[teacher Zhao Yuqiang] redis's slow query log

Redis Slow query log helps developers and operation and maintenance personnel locate the slow operation of the system . Slow query log is that the system calculates the execution time of each command before and after the execution of the command , When the default threshold is exceeded , This command will be the relevant information （ The slow query ID, Occurrence timestamp , Time consuming , Command details ） recorded .

Redis A client command is divided into four parts as follows ：

It should be noted that , Slow query log is just a statistical step 3） Time of execution of order , So slow query does not mean that the client has no timeout problem . It should be noted that , Slow query log is just a statistical step 3） Time of execution of order , So slow query does not mean that the client has no timeout problem .

One 、 Configuration parameters for slow queries ：

• Default threshold for slow query   slowlog-log-slower-than

slowlog-log-slower-than The parameter is the default threshold , The unit is microseconds , The default value is 10000, If the execution time of a command exceeds 10000 subtle , Then it will be recorded in the slow query log .
If slowlog-log-slower-than The value of is 0, All orders will be recorded .
If slowlog-log-slower-than The value is less than 0, Then no command will log .

• Slow query log length slowlog-max-len

slowlog-max-len It just shows how many slow query logs can be stored at most .Redis Use a list to store slow query logs ,showlog-max-len Is the maximum length of the list . When the slow query log has reached the maximum length of the list , Another slow query log to enter the list , The first log inserted into the list will be removed from the list , The new log is inserted at the end of the list .

Two 、 Composition of slow query log

The slow query log consists of the following four properties ：
identification ID, Occurrence timestamp , The command takes time , Execute commands and parameters

3、 ... and 、 Slow query log access and management

Get slow query log slowlog get [n]
command ：slowlog get [N]
The selection ：N, Optional , Represents the number of log entries obtained
for example ：showlog get 5

Four 、 Slow query log best practices

• slowlog-max-len Suggestions for setting up

It is recommended to increase and slow down the list of query logs in the online environment , When logging slow queries Redis Will truncate long commands , It doesn't take up a lot of memory . Increasing the list of slow queries can slow down the possibility that slow queries will be removed from the list . For example, the line can be set to 1000 above .

• slowlog-log-lower-than Suggestions for setting up

Need basis redis The concurrency value is adjusted . because redis Using single thread response fame and fortune , For high traffic scenarios , If the order is executed at 1 Milliseconds or more , that redis Support at most OPS（ Number of operations per second ） Less than 1000, So high OPS Of the scene REDIS Recommended setting is 1 millisecond .

• Slow queries only record command execution time , Does not include command queuing time and network transmission time .

Therefore, the execution time of client command is longer than redis The time the server actually executed the command . Because command execution queuing is extreme , Slow queries can cause command cascading blocking , So when the client has a request timeout , You need to check whether there is a corresponding slow query at this time point , So as to analyze whether the command cascade blocking caused by slow query

• The slow query log is a first in, first out queue

When there are more slow queries , Some slow query commands may be lost , It can be carried out on a regular basis slow get Command to persist slow query logs to other stores . Then make a visual interface query .