Redis删除大Key

原文链接:https://www.dubby.cn/detail.html?id=9112

这里说的大key是指包含很多元素的set,sorted set,list和hash。

删除操作,我们一般想到有2种,delexpire

DEL

Time complexity: O(N) where N is the number of keys that will be removed. When a key to remove holds a value other than a string, the individual complexity for this key is O(M) where M is the number of elements in the list, set, sorted set or hash. Removing a single key that holds a string value is O(1).

如果要删除的key是一个集合,包含了很多元素,那么DEL时的耗时和元素个数成正比,所以如果直接DEL,会很慢。

EXPIRE

Note that calling EXPIRE/PEXPIRE with a non-positive timeout or EXPIREAT/PEXPIREAT with a time in the past will result in the key being deleted rather than expired (accordingly, the emitted key event will be del, not expired).

想着expire会不会可以不是直接删除,可惜官网的描述让我心灰意冷,如果expire后指定的timeout不是正数,也就是<=0,那其实就是DEL

一点一点删

我们知道Redis的工作线程是单线程的,如果一个command堵塞了,那所有请求都会超时,这时候,一些骚操作也许可以帮助你。

其实如果想删除key,可以分解成2个目的,1:不想让其他人访问到这个key,2:释放空间。

那其实我们可以分解成两步,先用RENAME把原先的key rename成另一个key,比如:

RENAME userInfo:123 "deleteKey:userInfo:123"

然后可以慢慢去删"deleteKey:userInfo:123",如果是set,那么可以用SREM慢慢删,最后再用DEL彻底删掉。

这里可以搞个task去SCAN deleteKey:*,然后慢慢删除。

UNLINK

Redis 4.0.0提供了一个更加方便的命令

Available since 4.0.0.

Time complexity: O(1) for each key removed regardless of its size. Then the command does O(N) work in a different thread in order to reclaim memory, where N is the number of allocations the deleted objects where composed of.

UNLINK其实是直接返回,然后在后台线程慢慢删除。

如果你的Redis版本>=4.0.0,那么强烈建议使用UNLINK来删除。

删除耗时测试结果

单位:微秒

Set个数 DEL EXPIRE UNLINK
1 90 97 75
10 79 67 100
100 51 49 47
1000 303 296 49
10000 2773 2592 52
100000 31210 33157 51
1000000 549388 501536 62
package main

import (
    "github.com/go-redis/redis"
    "fmt"
    "time"
)

func main() {
    client := redis.NewClient(&redis.Options{
        Addr:         "localhost:6379",
        Password:     "",
        DB:           0,
        ReadTimeout:  1000 * 1000 * 1000 * 60 * 60 * 24,
        WriteTimeout: 1000 * 1000 * 1000 * 60 * 60 * 24,
    })

    maxLength := int64(10000 * 100)

    for n := int64(1); n <= maxLength; n *= 10 {
        fmt.Println("Set个数", n)
        TestDelBigSet(client, n)
        TestExpireBigSet(client, n)
        TestUnlinkBigSet(client, n)
        fmt.Println()
    }
}

func TestDelBigSet(client *redis.Client, count int64) {
    redisKey := fmt.Sprintf("%s%d", "del:", time.Now().Nanosecond())

    for n := int64(0); n < count; n++ {
        err := client.SAdd(redisKey, fmt.Sprintf("%d", n)).Err()
        if err != nil {
            panic(err)
        }
    }

    startTime := CurrentTimestampInMicroSecond()
    client.Del(redisKey)
    endTime := CurrentTimestampInMicroSecond()

    fmt.Println("Del", endTime-startTime)
}

func TestUnlinkBigSet(client *redis.Client, count int64) {
    redisKey := fmt.Sprintf("%s%d", "unlink:", time.Now().Nanosecond())

    for n := int64(0); n < count; n++ {
        err := client.SAdd(redisKey, fmt.Sprintf("%d", n)).Err()
        if err != nil {
            panic(err)
        }
    }
    startTime := CurrentTimestampInMicroSecond()
    client.Unlink(redisKey)
    endTime := CurrentTimestampInMicroSecond()

    fmt.Println("Unlink", endTime-startTime)
}

func TestExpireBigSet(client *redis.Client, count int64) {
    redisKey := fmt.Sprintf("%s%d", "expire:", time.Now().Nanosecond())

    for n := int64(0); n < count; n++ {
        err := client.SAdd(redisKey, fmt.Sprintf("%d", n)).Err()
        if err != nil {
            panic(err)
        }
    }
    startTime := CurrentTimestampInMicroSecond()
    client.Expire(redisKey, 0)
    endTime := CurrentTimestampInMicroSecond()

    fmt.Println("Expire", endTime-startTime)
}

func CurrentTimestampInMicroSecond() int64 {
    return time.Now().UnixNano() / 1000
}

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