16_ Redis_ Redis persistence

Redis Persistence

Redis Is a memory database , If you do not save the database state in memory to disk , So once the server process exits , The database state in the server also disappears . therefore Redis Provides persistence !

RDB (Redis DataBase)

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

Redis Will create... Separately （ fork ) A subprocess to persist , Write the data 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 is that the data may be lost after the last persistence . Our default is RDB, In general, you don't need to modify this configuration !

Sometimes in the production environment, we Backup this file !

rdb The saved file is dump.rdb All of them are configured in our configuration file !

Trigger mechanism

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 !

The backup automatically generates one dump.rdb

How to restore rdb file

1、 Only need to rdb The documents are in us redis Just start the directory ,redis It will check automatically when it starts dump.rdb Recover the data !
2、 See where you need to be

advantage ：

1、 Suitable for large-scale data recovery !
2、 The integrity of the data is not high !

shortcoming ：

1、 Need a certain time interval process operation ! If redis Unexpected downtime , There is no need to modify the data for the last time !
2、fork Process time , Will take up a certain amount of content space !!

AOF(Append Only File)

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

Log every write operation in the form of a log , 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

Not on by default , We need to configure it manually ! We just need to put appendonly Change it to yes It opens. aof !

restart ,redis It's going to take effect !

If this aof There is an error in the file , Now redis The server cannot be started , We need to fix this aof file

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

If the file is normal , Restart and you can recover directly !

aof To configure ：

Redis And AOF Rewriting and its implementation principles

Rewriting rule ：aof The default is unlimited appending of files , The file will get bigger and bigger !
If aof File is larger than 64mb, Indicates that the file is too large ! redis will fork A new process to rewrite our files

advantage ：

1、 Every change is synchronized , The integrity of the document would be better !
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, It's faster than rdb slow !
2、Aof It's more efficient than rdb slow , So we redis The default configuration is rdb Persistence !

Expand

1、RDB Persistence mode can snapshot your data in a specified time interval

2、AOF Persist to record every write to the server , These commands are reexecuted when the server restarts to restore the original data ,AOF Command to Redis Protocol append saves each write to the end of the file ,Redis Also able to AOF The file is rewritten in the background , bring AOF The size of the file should not be too large .

3、 Cache only , If you only want your data to exist when the server is running , You can also use no persistence

4、 Open two persistence methods at the same time

• under these circumstances , When redis It will be loaded prior to restart AOF File to restore the original data , Because in general AOF The data set saved by the file is better than RDB The data set of the file should be complete .
• RDB The data is not real-time , When using both, the server will only look for AOF file , Do you want to use only AOF Well ? The author suggests not to , because RDB Better for backing up databases （AOF It's not easy to backup in constant change ）, A quick restart , And there won't be AOF May be latent Bug, Keep it as a means in case .

5、 Performance Suggestions

• because RDB The documents are for backup purposes only , The advice is only in Slave On persistence RDB file , And as long as 15 One minute backup is enough , Only keep save 900 1 This rule .
• If Enable AOF, The advantage is that in the worst case, it will only lose no more than two seconds of data , The startup script is simple load Their own AOF Just file it , The first is to bring about continuous IO, Two is AOF rewrite At the end of rewrite The blocking caused by writing new data to new files is almost inevitable . As long as the hard disk is licensed , We should try to minimize AOF rewrite The frequency of ,AOF Overridden base size default 64M Is too small , It can be set to 5G above , Default over original size 100% The size rewrite can be changed to the appropriate value .
• If you don't Enable AOF, Only by Master-Slave Replcation High availability can also be achieved , Can save a lot of IO, And less rewrite The fluctuation of the system caused by . The price is if Master/Slave At the same time （ power failure ）, It will lose more than ten minutes of data , There are also two startup scripts to compare Master/Slave Medium RDB file , Load the newer one , Microblogging is this kind of Architecture .