redis配置文件样例

阳光穿透心脏的1/2处 2022-05-24 02:49 152阅读 0赞

# Redis配置文件样例
    
    # Note on units: when memory size is needed, it is possible to specifiy
    # 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.
    
    # Redis默认不是以守护进程的方式运行，可以通过该配置项修改，使用yes启用守护进程
    # 启用守护进程后，Redis会把pid写到一个pidfile中，在/var/run/redis.pid
    daemonize no
    
    # 当Redis以守护进程方式运行时，Redis默认会把pid写入/var/run/redis.pid文件，可以通过pidfile指定
    pidfile /var/run/redis.pid
    
    # 指定Redis监听端口，默认端口为6379
    # 如果指定0端口，表示Redis不监听TCP连接
    port 6379
    
    # 绑定的主机地址
    # 你可以绑定单一接口，如果没有绑定，所有接口都会监听到来的连接
    # bind 127.0.0.1
    
    # 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 755
    
    # 当客户端闲置多长时间后关闭连接，如果指定为0，表示关闭该功能
    timeout 0
    
    # 指定日志记录级别，Redis总共支持四个级别：debug、verbose、notice、warning，默认为verbose
    # debug (很多信息, 对开发／测试比较有用)
    # 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)
    loglevel verbose
    
    # 日志记录方式，默认为标准输出，如果配置为redis为守护进程方式运行，而这里又配置为标准输出，则日志将会发送给/dev/null
    logfile stdout
    
    # To enable logging to the system logger, just set 'syslog-enabled' to yes,
    # and optionally update the other syslog parameters to suit your needs.
    # syslog-enabled no
    
    # Specify the syslog identity.
    # syslog-ident redis
    
    # Specify the syslog facility.  Must be USER or between LOCAL0-LOCAL7.
    # syslog-facility local0
    
    # 设置数据库的数量，默认数据库为0，可以使用select <dbid>命令在连接上指定数据库id
    # dbid是从0到‘databases’-1的数目
    databases 16
    
    ################################ SNAPSHOTTING  #################################
    # 指定在多长时间内，有多少次更新操作，就将数据同步到数据文件，可以多个条件配合
    # 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.
    #
    #   满足以下条件将会同步数据:
    #   900秒（15分钟）内有1个更改
    #   300秒（5分钟）内有10个更改
    #   60秒内有10000个更改
    #   Note: 可以把所有“save”行注释掉，这样就取消同步操作了
    
    save 900 1
    save 300 10
    save 60 10000
    
    # 指定存储至本地数据库时是否压缩数据，默认为yes，Redis采用LZF压缩，如果为了节省CPU时间，可以关闭该选项，但会导致数据库文件变的巨大
    rdbcompression yes
    
    # 指定本地数据库文件名，默认值为dump.rdb
    dbfilename dump.rdb
    
    # 工作目录.
    # 指定本地数据库存放目录，文件名由上一个dbfilename配置项指定
    # 
    # Also the Append Only File will be created inside this directory.
    # 
    # 注意，这里只能指定一个目录，不能指定文件名
    dir ./
    
    ################################# REPLICATION #################################
    
    # 主从复制。使用slaveof从 Redis服务器复制一个Redis实例。注意，该配置仅限于当前slave有效
    # so for example it is possible to configure the slave to save the DB with a
    # different interval, or to listen to another port, and so on.
    # 设置当本机为slav服务时，设置master服务的ip地址及端口，在Redis启动时，它会自动从master进行数据同步
    # slaveof <masterip> <masterport>
    
    
    # 当master服务设置了密码保护时，slav服务连接master的密码
    # 下文的“requirepass”配置项可以指定密码
    # masterauth <master-password>
    
    # When a slave lost the 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 data 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.
    #
    slave-serve-stale-data yes
    
    # 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.
    #
    # repl-ping-slave-period 10
    
    # The following option sets a timeout for both Bulk transfer I/O timeout and
    # master data or ping response timeout. The default value is 60 seconds.
    #
    # 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.
    #
    # repl-timeout 60
    
    ################################## SECURITY ###################################
    
    # 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Redis连接密码，如果配置了连接密码，客户端在连接Redis时需要通过auth <password>命令提供密码，默认关闭
    # requirepass foobared
    
    # Command renaming.
    #
    # It is possilbe to change the name of dangerous commands in a shared
    # environment. For instance the CONFIG command may be renamed into something
    # of hard to guess so that it will be still available for internal-use
    # tools but not available for general clients.
    #
    # Example:
    #
    # rename-command CONFIG b840fc02d524045429941cc15f59e41cb7be6c52
    #
    # It is also possilbe to completely kill a command renaming it into
    # an empty string:
    #
    # rename-command CONFIG ""
    
    ################################### LIMITS ####################################
    
    # 设置同一时间最大客户端连接数，默认无限制，Redis可以同时打开的客户端连接数为Redis进程可以打开的最大文件描述符数，
    # 如果设置maxclients 0，表示不作限制。当客户端连接数到达限制时，Redis会关闭新的连接并向客户端返回max Number of clients reached错误信息
    # maxclients 128
    
    # Don't use more memory than the specified amount of bytes.
    # When the memory limit is reached Redis will try to remove keys with an
    # EXPIRE set. It will try to start freeing keys that are going to expire
    # in little time and preserve keys with a longer time to live.
    # Redis will also try to remove objects from free lists if possible.
    #
    # If all this fails, Redis will start to reply with errors to commands
    # that will use more memory, like SET, LPUSH, and so on, and will continue
    # to reply to most read-only commands like GET.
    #
    # WARNING: maxmemory can be a good idea mainly if you want to use Redis as a
    # 'state' server or cache, not as a real DB. When Redis is used as a real
    # database the memory usage will grow over the weeks, it will be obvious if
    # it is going to use too much memory in the long run, and you'll have the time
    # to upgrade. With maxmemory after the limit is reached you'll start to get
    # errors for write operations, and this may even lead to DB inconsistency.
    # 指定Redis最大内存限制，Redis在启动时会把数据加载到内存中，达到最大内存后，Redis会先尝试清除已到期或即将到期的Key，
    # 当此方法处理后，仍然到达最大内存设置，将无法再进行写入操作，但仍然可以进行读取操作。
    # Redis新的vm机制，会把Key存放内存，Value会存放在swap区
    # maxmemory <bytes>
    
    # MAXMEMORY POLICY: how Redis will select what to remove when maxmemory
    # is reached? You can select among five behavior:
    # 
    # volatile-lru -> remove the key with an expire set using an LRU algorithm
    # allkeys-lru -> remove any key accordingly 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 all the kind of policies, Redis will return an error on write
    #       operations, when there are not suitable keys for eviction.
    #
    #       At the date of writing this 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 volatile-lru
    
    # LRU and minimal TTL algorithms are not precise algorithms but approximated
    # algorithms (in order to save memory), so you can select as well the sample
    # size to check. For instance for default Redis will check three keys and
    # pick the one that was used less recently, you can change the sample size
    # using the following configuration directive.
    #
    # maxmemory-samples 3
    
    ############################## APPEND ONLY MODE ###############################
    
    # 
    # Note that you can have both the async dumps and the append only file if you
    # like (you have to comment the "save" statements above to disable the dumps).
    # Still if append only mode is enabled Redis will load the data from the
    # log file at startup ignoring the dump.rdb file.
    # 指定是否在每次更新操作后进行日志记录，Redis在默认情况下是异步的把数据写入磁盘，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。默认为no
    # IMPORTANT: Check the BGREWRITEAOF to check how to rewrite the append
    # log file in background when it gets too big.
    
    appendonly no
    
    # 指定更新日志文件名，默认为appendonly.aof
    # appendfilename appendonly.aof
    
    # The fsync() call tells the Operating System to actually write data on disk
    # instead to wait 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.
    
    # 指定更新日志条件，共有3个可选值：
    # no:表示等操作系统进行数据缓存同步到磁盘（快）
    # always:表示每次更新操作后手动调用fsync()将数据写到磁盘（慢，安全）
    # everysec:表示每秒同步一次（折衷，默认值）
    
    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", that in pratical terms means that it is
    # possible to lost 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.
    no-appendfsync-on-rewrite no
    
    # Automatic rewrite of the append only file.
    # Redis is able to automatically rewrite the log file implicitly calling
    # BGREWRITEAOF when the AOF log size will growth by the specified percentage.
    # 
    # This is how it works: Redis remembers the size of the AOF file after the
    # latest rewrite (or if no rewrite 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 precentage of zero in order to disable the automatic AOF
    # rewrite feature.
    
    auto-aof-rewrite-percentage 100
    auto-aof-rewrite-min-size 64mb
    
    ################################## 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 1024
    
    ################################ VIRTUAL MEMORY ###############################
    
    ### WARNING! Virtual Memory is deprecated in Redis 2.4
    ### The use of Virtual Memory is strongly discouraged.
    
    ### WARNING! Virtual Memory is deprecated in Redis 2.4
    ### The use of Virtual Memory is strongly discouraged.
    
    # Virtual Memory allows Redis to work with datasets bigger than the actual
    # amount of RAM needed to hold the whole dataset in memory.
    # In order to do so very used keys are taken in memory while the other keys
    # are swapped into a swap file, similarly to what operating systems do
    # with memory pages.
    # 指定是否启用虚拟内存机制，默认值为no，
    # VM机制将数据分页存放，由Redis将访问量较少的页即冷数据swap到磁盘上，访问多的页面由磁盘自动换出到内存中
    # 把vm-enabled设置为yes，根据需要设置好接下来的三个VM参数，就可以启动VM了
    vm-enabled no
    # vm-enabled yes
    
    # This is the path of the Redis swap file. As you can guess, swap files
    # can't be shared by different Redis instances, so make sure to use a swap
    # file for every redis process you are running. Redis will complain if the
    # swap file is already in use.
    #
    # Redis交换文件最好的存储是SSD（固态硬盘）
    # 虚拟内存文件路径，默认值为/tmp/redis.swap，不可多个Redis实例共享
    # *** WARNING *** if you are using a shared hosting the default of putting
    # the swap file under /tmp is not secure. Create a dir with access granted
    # only to Redis user and configure Redis to create the swap file there.
    vm-swap-file /tmp/redis.swap
    
    # With vm-max-memory 0 the system will swap everything it can. Not a good
    # default, just specify the max amount of RAM you can in bytes, but it's
    # better to leave some margin. For instance specify an amount of RAM
    # that's more or less between 60 and 80% of your free RAM.
    # 将所有大于vm-max-memory的数据存入虚拟内存，无论vm-max-memory设置多少，所有索引数据都是内存存储的（Redis的索引数据就是keys）
    # 也就是说当vm-max-memory设置为0的时候，其实是所有value都存在于磁盘。默认值为0
    vm-max-memory 0
    
    # Redis swap文件分成了很多的page，一个对象可以保存在多个page上面，但一个page上不能被多个对象共享，vm-page-size是要根据存储的数据大小来设定的。
    # 建议如果存储很多小对象，page大小最后设置为32或64bytes；如果存储很大的对象，则可以使用更大的page，如果不确定，就使用默认值
    vm-page-size 32
    
    # 设置swap文件中的page数量由于页表（一种表示页面空闲或使用的bitmap）是存放在内存中的，在磁盘上每8个pages将消耗1byte的内存
    # swap空间总容量为 vm-page-size * vm-pages
    #
    # With the default of 32-bytes memory pages and 134217728 pages Redis will
    # use a 4 GB swap file, that will use 16 MB of RAM for the page table.
    #
    # It's better to use the smallest acceptable value for your application,
    # but the default is large in order to work in most conditions.
    vm-pages 134217728
    
    # Max number of VM I/O threads running at the same time.
    # This threads are used to read/write data from/to swap file, since they
    # also encode and decode objects from disk to memory or the reverse, a bigger
    # number of threads can help with big objects even if they can't help with
    # I/O itself as the physical device may not be able to couple with many
    # reads/writes operations at the same time.
    # 设置访问swap文件的I/O线程数，最后不要超过机器的核数，如果设置为0，那么所有对swap文件的操作都是串行的，可能会造成比较长时间的延迟，默认值为4
    vm-max-threads 4
    
    ############################### ADVANCED CONFIG ###############################
    
    # Hashes are encoded in a special way (much more memory efficient) when they
    # have at max a given numer of elements, and the biggest element does not
    # exceed a given threshold. You can configure this limits with the following
    # configuration directives.
    # 指定在超过一定的数量或者最大的元素超过某一临界值时，采用一种特殊的哈希算法
    hash-max-zipmap-entries 512
    hash-max-zipmap-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-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 happens 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-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:
    zset-max-ziplist-entries 128
    zset-max-ziplist-value 64
    
    # 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 an hash table
    # that is rhashing, 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
    # active rehashing 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 form time to time
    # to queries with 2 milliseconds delay.
    # 指定是否激活重置哈希，默认为开启
    activerehashing yes
    
    ################################## INCLUDES ###################################
    
    # 指定包含其他的配置文件，可以在同一主机上多个Redis实例之间使用同一份配置文件，而同时各实例又拥有自己的特定配置文件
    # include /path/to/local.conf
    # include /path/to/other.conf

**Redis中数据存储模式有2种：cache-only,persistence;**

>  *  cache-only即只做为“缓存”服务，不持久数据，数据在服务终止后将消失，此模式下也将不存在“数据恢复”的手段，是一种安全性低/效率高/容易扩展的方式；
>  *  persistence即为内存中的数据持久备份到磁盘文件，在服务重启后可以恢复，此模式下数据相对安全。

对于persistence持久化存储，Redis提供了两种持久化方法：

>  *  Redis DataBase(简称RDB)
>  *  Append-only file (简称AOF)

除了这两种方法，Redis在早起的版本还存在虚拟内存的方法，现在已经被废弃。

一、RDB概述

RDB是在某个时间点将数据写入一个临时文件，持久化结束后，用这个临时文件替换上次持久化的文件，达到数据恢复。  
优点：**使用单独子进程来进行持久化，主进程不会进行任何IO操作，保证了redis的高性能**   
缺点：**RDB是间隔一段时间进行持久化，如果持久化之间redis发生故障，会发生数据丢失。所以这种方式更适合数据要求不严谨的时候**

这里说的这个执行数据写入到临时文件的时间点是可以通过配置来自己确定的，通过配置**redis在n秒内如果超过m个key被修改**这执行一次RDB操作。这个操作就类似于在这个时间点来保存一次Redis的所有数据，一次快照数据。所有这个持久化方法也通常叫做snapshots。

RDB默认开启，redis.conf中的具体配置参数如下；

#dbfilename：持久化数据存储在本地的文件
    dbfilename dump.rdb
    #dir：持久化数据存储在本地的路径，如果是在/redis/redis-3.0.6/src下启动的redis-cli，则数据会存储在当前src目录下
    dir ./
    ##snapshot触发的时机，save <seconds> <changes>  
    ##如下为900秒后，至少有一个变更操作，才会snapshot  
    ##对于此值的设置，需要谨慎，评估系统的变更操作密集程度  
    ##可以通过“save “””来关闭snapshot功能  
    #save时间，以下分别表示更改了1个key时间隔900s进行持久化存储；更改了10个key300s进行存储；更改10000个key60s进行存储。
    save 900 1
    save 300 10
    save 60 10000
    ##当snapshot时出现错误无法继续时，是否阻塞客户端“变更操作”，“错误”可能因为磁盘已满/磁盘故障/OS级别异常等  
    stop-writes-on-bgsave-error yes  
    ##是否启用rdb文件压缩，默认为“yes”，压缩往往意味着“额外的cpu消耗”，同时也意味这较小的文件尺寸以及较短的网络传输时间  
    rdbcompression yes

snapshot触发的时机，是有“间隔时间”和“变更次数”共同决定，同时符合2个条件才会触发snapshot,否则“变更次数”会被继续累加到下一个“间隔时间”上。snapshot过程中并不阻塞客户端请求。snapshot首先将数据写入临时文件，当成功结束后，将临时文件重名为dump.rdb。

使用RDB恢复数据：   
自动的持久化数据存储到dump.rdb后。实际只要重启redis服务即可完成（启动redis的server时会从dump.rdb中先同步数据）

客户端使用命令进行持久化save存储：

./redis-cli -h ip -p port save
    ./redis-cli -h ip -p port bgsave

一个是在前台进行存储，一个是在后台进行存储。我的client就在server这台服务器上，所以不需要连其他机器，直接./redis-cli bgsave。由于redis是用一个主线程来处理所有 client的请求，这种方式会阻塞所有client请求。所以不推荐使用。另一点需要注意的是，每次快照持久化都是将内存数据完整写入到磁盘一次，并不是增量的只同步脏数据。如果数据量大的话，而且写操作比较多，必然会引起大量的磁盘io操作，可能会严重影响性能。

二、AOF概述

Append-only file，将“操作 + 数据”以格式化指令的方式追加到操作日志文件的尾部，在append操作返回后(已经写入到文件或者即将写入)，才进行实际的数据变更，“日志文件”保存了历史所有的操作过程；当server需要数据恢复时，可以直接replay此日志文件，即可还原所有的操作过程。AOF相对可靠，它和mysql中bin.log、apache.log、zookeeper中txn-log简直异曲同工。AOF文件内容是字符串，非常容易阅读和解析。   
优点：**可以保持更高的数据完整性，如果设置追加file的时间是1s，如果redis发生故障，最多会丢失1s的数据；且如果日志写入不完整支持redis-check-aof来进行日志修复；AOF文件没被rewrite之前（文件过大时会对命令进行合并重写），可以删除其中的某些命令（比如误操作的flushall）。**   
缺点：**AOF文件比RDB文件大，且恢复速度慢。**

我们可以简单的认为AOF就是日志文件，此文件只会记录“变更操作”(例如：set/del等)，如果server中持续的大量变更操作，将会导致AOF文件非常的庞大，意味着server失效后，数据恢复的过程将会很长；事实上，一条数据经过多次变更，将会产生多条AOF记录，其实只要保存当前的状态，历史的操作记录是可以抛弃的；因为AOF持久化模式还伴生了“AOF rewrite”。   
AOF的特性决定了它相对比较安全，如果你期望数据更少的丢失，那么可以采用AOF模式。如果AOF文件正在被写入时突然server失效，有可能导致文件的最后一次记录是不完整，你可以通过手工或者程序的方式去检测并修正不完整的记录，以便通过aof文件恢复能够正常；同时需要提醒，如果你的redis持久化手段中有aof，那么在server故障失效后再次启动前，需要检测aof文件的完整性。

AOF默认关闭，开启方法，修改配置文件reds.conf：appendonly yes

##此选项为aof功能的开关，默认为“no”，可以通过“yes”来开启aof功能  
    ##只有在“yes”下，aof重写/文件同步等特性才会生效  
    appendonly yes  
    
    ##指定aof文件名称  
    appendfilename appendonly.aof  
    
    ##指定aof操作中文件同步策略，有三个合法值：always everysec no,默认为everysec  
    appendfsync everysec  
    ##在aof-rewrite期间，appendfsync是否暂缓文件同步，"no"表示“不暂缓”，“yes”表示“暂缓”，默认为“no”  
    no-appendfsync-on-rewrite no  
    
    ##aof文件rewrite触发的最小文件尺寸(mb,gb),只有大于此aof文件大于此尺寸是才会触发rewrite，默认“64mb”，建议“512mb”  
    auto-aof-rewrite-min-size 64mb  
    
    ##相对于“上一次”rewrite，本次rewrite触发时aof文件应该增长的百分比。  
    ##每一次rewrite之后，redis都会记录下此时“新aof”文件的大小(例如A)，那么当aof文件增长到A*(1 + p)之后  
    ##触发下一次rewrite，每一次aof记录的添加，都会检测当前aof文件的尺寸。  
    auto-aof-rewrite-percentage 100

AOF是文件操作，对于变更操作比较密集的server，那么必将造成磁盘IO的负荷加重；此外linux对文件操作采取了“延迟写入”手段，即并非每次write操作都会触发实际磁盘操作，而是进入了buffer中，当buffer数据达到阀值时触发实际写入(也有其他时机)，这是linux对文件系统的优化，但是这却有可能带来隐患，如果buffer没有刷新到磁盘，此时物理机器失效(比如断电)，那么有可能导致最后一条或者多条aof记录的丢失。通过上述配置文件，可以得知redis提供了3中aof记录同步选项：

>  *  always：每一条aof记录都立即同步到文件，这是最安全的方式，也以为更多的磁盘操作和阻塞延迟，是IO开支较大。
>  *  everysec：每秒同步一次，性能和安全都比较中庸的方式，也是redis推荐的方式。如果遇到物理服务器故障，有可能导致最近一秒内aof记录丢失(可能为部分丢失)。
>  *  no：redis并不直接调用文件同步，而是交给操作系统来处理，操作系统可以根据buffer填充情况/通道空闲时间等择机触发同步；这是一种普通的文件操作方式。性能较好，在物理服务器故障时，数据丢失量会因OS配置有关。

其实，我们可以选择的太少，everysec是最佳的选择。如果你非常在意每个数据都极其可靠，建议你选择一款“关系性数据库”吧。   
AOF文件会不断增大，它的大小直接影响“故障恢复”的时间,而且AOF文件中历史操作是可以丢弃的。**AOF rewrite操作就是“压缩”AOF文件的过程，当然redis并没有采用“基于原aof文件”来重写的方式，而是采取了类似snapshot的方式：基于copy-on-write，全量遍历内存中数据，然后逐个序列到aof文件中。因此AOF rewrite能够正确反应当前内存数据的状态，这正是我们所需要的；\*rewrite过程中，对于新的变更操作将仍然被写入到原AOF文件中，同时这些新的变更操作也会被redis收集起来(buffer，copy-on-write方式下，最极端的可能是所有的key都在此期间被修改，将会耗费2倍内存)，当内存数据被全部写入到新的aof文件之后，收集的新的变更操作也将会一并追加到新的aof文件中，此后将会重命名新的aof文件为appendonly.aof,此后所有的操作都将被写入新的aof文件。**如果在rewrite过程中，出现故障，将不会影响原AOF文件的正常工作，只有当rewrite完成之后才会切换文件，因为rewrite过程是比较可靠的。\*

触发rewrite的时机可以通过配置文件来声明，同时redis中可以通过bgrewriteaof指令人工干预。

redis-cli -h ip -p port bgrewriteaof

因为rewrite操作/aof记录同步/snapshot都消耗磁盘IO，redis采取了“schedule”策略：无论是“人工干预”还是系统触发，snapshot和rewrite需要逐个被执行。

AOF rewrite过程并不阻塞客户端请求。系统会开启一个子进程来完成。

三.总结：

AOF和RDB各有优缺点，这是有它们各自的特点所决定：

>  *  1) AOF更加安全，可以将数据更加及时的同步到文件中，但是AOF需要较多的磁盘IO开支，AOF文件尺寸较大，文件内容恢复数度相对较慢。   
>     \*2) snapshot，安全性较差，它是“正常时期”数据备份以及master-slave数据同步的最佳手段，文件尺寸较小，恢复数度较快。

可以通过配置文件来指定它们中的一种，或者同时使用它们(不建议同时使用)，或者全部禁用，在架构良好的环境中，master通常使用AOF，slave使用snapshot，主要原因是master需要首先确保数据完整性，它作为数据备份的第一选择；slave提供只读服务(目前slave只能提供读取服务)，它的主要目的就是快速响应客户端read请求；但是如果你的redis运行在网络稳定性差/物理环境糟糕情况下，建议你master和slave均采取AOF，这个在master和slave角色切换时，可以减少“人工数据备份”/“人工引导数据恢复”的时间成本；如果你的环境一切非常良好，且服务需要接收密集性的write操作，那么建议master采取snapshot，而slave采用AOF。