to operate If redis Unexpected downtime , The last modified data is gone

​ 2.fork Process time , It will occupy a certain amount of memory space

AOF（append only file）

Record all our orders ,history, When restoring, execute the file all over again ！

Log each read and write operation , take Redis All 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 .

 Rewrite rule description 
	aof The default is unlimited appending of files , The file will get bigger and bigger 
	auto-aof-rewirte-percentage 100
	auto-aof-rewirte-min-size 64mb
 If aof File is larger than 64m. will fork A new process will rewrite our files .

AOF What is kept is appendonly.aof file

appendonly  Not on by default . We need to configure it manually   Change it to yes It opens. aof
 restart redis It will take effect 

If aof There is an error in the file , This is the time redis It can't start up , We need to fix aof file

redis Provides us with a tool redis-check-aof --fix

If the file is normal , Restart to recover .

advantage ：

​ 1. Every change is synchronized , The file will be more complete

​ 2. Synchronize once per second , One second of data could be lost

​ 3. No synchronization , The most efficient

shortcoming ：

​ 1. Relative to data files ,aof Far greater than rdb, The repaired data speed is low

​ 2.aof It is also more efficient than rdb slow , So we redis The default configuration is rdb Persistence