redis配置详解说明

待我称王封你为后i 2024-02-18 14:35 35阅读 0赞

##redis配置详解
    
    # Redis configuration file example.
    #
    # Note that in order to read the configuration file, Redis must be
    # started with the file path as first argument:
    #
    # ./redis-server /path/to/redis.conf
    
    # Note on units: when memory size is needed, it is possible to specify
    # it in the usual form of 1k 5GB 4M and so forth:
    #
    # 1k => 1000 bytes
    # 1kb => 1024 bytes
    # 1m => 1000000 bytes
    # 1mb => 1024*1024 bytes
    # 1g => 1000000000 bytes
    # 1gb => 1024*1024*1024 bytes
    #
    # units are case insensitive so 1GB 1Gb 1gB are all the same.
    
    ################################## INCLUDES ###################################
    ################################## 包含     ###################################
    
    # Include one or more other config files here.  This is useful if you
    # have a standard template that goes to all Redis servers but also need
    # to customize a few per-server settings.  Include files can include
    # other files, so use this wisely.
    #
    # Notice option "include" won't be rewritten by command "CONFIG REWRITE"
    # from admin or Redis Sentinel. Since Redis always uses the last processed
    # line as value of a configuration directive, you'd better put includes
    # at the beginning of this file to avoid overwriting config change at runtime.
    #
    # If instead you are interested in using includes to override configuration
    # options, it is better to use include as the last line.
    #
    # 假如说你有一个可用于所有的 redis server 的标准配置模板，
    # 但针对某些 server 又需要一些个性化的设置，
    # 你可以使用 include 来包含一些其他的配置文件，这对你来说是非常有用的。
    #
    # 但是要注意哦，include 是不能被 config rewrite 命令改写的
    # 由于 redis 总是以最后的加工线作为一个配置指令值，所以你最好是把 include 放在这个文件的最前面，
    # 以避免在运行时覆盖配置的改变，相反，你就把它放在后面
    # include /path/to/local.conf
    # include /path/to/other.conf
    
    ################################ GENERAL  #####################################
    ################################ 常用     #####################################
    
    # By default Redis does not run as a daemon. Use 'yes' if you need it.
    # Note that Redis will write a pid file in /var/run/redis.pid when daemonized.
    # 默认情况下 redis 不是作为守护进程运行的，如果你想让它在后台运行，你就把它改成 yes。
    # 当redis作为守护进程运行的时候，它会写一个 pid 到 /var/run/redis.pid 文件里面。
    daemonize yes
    
    # When running daemonized, Redis writes a pid file in /var/run/redis.pid by
    # default. You can specify a custom pid file location here.
    # 当 Redis 以守护进程的方式运行的时候，Redis 默认会把 pid 文件放在/var/run/redis.pid
    # 可配置到其他地址，当运行多个 redis 服务时，需要指定不同的 pid 文件和端口
    # 指定存储Redis进程号的文件路径
    pidfile /var/run/redis.pid
    
    # Accept connections on the specified port, default is 6379.
    # If port 0 is specified Redis will not listen on a TCP socket.
    # 端口，默认端口是6379，生产环境中建议更改端口号，安全性更高
    # 如果你设为 0 ，redis 将不在 socket 上监听任何客户端连接。
    port 9966
    
    # TCP listen() backlog.
    #
    # In high requests-per-second environments you need an high backlog in order
    # to avoid slow clients connections issues. Note that the Linux kernel
    # will silently truncate it to the value of /proc/sys/net/core/somaxconn so
    # make sure to raise both the value of somaxconn and tcp_max_syn_backlog
    # in order to get the desired effect.
    # TCP 监听的最大容纳数量
    # 此参数确定了TCP连接中已完成队列(完成三次握手之后)的长度，
    # 当系统并发量大并且客户端速度缓慢的时候，你需要把这个值调高以避免客户端连接缓慢的问题。
    # Linux 内核会一声不响的把这个值缩小成 /proc/sys/net/core/somaxconn 对应的值，默认是511，而Linux的默认参数值是128。
    # 所以可以将这二个参数一起参考设定，你以便达到你的预期。
    #  
    tcp-backlog 511
    
    # By default Redis listens for connections from all the network interfaces
    # available on the server. It is possible to listen to just one or multiple
    # interfaces using the "bind" configuration directive, followed by one or
    # more IP addresses.
    #
    # Examples:
    #
    # bind 192.168.1.100 10.0.0.1
    # 有时候为了安全起见，redis一般都是监听127.0.0.1 但是有时候又有同网段能连接的需求，当然可以绑定0.0.0.0 用iptables来控制访问权限，或者设置redis访问密码来保证数据安全
    
    # 不设置将处理所有请求,建议生产环境中设置，有个误区：bind是用来限制外网IP访问的，其实不是，限制外网ip访问可以通过iptables；如：-A INPUT -s 10.10.1.0/24 -p tcp -m state --state NEW -m tcp --dport 9966 -j ACCEPT ；
    # 实际上，bind ip 绑定的是redis所在服务器网卡的ip，当然127.0.0.1也是可以的
    #如果绑定一个外网ip，就会报错：Creating Server TCP listening socket xxx.xxx.xxx.xxx:9966: bind: Cannot assign requested address
    
    # bind 127.0.0.1
    bind 127.0.0.1 10.10.1.3
    
    # 假设绑定是以上ip，使用 netstat -anp|grep 9966 会发现，这两个ip被bind，其中10.10.1.3是服务器网卡的ip
    # tcp        0      0 10.10.1.3:9966         0.0.0.0:*                   LISTEN      11188/redis-server  
    # tcp        0      0 127.0.0.1:9966         0.0.0.0:*                   LISTEN      11188/redis-server 
    
    
    # Specify the path for the Unix socket that will be used to listen for
    # incoming connections. There is no default, so Redis will not listen
    # on a unix socket when not specified.
    #
    # unixsocket /tmp/redis.sock
    # unixsocketperm 700
    
    # Close the connection after a client is idle for N seconds (0 to disable)
    # 客户端和Redis服务端的连接超时时间，默认是0，表示永不超时。
    timeout 0
    
    # TCP keepalive.
    #
    # If non-zero, use SO_KEEPALIVE to send TCP ACKs to clients in absence
    # of communication. This is useful for two reasons:
    #
    # 1) Detect dead peers.
    # 2) Take the connection alive from the point of view of network
    #    equipment in the middle.
    #
    # On Linux, the specified value (in seconds) is the period used to send ACKs.
    # Note that to close the connection the double of the time is needed.
    # On other kernels the period depends on the kernel configuration.
    #
    # A reasonable value for this option is 60 seconds.
    
    # tcp 心跳包。
    #
    # 如果设置为非零，则在与客户端缺乏通讯的时候使用 SO_KEEPALIVE 发送 tcp acks 给客户端。
    # 这个之所有有用，主要由两个原因：
    #
    # 1) 防止死的 peers
    # 2) Take the connection alive from the point of view of network
    #    equipment in the middle.
    #
    # 推荐一个合理的值就是60秒
    tcp-keepalive 0
    
    # Specify the server verbosity level.
    # This can be one of:
    # debug (a lot of information, useful for development/testing)
    # verbose (many rarely useful info, but not a mess like the debug level)
    # notice (moderately verbose, what you want in production probably)
    # warning (only very important / critical messages are logged)
    # 日志记录等级，4个可选值debug,verbose,notice,warning
    # 可以是下面的这些值：
    # debug (适用于开发或测试阶段)
    # verbose (many rarely useful info, but not a mess like the debug level)
    # notice (适用于生产环境)
    # warning (仅仅一些重要的消息被记录)
    loglevel notice
    
    # Specify the log file name. Also the empty string can be used to force
    # Redis to log on the standard output. Note that if you use standard
    # output for logging but daemonize, logs will be sent to /dev/null
    #配置 log 文件地址,默认打印在命令行终端的窗口上，也可设为/dev/null屏蔽日志、
    logfile "/data/logs/redis/redis.log"
    
    # To enable logging to the system logger, just set 'syslog-enabled' to yes,
    # and optionally update the other syslog parameters to suit your needs.
    # 要想把日志记录到系统日志，就把它改成 yes，
    # 也可以可选择性的更新其他的syslog 参数以达到你的要求
    # syslog-enabled no
    
    # Specify the syslog identity.
    # 设置 syslog 的 identity。
    # syslog-ident redis
    
    # Specify the syslog facility. Must be USER or between LOCAL0-LOCAL7.
    # syslog-facility local0
    
    # Set the number of databases. The default database is DB 0, you can select
    # a different one on a per-connection basis using SELECT <dbid> where
    # dbid is a number between 0 and 'databases'-1
    # 可用的数据库数，默认值为16，默认数据库为0，数据库范围在0-（database-1）之间
    databases 16
    
    ################################ SNAPSHOTTING  ################################
    ################################ 快照          ################################
    #
    # Save the DB on disk:
    #
    #   save <seconds> <changes>
    #
    #   Will save the DB if both the given number of seconds and the given
    #   number of write operations against the DB occurred.
    #
    #   In the example below the behaviour will be to save:
    #   after 900 sec (15 min) if at least 1 key changed
    #   after 300 sec (5 min) if at least 10 keys changed
    #   after 60 sec if at least 10000 keys changed
    #
    #   Note: you can disable saving completely by commenting out all "save" lines.
    #
    #   It is also possible to remove all the previously configured save
    #   points by adding a save directive with a single empty string argument
    #   like in the following example:
    #
    #   save ""
    # 在 900 秒内最少有 1 个 key 被改动，或者 300 秒内最少有 10 个 key 被改动，又或者 60 秒内最少有 1000 个 key 被改动，以上三个条件随便满足一个，就触发一次保存操作。
    
    #    if(在60秒之内有10000个keys发生变化时){
    #      进行镜像备份
    #    }else if(在300秒之内有10个keys发生了变化){
    #      进行镜像备份
    #    }else if(在900秒之内有1个keys发生了变化){
    #      进行镜像备份
    #    }
    
    save 900 1
    save 300 10
    save 60 10000
    
    # By default Redis will stop accepting writes if RDB snapshots are enabled
    # (at least one save point) and the latest background save failed.
    # This will make the user aware (in a hard way) that data is not persisting
    # on disk properly, otherwise chances are that no one will notice and some
    #:/ disaster will happen.
    #
    # If the background saving process will start working again Redis will
    # automatically allow writes again.
    #
    # However if you have setup your proper monitoring of the Redis server
    # and persistence, you may want to disable this feature so that Redis will
    # continue to work as usual even if there are problems with disk,
    # permissions, and so forth.
    # 默认情况下，如果 redis 最后一次的后台保存失败，redis 将停止接受写操作，
    # 这样以一种强硬的方式让用户知道数据不能正确的持久化到磁盘，
    # 否则就会没人注意到灾难的发生。
    #
    # 如果后台保存进程重新启动工作了，redis 也将自动的允许写操作。
    #
    # 然而你要是安装了靠谱的监控，你可能不希望 redis 这样做，那你就改成 no 好
    stop-writes-on-bgsave-error yes
    
    # Compress string objects using LZF when dump .rdb databases?
    # For default that's set to 'yes' as it's almost always a win.
    # If you want to save some CPU in the saving child set it to 'no' but
    # the dataset will likely be bigger if you have compressible values or keys.
    # 在进行备份时,是否进行压缩
    # 是否在 dump .rdb 数据库的时候使用 LZF 压缩字符串
    # 默认都设为 yes
    # 如果你希望保存子进程节省点 cpu ，你就设置它为 no ，
    # 不过这个数据集可能就会比较大
    rdbcompression yes
    
    # Since version 5 of RDB a CRC64 checksum is placed at the end of the file.
    # This makes the format more resistant to corruption but there is a performance
    # hit to pay (around 10%) when saving and loading RDB files, so you can disable it
    # for maximum performances.
    #
    # RDB files created with checksum disabled have a checksum of zero that will
    # tell the loading code to skip the check.    
    # 读取和写入的时候是否支持CRC64校验，默认是开启的
    rdbchecksum yes
    
    # The filename where to dump the DB
    # 备份文件的文件名
    dbfilename dump.rdb
    
    # The working directory.
    #
    # The DB will be written inside this directory, with the filename specified
    # above using the 'dbfilename' configuration directive.
    #
    # The Append Only File will also be created inside this directory.
    #
    # Note that you must specify a directory here, not a file name.
    # 数据库备份的文件放置的路径
    # 路径跟文件名分开配置是因为 Redis 备份时，先会将当前数据库的状态写入到一个临时文件
    # 等备份完成时，再把该临时文件替换为上面所指定的文件
    # 而临时文件和上面所配置的备份文件都会放在这个指定的路径当中
    # 默认值为 ./
    dir /data/data/redis/
    
    ################################# REPLICATION #################################
    ################################# 主从复制    #################################
    # Master-Slave replication. Use slaveof to make a Redis instance a copy of
    # another Redis server. A few things to understand ASAP about Redis replication.
    #
    # 1) Redis replication is asynchronous, but you can configure a master to
    #    stop accepting writes if it appears to be not connected with at least
    #    a given number of slaves.
    # 2) Redis slaves are able to perform a partial resynchronization with the
    #    master if the replication link is lost for a relatively small amount of
    #    time. You may want to configure the replication backlog size (see the next
    #    sections of this file) with a sensible value depending on your needs.
    # 3) Replication is automatic and does not need user intervention. After a
    #    network partition slaves automatically try to reconnect to masters
    #    and resynchronize with them.
    #
    # 设置该数据库为其他数据库的从数据库
    # slaveof <masterip> <masterport> 当本机为从服务时，设置主服务的IP及端口
    # slaveof <masterip> <masterport>
    
    # If the master is password protected (using the "requirepass" configuration
    # directive below) it is possible to tell the slave to authenticate before
    # starting the replication synchronization process, otherwise the master will
    # refuse the slave request.
    #
    # 指定与主数据库连接时需要的密码验证
    # masterauth <master-password> 当本机为从服务时，设置访问master服务器的密码
    # masterauth <master-password>
    
    # When a slave loses its connection with the master, or when the replication
    # is still in progress, the slave can act in two different ways:
    #
    # 1) if slave-serve-stale-data is set to 'yes' (the default) the slave will
    #    still reply to client requests, possibly with out of date data, or the
    #    data set may just be empty if this is the first synchronization.
    #
    # 2) if slave-serve-stale-data is set to 'no' the slave will reply with
    #    an error "SYNC with master in progress" to all the kind of commands
    #    but to INFO and SLAVEOF.
    #
    # 当slave服务器和master服务器失去连接后，或者当数据正在复制传输的时候，如果此参数值设置“yes”，slave服务器可以继续接受客户端的请求，否则，会返回给请求的客户端如下信息“SYNC with master in progress”,除了INFO，SLAVEOF这两个命令
    slave-serve-stale-data yes
    
    # You can configure a slave instance to accept writes or not. Writing against
    # a slave instance may be useful to store some ephemeral data (because data
    # written on a slave will be easily deleted after resync with the master) but
    # may also cause problems if clients are writing to it because of a
    # misconfiguration.
    #
    # Since Redis 2.6 by default slaves are read-only.
    #
    # Note: read only slaves are not designed to be exposed to untrusted clients
    # on the internet. It's just a protection layer against misuse of the instance.
    # Still a read only slave exports by default all the administrative commands
    # such as CONFIG, DEBUG, and so forth. To a limited extent you can improve
    # security of read only slaves using 'rename-command' to shadow all the
    # administrative / dangerous commands.
    # 是否允许slave服务器节点只提供读服务
    slave-read-only yes
    
    # Replication SYNC strategy: disk or socket.
    #
    # -------------------------------------------------------
    # WARNING: DISKLESS REPLICATION IS EXPERIMENTAL CURRENTLY
    # -------------------------------------------------------
    #
    # New slaves and reconnecting slaves that are not able to continue the replication
    # process just receiving differences, need to do what is called a "full
    # synchronization". An RDB file is transmitted from the master to the slaves.
    # The transmission can happen in two different ways:
    #
    # 1) Disk-backed: The Redis master creates a new process that writes the RDB
    #                 file on disk. Later the file is transferred by the parent
    #                 process to the slaves incrementally.
    # 2) Diskless: The Redis master creates a new process that directly writes the
    #              RDB file to slave sockets, without touching the disk at all.
    #
    # With disk-backed replication, while the RDB file is generated, more slaves
    # can be queued and served with the RDB file as soon as the current child producing
    # the RDB file finishes its work. With diskless replication instead once
    # the transfer starts, new slaves arriving will be queued and a new transfer
    # will start when the current one terminates.
    #
    # When diskless replication is used, the master waits a configurable amount of
    # time (in seconds) before starting the transfer in the hope that multiple slaves
    # will arrive and the transfer can be parallelized.
    #
    # With slow disks and fast (large bandwidth) networks, diskless replication
    # works better.
    repl-diskless-sync no
    
    # When diskless replication is enabled, it is possible to configure the delay
    # the server waits in order to spawn the child that transfers the RDB via socket
    # to the slaves.
    #
    # This is important since once the transfer starts, it is not possible to serve
    # new slaves arriving, that will be queued for the next RDB transfer, so the server
    # waits a delay in order to let more slaves arrive.
    #
    # The delay is specified in seconds, and by default is 5 seconds. To disable
    # it entirely just set it to 0 seconds and the transfer will start ASAP.
    repl-diskless-sync-delay 5
    
    # Slaves send PINGs to server in a predefined interval. It's possible to change
    # this interval with the repl_ping_slave_period option. The default value is 10
    # seconds.
    #
    # Slaves 在一个预定义的时间间隔内发送 ping 命令到 server 。
    # 你可以改变这个时间间隔。默认为 10 秒。
    # repl-ping-slave-period 10
    
    # The following option sets the replication timeout for:
    #
    # 1) Bulk transfer I/O during SYNC, from the point of view of slave.
    # 2) Master timeout from the point of view of slaves (data, pings).
    # 3) Slave timeout from the point of view of masters (REPLCONF ACK pings).
    #
    # It is important to make sure that this value is greater than the value
    # specified for repl-ping-slave-period otherwise a timeout will be detected
    # every time there is low traffic between the master and the slave.
    #
    # 设置主从复制过期时间
    # 这个值一定要比 repl-ping-slave-period 大
    # repl-timeout 60
    
    # Disable TCP_NODELAY on the slave socket after SYNC?
    #
    # If you select "yes" Redis will use a smaller number of TCP packets and
    # less bandwidth to send data to slaves. But this can add a delay for
    # the data to appear on the slave side, up to 40 milliseconds with
    # Linux kernels using a default configuration.
    #
    # If you select "no" the delay for data to appear on the slave side will
    # be reduced but more bandwidth will be used for replication.
    #
    # By default we optimize for low latency, but in very high traffic conditions
    # or when the master and slaves are many hops away, turning this to "yes" may
    # be a good idea.
    # 指定向slave同步数据时，是否禁用socket的NO_DELAY选 项。若配置为“yes”，则禁用NO_DELAY，则TCP协议栈会合并小包统一发送，这样可以减少主从节点间的包数量并节省带宽，但会增加数据同步到 slave的时间。若配置为“no”，表明启用NO_DELAY，则TCP协议栈不会延迟小包的发送时机，这样数据同步的延时会减少，但需要更大的带宽。 通常情况下，应该配置为no以降低同步延时，但在主从节点间网络负载已经很高的情况下，可以配置为yes。
    repl-disable-tcp-nodelay no
    
    # Set the replication backlog size. The backlog is a buffer that accumulates
    # slave data when slaves are disconnected for some time, so that when a slave
    # wants to reconnect again, often a full resync is not needed, but a partial
    # resync is enough, just passing the portion of data the slave missed while
    # disconnected.
    #
    # The bigger the replication backlog, the longer the time the slave can be
    # disconnected and later be able to perform a partial resynchronization.
    #
    # The backlog is only allocated once there is at least a slave connected.
    #
    # 设置主从复制容量大小。这个 backlog 是一个用来在 slaves 被断开连接时
    # 存放 slave 数据的 buffer，所以当一个 slave 想要重新连接，通常不希望全部重新同步，
    # 只是部分同步就够了，仅仅传递 slave 在断开连接时丢失的这部分数据。
    #
    # The biggest the replication backlog, the longer the time the slave can be
    # disconnected and later be able to perform a partial resynchronization.
    # 这个值越大，salve 可以断开连接的时间就越长。
    
    # repl-backlog-size 1mb
    
    # After a master has no longer connected slaves for some time, the backlog
    # will be freed. The following option configures the amount of seconds that
    # need to elapse, starting from the time the last slave disconnected, for
    # the backlog buffer to be freed.
    #
    # A value of 0 means to never release the backlog.
    #
    # 在某些时候，master 不再连接 slaves，backlog 将被释放。
    # 如果设置为 0 ，意味着绝不释放 backlog 。
    # repl-backlog-ttl 3600
    
    # The slave priority is an integer number published by Redis in the INFO output.
    # It is used by Redis Sentinel in order to select a slave to promote into a
    # master if the master is no longer working correctly.
    #
    # A slave with a low priority number is considered better for promotion, so
    # for instance if there are three slaves with priority 10, 100, 25 Sentinel will
    # pick the one with priority 10, that is the lowest.
    #
    # However a special priority of 0 marks the slave as not able to perform the
    # role of master, so a slave with priority of 0 will never be selected by
    # Redis Sentinel for promotion.
    #
    # By default the priority is 100.
    # 指定slave的优先级。在不只1个slave存在的部署环境下，当master宕机时，Redis
    # Sentinel会将priority值最小的slave提升为master。
    # 这个值越小，就越会被优先选中，需要注意的是，
    # 若该配置项为0，则对应的slave永远不会自动提升为master。
    slave-priority 100
    
    # It is possible for a master to stop accepting writes if there are less than
    # N slaves connected, having a lag less or equal than M seconds.
    #
    # The N slaves need to be in "online" state.
    #
    # The lag in seconds, that must be <= the specified value, is calculated from
    # the last ping received from the slave, that is usually sent every second.
    #
    # This option does not GUARANTEE that N replicas will accept the write, but
    # will limit the window of exposure for lost writes in case not enough slaves
    # are available, to the specified number of seconds
    #
    # For example to require at least 3 slaves with a lag <= 10 seconds use:
    #
    # min-slaves-to-write 3
    # min-slaves-max-lag 10
    #
    # Setting one or the other to 0 disables the feature.
    #
    # By default min-slaves-to-write is set to 0 (feature disabled) and
    # min-slaves-max-lag is set to 10.
    
    ################################## SECURITY ###################################
    ################################## 安全     ###################################
    
    # Require clients to issue AUTH <PASSWORD> before processing any other
    # commands.  This might be useful in environments in which you do not trust
    # others with access to the host running redis-server.
    #
    # This should stay commented out for backward compatibility and because most
    # people do not need auth (e.g. they run their own servers).
    #
    # Warning: since Redis is pretty fast an outside user can try up to
    # 150k passwords per second against a good box. This means that you should
    # use a very strong password otherwise it will be very easy to break.
    #
    # 设置连接redis的密码
    # redis速度相当快，一个外部用户在一秒钟进行150K次密码尝试，需指定强大的密码来防止暴力破解
    requirepass set_enough_strong_passwd
    
    # Command renaming.
    #
    # It is possible to change the name of dangerous commands in a shared
    # environment. For instance the CONFIG command may be renamed into something
    # hard to guess so that it will still be available for internal-use tools
    # but not available for general clients.
    #
    # Example:
    #
    # rename-command CONFIG b840fc02d524045429941cc15f59e41cb7be6c52
    #
    # It is also possible to completely kill a command by renaming it into
    # an empty string:
    #
    # rename-command CONFIG ""
    #
    # Please note that changing the name of commands that are logged into the
    # AOF file or transmitted to slaves may cause problems.
    # 重命名一些高危命令，用来禁止高危命令
    rename-command FLUSHALL ZYzv6FOBdwflW2nX
    rename-command CONFIG aI7zwm1GDzMMrEi
    rename-command EVAL S9UHPKEpSvUJMM
    rename-command FLUSHDB D60FPVDJuip7gy6l
    
    ################################### LIMITS ####################################
    ################################### 限制   ####################################
    
    # Set the max number of connected clients at the same time. By default
    # this limit is set to 10000 clients, however if the Redis server is not
    # able to configure the process file limit to allow for the specified limit
    # the max number of allowed clients is set to the current file limit
    # minus 32 (as Redis reserves a few file descriptors for internal uses).
    #
    # Once the limit is reached Redis will close all the new connections sending
    # an error 'max number of clients reached'.
    #
    # 限制同时连接的客户数量,默认是10000
    # 当连接数超过这个值时，redis 将不再接收其他连接请求，客户端尝试连接时将收到 error 信息
    # maxclients 10000
    
    # Don't use more memory than the specified amount of bytes.
    # When the memory limit is reached Redis will try to remove keys
    # according to the eviction policy selected (see maxmemory-policy).
    #
    # If Redis can't remove keys according to the policy, or if the policy is
    # set to 'noeviction', Redis will start to reply with errors to commands
    # that would use more memory, like SET, LPUSH, and so on, and will continue
    # to reply to read-only commands like GET.
    #
    # This option is usually useful when using Redis as an LRU cache, or to set
    # a hard memory limit for an instance (using the 'noeviction' policy).
    #
    # WARNING: If you have slaves attached to an instance with maxmemory on,
    # the size of the output buffers needed to feed the slaves are subtracted
    # from the used memory count, so that network problems / resyncs will
    # not trigger a loop where keys are evicted, and in turn the output
    # buffer of slaves is full with DELs of keys evicted triggering the deletion
    # of more keys, and so forth until the database is completely emptied.
    #
    # In short... if you have slaves attached it is suggested that you set a lower
    # limit for maxmemory so that there is some free RAM on the system for slave
    # output buffers (but this is not needed if the policy is 'noeviction').
    #
    # 设置redis能够使用的最大内存。
    # 达到最大内存设置后，Redis会先尝试清除已到期或即将到期的Key（设置过expire信息的key）
    # 在删除时,按照过期时间进行删除，最早将要被过期的key将最先被删除
    # 如果已到期或即将到期的key删光，仍进行set操作，那么将返回错误
    # 此时redis将不再接收写请求,只接收get请求。
    # maxmemory的设置比较适合于把redis当作于类似memcached 的缓存来使用
    # maxmemory <bytes>
    
    # MAXMEMORY POLICY: how Redis will select what to remove when maxmemory
    # is reached. You can select among five behaviors:
    #
    # volatile-lru -> remove the key with an expire set using an LRU algorithm
    # allkeys-lru -> remove any key according to the LRU algorithm
    # volatile-random -> remove a random key with an expire set
    # allkeys-random -> remove a random key, any key
    # volatile-ttl -> remove the key with the nearest expire time (minor TTL)
    # noeviction -> don't expire at all, just return an error on write operations
    #
    # Note: with any of the above policies, Redis will return an error on write
    #       operations, when there are no suitable keys for eviction.
    #
    #       At the date of writing these commands are: set setnx setex append
    #       incr decr rpush lpush rpushx lpushx linsert lset rpoplpush sadd
    #       sinter sinterstore sunion sunionstore sdiff sdiffstore zadd zincrby
    #       zunionstore zinterstore hset hsetnx hmset hincrby incrby decrby
    #       getset mset msetnx exec sort
    #
    # The default is:
    #
    # maxmemory-policy noeviction
    
    # LRU and minimal TTL algorithms are not precise algorithms but approximated
    # algorithms (in order to save memory), so you can tune it for speed or
    # accuracy. For default Redis will check five keys and pick the one that was
    # used less recently, you can change the sample size using the following
    # configuration directive.
    #
    # The default of 5 produces good enough results. 10 Approximates very closely
    # true LRU but costs a bit more CPU. 3 is very fast but not very accurate.
    #
    # maxmemory-samples 5
    
    ############################## APPEND ONLY MODE ###############################
    
    # By default Redis asynchronously dumps the dataset on disk. This mode is
    # good enough in many applications, but an issue with the Redis process or
    # a power outage may result into a few minutes of writes lost (depending on
    # the configured save points).
    #
    # The Append Only File is an alternative persistence mode that provides
    # much better durability. For instance using the default data fsync policy
    # (see later in the config file) Redis can lose just one second of writes in a
    # dramatic event like a server power outage, or a single write if something
    # wrong with the Redis process itself happens, but the operating system is
    # still running correctly.
    #
    # AOF and RDB persistence can be enabled at the same time without problems.
    # If the AOF is enabled on startup Redis will load the AOF, that is the file
    # with the better durability guarantees.
    #
    # Please check http://redis.io/topics/persistence for more information.
    
    # redis 默认每次更新操作后会在后台异步的把数据库镜像备份到磁盘，但该备份非常耗时，且备份不宜太频繁
    # redis 同步数据文件是按上面save条件来同步的
    # 如果发生诸如拉闸限电、拔插头等状况,那么将造成比较大范围的数据丢失
    # 所以redis提供了另外一种更加高效的数据库备份及灾难恢复方式
    # 开启append only 模式后,redis 将每一次写操作请求都追加到appendonly.aof 文件中
    # redis重新启动时,会从该文件恢复出之前的状态。
    # 但可能会造成 appendonly.aof 文件过大，所以redis支持BGREWRITEAOF 指令，对appendonly.aof重新整理,默认是不开启的。
    
    appendonly no
    
    # The name of the append only file (default: "appendonly.aof")
    # 默认为appendonly.aof。
    appendfilename "appendonly.aof"
    
    # The fsync() call tells the Operating System to actually write data on disk
    # instead of waiting for more data in the output buffer. Some OS will really flush
    # data on disk, some other OS will just try to do it ASAP.
    #
    # Redis supports three different modes:
    #
    # no: don't fsync, just let the OS flush the data when it wants. Faster.
    # always: fsync after every write to the append only log. Slow, Safest.
    # everysec: fsync only one time every second. Compromise.
    #
    # The default is "everysec", as that's usually the right compromise between
    # speed and data safety. It's up to you to understand if you can relax this to
    # "no" that will let the operating system flush the output buffer when
    # it wants, for better performances (but if you can live with the idea of
    # some data loss consider the default persistence mode that's snapshotting),
    # or on the contrary, use "always" that's very slow but a bit safer than
    # everysec.
    #
    # More details please check the following article:
    # http://antirez.com/post/redis-persistence-demystified.html
    #
    # If unsure, use "everysec".
    
    # 设置对 appendonly.aof 文件进行同步的频率,有三种选择always、everysec、no，默认是everysec表示每秒同步一次。
    # always 表示每次有写操作都进行同步,everysec 表示对写操作进行累积,每秒同步一次。
    # no表示等操作系统进行数据缓存同步到磁盘，都进行同步,everysec 表示对写操作进行累积,每秒同步一次
    # appendfsync always
    # appendfsync everysec
    # appendfsync no
    
    # When the AOF fsync policy is set to always or everysec, and a background
    # saving process (a background save or AOF log background rewriting) is
    # performing a lot of I/O against the disk, in some Linux configurations
    # Redis may block too long on the fsync() call. Note that there is no fix for
    # this currently, as even performing fsync in a different thread will block
    # our synchronous write(2) call.
    #
    # In order to mitigate this problem it's possible to use the following option
    # that will prevent fsync() from being called in the main process while a
    # BGSAVE or BGREWRITEAOF is in progress.
    #
    # This means that while another child is saving, the durability of Redis is
    # the same as "appendfsync none". In practical terms, this means that it is
    # possible to lose up to 30 seconds of log in the worst scenario (with the
    # default Linux settings).
    #
    # If you have latency problems turn this to "yes". Otherwise leave it as
    # "no" that is the safest pick from the point of view of durability.
    # 指定是否在后台aof文件rewrite期间调用fsync，默认为no，表示要调用fsync（无论后台是否有子进程在刷盘）。Redis在后台写RDB文件或重写afo文件期间会存在大量磁盘IO，此时，在某些linux系统中，调用fsync可能会阻塞。
    no-appendfsync-on-rewrite yes
    
    # Automatic rewrite of the append only file.
    # Redis is able to automatically rewrite the log file implicitly calling
    # BGREWRITEAOF when the AOF log size grows by the specified percentage.
    #
    # This is how it works: Redis remembers the size of the AOF file after the
    # latest rewrite (if no rewrite has happened since the restart, the size of
    # the AOF at startup is used).
    #
    # This base size is compared to the current size. If the current size is
    # bigger than the specified percentage, the rewrite is triggered. Also
    # you need to specify a minimal size for the AOF file to be rewritten, this
    # is useful to avoid rewriting the AOF file even if the percentage increase
    # is reached but it is still pretty small.
    #
    # Specify a percentage of zero in order to disable the automatic AOF
    # rewrite feature.
    # 指定Redis重写aof文件的条件，默认为100，表示与上次rewrite的aof文件大小相比，当前aof文件增长量超过上次afo文件大小的100%时，就会触发background rewrite。若配置为0，则会禁用自动rewrite
    auto-aof-rewrite-percentage 100
    
    # 指定触发rewrite的aof文件大小。若aof文件小于该值，即使当前文件的增量比例达到auto-aof-rewrite-percentage的配置值，也不会触发自动rewrite。即这两个配置项同时满足时，才会触发rewrite。
    auto-aof-rewrite-min-size 64mb
    
    # An AOF file may be found to be truncated at the end during the Redis
    # startup process, when the AOF data gets loaded back into memory.
    # This may happen when the system where Redis is running
    # crashes, especially when an ext4 filesystem is mounted without the
    # data=ordered option (however this can't happen when Redis itself
    # crashes or aborts but the operating system still works correctly).
    #
    # Redis can either exit with an error when this happens, or load as much
    # data as possible (the default now) and start if the AOF file is found
    # to be truncated at the end. The following option controls this behavior.
    #
    # If aof-load-truncated is set to yes, a truncated AOF file is loaded and
    # the Redis server starts emitting a log to inform the user of the event.
    # Otherwise if the option is set to no, the server aborts with an error
    # and refuses to start. When the option is set to no, the user requires
    # to fix the AOF file using the "redis-check-aof" utility before to restart
    # the server.
    #
    # Note that if the AOF file will be found to be corrupted in the middle
    # the server will still exit with an error. This option only applies when
    # Redis will try to read more data from the AOF file but not enough bytes
    # will be found.
    aof-load-truncated yes
    
    ################################ LUA SCRIPTING  ###############################
    
    # Max execution time of a Lua script in milliseconds.
    #
    # If the maximum execution time is reached Redis will log that a script is
    # still in execution after the maximum allowed time and will start to
    # reply to queries with an error.
    #
    # When a long running script exceeds the maximum execution time only the
    # SCRIPT KILL and SHUTDOWN NOSAVE commands are available. The first can be
    # used to stop a script that did not yet called write commands. The second
    # is the only way to shut down the server in the case a write command was
    # already issued by the script but the user doesn't want to wait for the natural
    # termination of the script.
    #
    # Set it to 0 or a negative value for unlimited execution without warnings.
    # 一个Lua脚本最长的执行时间，单位为毫秒，如果为0或负数表示无限执行时间，默认为5000
    lua-time-limit 5000
    
    ################################ REDIS CLUSTER  ###############################
    #
    # ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
    # WARNING EXPERIMENTAL: Redis Cluster is considered to be stable code, however
    # in order to mark it as "mature" we need to wait for a non trivial percentage
    # of users to deploy it in production.
    # ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
    #
    # Normal Redis instances can't be part of a Redis Cluster; only nodes that are
    # started as cluster nodes can. In order to start a Redis instance as a
    # cluster node enable the cluster support uncommenting the following:
    #
    # cluster-enabled yes
    
    # Every cluster node has a cluster configuration file. This file is not
    # intended to be edited by hand. It is created and updated by Redis nodes.
    # Every Redis Cluster node requires a different cluster configuration file.
    # Make sure that instances running in the same system do not have
    # overlapping cluster configuration file names.
    #
    # cluster-config-file nodes-6379.conf
    
    # Cluster node timeout is the amount of milliseconds a node must be unreachable
    # for it to be considered in failure state.
    # Most other internal time limits are multiple of the node timeout.
    #
    # cluster-node-timeout 15000
    
    # A slave of a failing master will avoid to start a failover if its data
    # looks too old.
    #
    # There is no simple way for a slave to actually have a exact measure of
    # its "data age", so the following two checks are performed:
    #
    # 1) If there are multiple slaves able to failover, they exchange messages
    #    in order to try to give an advantage to the slave with the best
    #    replication offset (more data from the master processed).
    #    Slaves will try to get their rank by offset, and apply to the start
    #    of the failover a delay proportional to their rank.
    #
    # 2) Every single slave computes the time of the last interaction with
    #    its master. This can be the last ping or command received (if the master
    #    is still in the "connected" state), or the time that elapsed since the
    #    disconnection with the master (if the replication link is currently down).
    #    If the last interaction is too old, the slave will not try to failover
    #    at all.
    #
    # The point "2" can be tuned by user. Specifically a slave will not perform
    # the failover if, since the last interaction with the master, the time
    # elapsed is greater than:
    #
    #   (node-timeout * slave-validity-factor) + repl-ping-slave-period
    #
    # So for example if node-timeout is 30 seconds, and the slave-validity-factor
    # is 10, and assuming a default repl-ping-slave-period of 10 seconds, the
    # slave will not try to failover if it was not able to talk with the master
    # for longer than 310 seconds.
    #
    # A large slave-validity-factor may allow slaves with too old data to failover
    # a master, while a too small value may prevent the cluster from being able to
    # elect a slave at all.
    #
    # For maximum availability, it is possible to set the slave-validity-factor
    # to a value of 0, which means, that slaves will always try to failover the
    # master regardless of the last time they interacted with the master.
    # (However they'll always try to apply a delay proportional to their
    # offset rank).
    #
    # Zero is the only value able to guarantee that when all the partitions heal
    # the cluster will always be able to continue.
    #
    # cluster-slave-validity-factor 10
    
    # Cluster slaves are able to migrate to orphaned masters, that are masters
    # that are left without working slaves. This improves the cluster ability
    # to resist to failures as otherwise an orphaned master can't be failed over
    # in case of failure if it has no working slaves.
    #
    # Slaves migrate to orphaned masters only if there are still at least a
    # given number of other working slaves for their old master. This number
    # is the "migration barrier". A migration barrier of 1 means that a slave
    # will migrate only if there is at least 1 other working slave for its master
    # and so forth. It usually reflects the number of slaves you want for every
    # master in your cluster.
    #
    # Default is 1 (slaves migrate only if their masters remain with at least
    # one slave). To disable migration just set it to a very large value.
    # A value of 0 can be set but is useful only for debugging and dangerous
    # in production.
    #
    # cluster-migration-barrier 1
    
    # By default Redis Cluster nodes stop accepting queries if they detect there
    # is at least an hash slot uncovered (no available node is serving it).
    # This way if the cluster is partially down (for example a range of hash slots
    # are no longer covered) all the cluster becomes, eventually, unavailable.
    # It automatically returns available as soon as all the slots are covered again.
    #
    # However sometimes you want the subset of the cluster which is working,
    # to continue to accept queries for the part of the key space that is still
    # covered. In order to do so, just set the cluster-require-full-coverage
    # option to no.
    #
    # cluster-require-full-coverage yes
    
    # In order to setup your cluster make sure to read the documentation
    # available at http://redis.io web site.
    
    ################################## SLOW LOG ###################################
    
    # The Redis Slow Log is a system to log queries that exceeded a specified
    # execution time. The execution time does not include the I/O operations
    # like talking with the client, sending the reply and so forth,
    # but just the time needed to actually execute the command (this is the only
    # stage of command execution where the thread is blocked and can not serve
    # other requests in the meantime).
    #
    # You can configure the slow log with two parameters: one tells Redis
    # what is the execution time, in microseconds, to exceed in order for the
    # command to get logged, and the other parameter is the length of the
    # slow log. When a new command is logged the oldest one is removed from the
    # queue of logged commands.
    
    # The following time is expressed in microseconds, so 1000000 is equivalent
    # to one second. Note that a negative number disables the slow log, while
    # a value of zero forces the logging of every command.
    slowlog-log-slower-than 10000
    
    # There is no limit to this length. Just be aware that it will consume memory.
    # You can reclaim memory used by the slow log with SLOWLOG RESET.
    slowlog-max-len 128
    
    ################################ LATENCY MONITOR ##############################
    
    # The Redis latency monitoring subsystem samples different operations
    # at runtime in order to collect data related to possible sources of
    # latency of a Redis instance.
    #
    # Via the LATENCY command this information is available to the user that can
    # print graphs and obtain reports.
    #
    # The system only logs operations that were performed in a time equal or
    # greater than the amount of milliseconds specified via the
    # latency-monitor-threshold configuration directive. When its value is set
    # to zero, the latency monitor is turned off.
    #
    # By default latency monitoring is disabled since it is mostly not needed
    # if you don't have latency issues, and collecting data has a performance
    # impact, that while very small, can be measured under big load. Latency
    # monitoring can easily be enabled at runtime using the command
    # "CONFIG SET latency-monitor-threshold <milliseconds>" if needed.
    latency-monitor-threshold 0
    
    ############################# EVENT NOTIFICATION ##############################
    
    # Redis can notify Pub/Sub clients about events happening in the key space.
    # This feature is documented at http://redis.io/topics/notifications
    #
    # For instance if keyspace events notification is enabled, and a client
    # performs a DEL operation on key "foo" stored in the Database 0, two
    # messages will be published via Pub/Sub:
    #
    # PUBLISH __keyspace@0__:foo del
    # PUBLISH __keyevent@0__:del foo
    #
    # It is possible to select the events that Redis will notify among a set
    # of classes. Every class is identified by a single character:
    #
    #  K     Keyspace events, published with __keyspace@<db>__ prefix.
    #  E     Keyevent events, published with __keyevent@<db>__ prefix.
    #  g     Generic commands (non-type specific) like DEL, EXPIRE, RENAME, ...
    #  $     String commands
    #  l     List commands
    #  s     Set commands
    #  h     Hash commands
    #  z     Sorted set commands
    #  x     Expired events (events generated every time a key expires)
    #  e     Evicted events (events generated when a key is evicted for maxmemory)
    #  A     Alias for g$lshzxe, so that the "AKE" string means all the events.
    #
    #  The "notify-keyspace-events" takes as argument a string that is composed
    #  of zero or multiple characters. The empty string means that notifications
    #  are disabled.
    #
    #  Example: to enable list and generic events, from the point of view of the
    #           event name, use:
    #
    #  notify-keyspace-events Elg
    #
    #  Example 2: to get the stream of the expired keys subscribing to channel
    #             name __keyevent@0__:expired use:
    #
    #  notify-keyspace-events Ex
    #
    #  By default all notifications are disabled because most users don't need
    #  this feature and the feature has some overhead. Note that if you don't
    #  specify at least one of K or E, no events will be delivered.
    notify-keyspace-events ""
    
    ############################### ADVANCED CONFIG ###############################
    
    # Hashes are encoded using a memory efficient data structure when they have a
    # small number of entries, and the biggest entry does not exceed a given
    # threshold. These thresholds can be configured using the following directives.
    # 当hash中包含超过指定元素个数并且最大的元素没有超过临界时，
    # hash将以一种特殊的编码方式（大大减少内存使用）来存储，这里可以设置这两个临界值
    hash-max-ziplist-entries 512
    hash-max-ziplist-value 64
    
    # Similarly to hashes, small lists are also encoded in a special way in order
    # to save a lot of space. The special representation is only used when
    # you are under the following limits:
    # list数据类型多少节点以下会采用去指针的紧凑存储格式。
    # list数据类型节点值大小小于多少字节会采用紧凑存储格式。
    list-max-ziplist-entries 512
    list-max-ziplist-value 64
    
    # Sets have a special encoding in just one case: when a set is composed
    # of just strings that happen to be integers in radix 10 in the range
    # of 64 bit signed integers.
    # The following configuration setting sets the limit in the size of the
    # set in order to use this special memory saving encoding.
    # set数据类型内部数据如果全部是数值型，且包含多少节点以下会采用紧凑格式存储。
    set-max-intset-entries 512
    
    # Similarly to hashes and lists, sorted sets are also specially encoded in
    # order to save a lot of space. This encoding is only used when the length and
    # elements of a sorted set are below the following limits:
    
    # zsort数据类型多少节点以下会采用去指针的紧凑存储格式。
    # zsort数据类型节点值大小小于多少字节会采用紧凑存储格式。
    zset-max-ziplist-entries 128
    zset-max-ziplist-value 64
    
    # HyperLogLog sparse representation bytes limit. The limit includes the
    # 16 bytes header. When an HyperLogLog using the sparse representation crosses
    # this limit, it is converted into the dense representation.
    #
    # A value greater than 16000 is totally useless, since at that point the
    # dense representation is more memory efficient.
    #
    # The suggested value is ~ 3000 in order to have the benefits of
    # the space efficient encoding without slowing down too much PFADD,
    # which is O(N) with the sparse encoding. The value can be raised to
    # ~ 10000 when CPU is not a concern, but space is, and the data set is
    # composed of many HyperLogLogs with cardinality in the 0 - 15000 range.
    hll-sparse-max-bytes 3000
    
    # Active rehashing uses 1 millisecond every 100 milliseconds of CPU time in
    # order to help rehashing the main Redis hash table (the one mapping top-level
    # keys to values). The hash table implementation Redis uses (see dict.c)
    # performs a lazy rehashing: the more operation you run into a hash table
    # that is rehashing, the more rehashing "steps" are performed, so if the
    # server is idle the rehashing is never complete and some more memory is used
    # by the hash table.
    #
    # The default is to use this millisecond 10 times every second in order to
    # actively rehash the main dictionaries, freeing memory when possible.
    #
    # If unsure:
    # use "activerehashing no" if you have hard latency requirements and it is
    # not a good thing in your environment that Redis can reply from time to time
    # to queries with 2 milliseconds delay.
    #
    # use "activerehashing yes" if you don't have such hard requirements but
    # want to free memory asap when possible.
    
    # Redis将在每100毫秒时使用1毫秒的CPU时间来对redis的hash表进行重新hash，可以降低内存的使用
    # 当你的使用场景中，有非常严格的实时性需要，不能够接受Redis时不时的对请求有2毫秒的延迟的话，把这项配置为no。
    # 如果没有这么严格的实时性要求，可以设置为yes，以便能够尽可能快的释放内存
    activerehashing yes
    
    # The client output buffer limits can be used to force disconnection of clients
    # that are not reading data from the server fast enough for some reason (a
    # common reason is that a Pub/Sub client can't consume messages as fast as the
    # publisher can produce them).
    #
    # The limit can be set differently for the three different classes of clients:
    #
    # normal -> normal clients including MONITOR clients
    # slave  -> slave clients
    # pubsub -> clients subscribed to at least one pubsub channel or pattern
    #
    # The syntax of every client-output-buffer-limit directive is the following:
    #
    # client-output-buffer-limit <class> <hard limit> <soft limit> <soft seconds>
    #
    # A client is immediately disconnected once the hard limit is reached, or if
    # the soft limit is reached and remains reached for the specified number of
    # seconds (continuously).
    # So for instance if the hard limit is 32 megabytes and the soft limit is
    # 16 megabytes / 10 seconds, the client will get disconnected immediately
    # if the size of the output buffers reach 32 megabytes, but will also get
    # disconnected if the client reaches 16 megabytes and continuously overcomes
    # the limit for 10 seconds.
    #
    # By default normal clients are not limited because they don't receive data
    # without asking (in a push way), but just after a request, so only
    # asynchronous clients may create a scenario where data is requested faster
    # than it can read.
    #
    # Instead there is a default limit for pubsub and slave clients, since
    # subscribers and slaves receive data in a push fashion.
    #
    # Both the hard or the soft limit can be disabled by setting them to zero.
    client-output-buffer-limit normal 0 0 0
    client-output-buffer-limit slave 256mb 64mb 60
    client-output-buffer-limit pubsub 32mb 8mb 60
    
    # Redis calls an internal function to perform many background tasks, like
    # closing connections of clients in timeout, purging expired keys that are
    # never requested, and so forth.
    #
    # Not all tasks are performed with the same frequency, but Redis checks for
    # tasks to perform according to the specified "hz" value.
    #
    # By default "hz" is set to 10. Raising the value will use more CPU when
    # Redis is idle, but at the same time will make Redis more responsive when
    # there are many keys expiring at the same time, and timeouts may be
    # handled with more precision.
    #
    # The range is between 1 and 500, however a value over 100 is usually not
    # a good idea. Most users should use the default of 10 and raise this up to
    # 100 only in environments where very low latency is required.
    hz 10
    
    # When a child rewrites the AOF file, if the following option is enabled
    # the file will be fsync-ed every 32 MB of data generated. This is useful
    # in order to commit the file to the disk more incrementally and avoid
    # big latency spikes.
    # aof rewrite过程中,是否采取增量文件同步策略,默认为“yes”。 rewrite过程中,每32M数据进行一次文件同步,这样可以减少aof大文件写入对磁盘的操作次数
    aof-rewrite-incremental-fsync yes
    
    
    # redis数据存储
    redis的存储分为内存存储、磁盘存储和log文件三部分，配置文件中有三个参数对其进行配置。
    save seconds updates，save配置，指出在多长时间内，有多少次更新操作，就将数据同步到数据文件。可多个条件配合，默认配置了三个条件。
    appendonly yes/no ，appendonly配置，指出是否在每次更新操作后进行日志记录，如果不开启，可能会在断电时导致一段时间内的数据丢失。因为redis本身同步数据文件是按上面的save条件来同步的，所以有的数据会在一段时间内只存在于内存中。
    appendfsync no/always/everysec ，appendfsync配置，no表示等操作系统进行数据缓存同步到磁盘，always表示每次更新操作后手动调用fsync()将数据写到磁盘，everysec表示每秒同步一次。