redis(4)AOF持久化

AOF机制对每条写入命令作为日志，以append-only的模式写入一个日志文件中，在redis重启的时候，可以通过回放AOF日志中的写入指令来重新构建整个数据集。

2、AOF持久化机制的优点

1. AOF可以更好的保护数据不丢失，AOF默认每隔1秒，通过一个后台线程执行一次fsync操作，最多丢失1秒钟的数据每隔1秒，就执行一次fsync操作，保证os cache中的数据写入磁盘中redis进程挂了，最多丢掉1秒钟的数据；
2. AOF日志文件以append-only模式写入，所以没有任何磁盘寻址的开销，写入性能非常高，而且文件不容易破损，即使文件尾部破损，也很容易修复；
3. AOF日志文件即使过大的时候，出现后台重写操作，也不会影响客户端的读写。因为在rewrite log的时候，会对其中的指导进行压缩，创建出一份需要恢复数据的最小日志出来。再创建新日志文件的时候，老的日志文件还是照常写入。当新的merge后的日志文件ready的时候，再交换新老日志文件即可；
4. AOF日志文件的命令通过非常可读的方式进行记录，这个特性非常适合做灾难性的误删除的紧急恢复。比如不小心用flushall命令清空了所有数据，只要这个时候后台rewrite还没有发生，那么就可以立即拷贝AOF文件，将最后一条flushall命令删除，然后再将该AOF文件放回去，就可以通过恢复机制，自动恢复所有数据；

   3、AOF持久化机制的缺点

5. 对于同一份数据来说，AOF日志文件通常比RDB数据快照文件更大；
6. AOF开启后，支持的写QPS会比RDB支持的写QPS低，因为AOF默认配置每秒fsync一次日志文件；
7. 使用AOF做数据恢复时，会比较慢，还有做冷备，定期的备份，不太方便，可能要自己手写复杂的脚本去做，做冷备不太合适;

   4、数据恢复顺序

   当redis同时有RDB和AOF两种持久化备份时，redis优先使用AOF进行数据还原。若AOF文件不存在，redis则创建一个新的AOF文件，而不从RDB恢复数据。
   若需要从RDB恢复数据，应：
   1 . 停止redis；
   2 . 修改redis配置文件关闭AOF；
   3 . 拷贝RDB文件到redis目录下；
   4 . 启动redis，确保数据已经恢复；
   5 . 在redis命令行中执行以下命令热修改redis配置开启AOF持久化；

   config set appendonly yes
   

6 . 此时redis重新生成AOF持久化文件，且AOF和RDB的数据已经同步；
7 . 停止redis，修改redis配置文件打开AOF持久化，启动redis；

############################## 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.

appendonly no

# The name of the append only file (default: "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".

# appendfsync always
appendfsync everysec
# appendfsync no

1. appendonly no表示redis默认关闭AOF持久化，通过修改no为yes打开AOF持久化；
2. appendfsync always/everysec/no表示AOF的持久化频率，always表示redis每变化一个值就进行一次AOF持久化；everysec表示每秒进行一次AOF持久化，也是redis的默认配置；no表示redis不主动进行AOF持久化，完全由系统触发AOF持久化，可控性差；