[Redis] RDB and AOF

Redis is an in- memory database . If the state of the database in memory is not saved to disk , once the server process exits , the state of the database in the server will also disappear . So Redis provides persistence function!

RDB（Redis DataBase)

Write the snapshot of the data set in the memory to the disk within the specified time interval, which is the Snapshot in the jargon, and it reads the snapshot file directly into the memory when it is restored.

Redis will separately create (fork) a child process for persistence, and will first write the data to a temporary file. After the persistence process is over, this temporary file will replace the last persisted file. During the whole process, the main process does not perform any I0 operations. This ensures extremely high performance. If large-scale data recovery is required, and the integrity of data recovery is not very sensitive, the RDB method is more efficient than the AOF method. The disadvantage of RDB is that the data after the last persistence may be lost.

1 time in 900 seconds

300 seconds 10 times

1W times a minute

save 60 5 Mainly, if we modify it 5 times within 60s, it will trigger the generation of dump.rdb file

Trigger mechanism

1. When the save rule is met, the rdb rule will be triggered automatically
2. Execute the fluxhall command, which triggers our rdb rule
3. Exit redis, it will also generate a rdb file

The backup will automatically generate a dump.rdb

How to recover rdb files

1. Just put the rdb file in our redis startup directory. When redis starts, it will automatically check dump.rdb to restore the data!

2. See where it needs to be

   127.0.0.1:6379> config get dir
   1) "dir"
   2) "/usr/local/bin"# 如果在这个目录下存在 dump. rdb文件，启动就会自动恢复其中的数据
   
   

advantage:

1. Suitable for large-scale data recovery, dump.rdb
2. The integrity of the data is not high

Disadvantages:

1. It takes a certain time interval for process operations. If redis crashes unexpectedly, the last record will be gone
2. When forking the process, it will occupy a certain amount of memory space

AOF(Append Only File)

Record all our commands, history, and execute them again when you restore

Record each write operation in the form of a log, record all the instructions executed by Redis (read operations are not recorded), only append files but not rewrite files, redis will read the file to rebuild the data at the beginning of startup. In other words, if redis restarts, it will execute the write command from front to back according to the content of the log file to complete the data recovery work.

AOF saves appendonly.aof file

1. Not open by default, manually open, restart to take effect
2. You can choose the name of the generated file

Write once every second

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What if appendonly.aof goes wrong?

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Can not start our Redis

Solution

Use our Redis-check-aof --fixe

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If our aof is greater than 64M, we will fork a new process to rewrite our file

advantage:

1. Every modification is synchronized, the integrity of the file will be better
2. The default is to synchronize once per second
3. Never synchronize, efficiency is the highest

Disadvantages:

1. Compared with data files, AOF is much larger than RDB memory, and the repair speed is slower than RDB
2. Aof runs slower than RDB, so the default configuration of redis is our RDB persistence

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