A complete tutorial for getting started with redis: RDB persistence

RDB Persistence is the process of saving a snapshot of the current process data to the hard disk , Trigger RDB a
The aging process is divided into manual trigger and automatic trigger .

5.1.1　 Trigger mechanism
Manual trigger corresponds to save and bgsave command ：
·save command ： Block the current Redis The server , until RDB Until the process is complete , For memory
Larger examples can create long-term blockages , Online environment is not recommended . function save Command corresponding to the
Of Redis The log is as follows ：
* DB saved on disk
·bgsave command ：Redis Process execution fork Action create subprocess ,RDB The persistence process is the result of
Process responsible , It will automatically end when it is finished . The blockage only happens in fork Stage , The average time is very short . function
bgsave The command corresponds to Redis The log is as follows ：
* Background saving started by pid 3151
* DB saved on disk
* RDB: 0 MB of memory used by copy-on-write
* Background saving terminated with success
obviously bgsave The order is for save Optimization of blocking problem . therefore Redis All internal references
And RDB The operation of the bgsave The way , and save The order has been abandoned .
In addition to executing the command to trigger manually ,Redis There are also automatic triggers inside RDB The persistence of
Mechanism , For example, the following scenario ：
1） Use save Related configuration , Such as “save m n”. Express m Data set exists in seconds n Time modification
when , Automatic triggering bgsave.
2） If full replication is performed from a node , Automatic execution of master node bgsave Generate RDB writing
And send it to the slave node , See... For more details 6.3 The principle of replication described in Section .
3） perform debug reload Command reload Redis when , It will also trigger automatically save operation .
4） Execute by default shutdown On command , If it's not on AOF The persistence function is
Automatic execution bgsave.

5.1.2　 Process description
bgsave It's the mainstream trigger RDB Persistence mode , Now according to the picture 5-1 Understand its operation flow
cheng .

 1） perform bgsave command ,Redis The parent process determines whether there is an executing child in progress
cheng , Such as RDB/AOF Subprocesses , If there is bgsave The command returns directly to .
2） Parent process execution fork Action create subprocess ,fork Parent process will block during operation , through
too info stats Command view latest_fork_usec Options , You can get the latest fork Operation cost
when , In microseconds .
3） The parent process fork After completion ,bgsave Command return “Background saving started” Information
And no longer block the parent process , You can continue to respond to other commands .
4） Subprocess creation RDB file , Generate a temporary snapshot file based on the memory of the parent process , After completion
Atomic replacement of the original file . perform lastsave Command to get the last build RDB Of
Time , Corresponding info The statistical rdb_last_save_time Options .
5） The process signals the parent process that it's done , Parent process update statistics , Specific view
info Persistence Under the rdb_* Related options .

5.1.3　RDB Handling of documents
preservation ：RDB The file is kept in dir Configure the specified directory , File name passed dbfilename with
Set designation . Can be executed by config set dir{newDir} and config set
dbfilename{newFileName} Run time dynamic execution , When next run RDB The file will be saved to
New catalog .
Operation and maintenance tips
When a bad disk or disk is full , Can pass config set dir{newDir} On-line
Modify the file path to the available disk path , After performing bgsave Perform disk switching , The same applies
On AOF Persistent files .
Compress ：Redis By default LZF Algorithm pair generated RDB File compression , Compressed
The file size is much smaller than the memory size , Default on , You can use the parameter config set
rdbcompression{yes|no} Dynamic modification .
Operation and maintenance tips
Although compressed RDB Will consume CPU, But it can greatly reduce the volume of the file , Easy to save to hard disk
Or send it to the slave node through the network , Therefore, it is suggested to open .
check ： If Redis Load corrupted RDB File refused to start , And print the following log ：
# Short read or OOM loading DB. Unrecoverable error, aborting now.
You can use Redis Provided redis-check-dump Tool detection RDB File and get the right
Should be reported .

5.1.4　RDB Advantages and disadvantages
RDB The advantages of ：
·RDB Is a compact binary , representative Redis Data at a certain point in time
snapshot . Ideal for backup , Full scale replication and other scenarios . Like every 6 Hours to perform bgsave Backup ,
And put RDB Copy files to a remote machine or file system （ Such as hdfs）, For disaster recovery .
·Redis load RDB Recover data much faster than AOF The way .
RDB The shortcomings of ：
·RDB Modal data cannot be persisted in real time / Second persistence . because bgsave Every time
All trades must be carried out fork Action create subprocess , Is a heavyweight operation , Frequent execution cost is too high .
·RDB Files are saved in a specific binary format ,Redis There are multiple formats in the version evolution process
Of RDB edition , There is an old version Redis The service is not compatible with the new version RDB The problem of format .
in the light of RDB Problems that are not suitable for real-time persistence ,Redis Provides AOF Persistent way to solve
" .