How to use Redis slow query log

Slow query log

What is SLOW LOG?

Slow log is a log system used by Redis to record command requests that take longer than a given time to execute a query. Query execution time refers to the time spent executing a query command, excluding IO operations such as client response (talking) and sending replies. In addition, the slow log is stored in the memory and the read and write speed is very fast, so you can use it with confidence and don't have to worry about damaging the speed of Redis by turning on the slow log.

Setting and viewing SLOWLOG

The server configuration has two options related to the slow query log:

slowlog-log-slower- than: option specifies that command requests whose execution time exceeds how many microseconds (1 second is equal to 1,000,000 microseconds) will be recorded in the log. For example, if the value of this option is 100, then commands that take more than 100 microseconds to execute will be recorded in the slow query log; if the value of this option is 500, then commands that take more than 500 microseconds to execute will be logged. Logging to the slow query log; and so on.

The option slowlog-max-len is used to determine the maximum number of slow query logs retained by the server. The server uses a first-in, first-out method to save multiple slow query logs: When the number of slow query logs stored by the server is equal to the value of the slowlog-max-len option, the server will first add the oldest slow query log before adding a new slow query log. A slow query log is deleted. For example, if the value of slowlog-max-len of the server is 100, and assuming that the server has stored 100 slow query logs, then if the server intends to add a new log, it must first delete the oldest one currently saved. logs, and then add new logs.

Let us look at an example of the slow query log function. First, use the CONFIG_SET command to set the value of the slowlog-log-slower-than option to 0 microseconds, so that any command executed by the Redis server will be recorded. In the slow query log, then set the value of the slowlog-max-len option to 5, so that the server can only save up to 5 slow query logs:

redis> CONFIG SET slowlog-log-slower-than 0
OK

redis> CONFIG SET slowlog-max-len 5
OK

Next, we use the client to send several command requests:

redis> SET msg "hello world"OK

redis> SET number 10086
OK

redis> SET database "Redis"OK

Then use the SLOWLOG GET command to view the slow query log saved by the server:

redis> SLOWLOG GET
1) 1) (integer) 4               #日志的唯一标识符（uid）;  2) (integer) 1378781447      #命令执行时的UNIX时间戳;  3) (integer) 13              #命令执行的时长，以微秒计算;  4) 1) "SET"                  #命令以及命令参数;     2) "database"     3) "Redis"2) 1) (integer) 3
  2) (integer) 1378781439
  3) (integer) 10
  4) 1) "SET"     2) "number"     3) "10086"3) 1) (integer) 2
  2) (integer) 1378781436
  3) (integer) 18
  4) 1) "SET"     2) "msg"     3) "hello world"4) 1) (integer) 1
  2) (integer) 1378781425
  3) (integer) 11
  4) 1) "CONFIG"  2) "SET"  3) "slowlog-max-len"  4) "5"5) 1) (integer) 0
  2) (integer) 1378781415
  3) (integer) 53
  4) 1) "CONFIG"     2) "SET"     3) "slowlog-log-slower-than"     4) "0"

If we execute another SLOWLOG GET 1 command at this time, we will see that the last executed SLOWLOG GET The command has been recorded in the slow query log, and the oldest slow query log numbered 0 has been deleted. The number of slow query logs on the server is still 5:

redis> SLOWLOG GET 1
1) 1) (integer) 5
  2) (integer) 1378781521
  3) (integer) 61
  4) 1) "SLOWLOG"     2) "GET"

The unique ID of the log is only in It will only be reset when the Redis server is restarted, which can avoid repeated processing of logs (for example, you may want to send an email to notify you every time a new slow query is discovered).

Use the command SLOWLOG LEN to view the current number of logs.

Please note the difference between this value and slower-max-len. One of them is the number of current logs, and the other is the maximum number of logs allowed to be recorded.

redis> SLOWLOG LEN
(integer) 5

Clear the log

Use the command SLOWLOG RESET to clear the slow log.

redis> SLOWLOG RESET
OK

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