Redis之持久化

1、RDB（Redis DataBase）

    在指定时间间隔内将内存中的数据集快照写入磁盘。

    保存的是dump.rdb文件。

    快照设置后进行默认备份。

    命令save（会阻塞其他）或bgsave（后台异步）可立即进行备份。

    执行flushall命令，也会产生dump.rdb文件，但内容为空，无意义。

恢复：

将备份文件dump.rdb文件移动到redis安装目录并启动服务即可，使用config get dir 获取目录。

优势：适合大规模恢复，对数据完整性和一致性要求不高。

劣势：在一定时间间隔做备份，如果意外发生，最后一次快照数据将丢失。FORK进程，内存数据被克隆，数据膨胀耗费内存。

2、AOF（Append Only File）

    以日志的形式来记录每个写操作，将redis的所有写指令记录下来，只能追加文件但不能改写文件，redis启动之初会读取该文件重新构建数据。

    保存的是appendonly.aof文件。

    aof与rdb可以同时存在，但优先加载aof。

    aof文件损坏，可以使用redis-check-aof --fix appendonly.aof进行修复。

    重写：文件越来越大，对文件进行优化精简。内容压缩，只保留可以恢复数据的最小指令集，可以使用命令bgrewriteaof

    优势：每秒同步，每修改同步，不同步

    劣势：aof文件要远大于rdb文件，恢复速度慢于rdb，运行效率要慢于rdb。

    

############################## 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.（两者可以同时使用）
# If the AOF is enabled on startup Redis will load the AOF, that is the file（优先加载AOF）
# with the better durability guarantees.
#
# Please check http://redis.io/topics/persistence for more information.


appendonly no（默认关闭）


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


appendfilename "appendonly.aof"（默认名称）


# 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（同步持久化，每次发生变化立即记录到磁盘）
appendfsync everysec（默认配置，一步操作，每秒记录，一秒内宕机，数据有丢失）
# 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.


no-appendfsync-on-rewrite no（重写时是否可以运用appendfsync ，默认为no即可，）


# 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.


auto-aof-rewrite-percentage 100（设置重写的基准值，上次重写后的一倍时进行重写）
auto-aof-rewrite-min-size 64mb（设置重写的基准值，达到64mb时进行重写）


# 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