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目录：
1. RDB
2. AOF（append-only file）
3. RDB 和 AOF 特性比对
4. 混合持久化
redis 数据持久化共有两种方式：一种是RDB，另一个是AOF
1. RDB：
默认情况下，redis 将内存数据库快照保存在名字为 dump.rdb的二进制文件中。
redis.conf 配置中对 rdb 方式 的配置如下：
################################ SNAPSHOTTING ################################ # # 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 "" save 900 1 save 300 10 save 60 10000
对 Redis 进行设置， 让它在“ N 秒内数据集至少有 M 个改动”这一条件被满足时， 自动保存一次数据集到 rdb 文件中。
redis 在数据进行rdb 持久化时，支持
Redis 借助操作系统提供的写时复制技术（Copy-On-Write, COW），在生成快照的同时，依然可以正常处理写命令。简单来说，bgsave 子进程是由主线程 fork 生成的，可以共享主线程的所有内存数据。bgsave 子进程运行后，开始读取主线程的内存数据，并把它们写入 RDB 文件。此时，如果主线程对这些数据也都是读操作，那么，主线程和 bgsave 子进程相互不影响。但是，如果主线程要修改一块数据，那么，这块数据就会被复制一份，生成该数据的副本。然后，bgsave 子进程会把这个副本数据写入 RDB 文件，而在这个过程中，主线程仍然可以直接修改原来的数据。
其 配置如下：
# 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. 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. 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. rdbchecksum yes # The filename where to dump the DB dbfilename dump.rdb
2. AOF（append-only file）：
RDB 快照功能并不是非常耐久， 如果 Redis 因为某些原因而造成故障停机， 那么服务器将丢失最近写入、且仍未保存到快照中的那些数据。
Redis 增加了一种完全耐久的持久化方式： AOF 持久化，将修改的每一条指令记录进文件 appendonly.aof 中. (先写入os cache，每隔一段时间fsync到磁盘)
############################## 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"
AOF 也可配置redis 多久将数据 fsync 同步到磁盘一次。
appendfsync always：每次有新命令追加到 AOF 文件时就执行一次 fsync ，非常慢，也非常安全。 appendfsync everysec：每秒 fsync 一次，足够快，并且在故障时只会丢失 1 秒钟的数据。推荐（并且也是默认） appendfsync no：从不 fsync ，将数据交给操作系统来处理。更快，也更不安全的选择。
3. RDB 和 AOF 特性比对
命令 |
RDB |
AOF |
启动优先级 |
低 |
高 |
体积 |
小 |
大 |
恢复速度 |
快 |
慢 |
数据安全性 |
容易丢数据 |
根据策略决定 |
4. 混合持久化：
Redis 支持同时开启开启两种持久化方式，我们可以综合使用 AOF 和 RDB 两种持久化机制，用 AOF 来保证数据不丢失，作为数据恢复的第一选择；
用 RDB 来做不同程度的冷备，在 AOF 文件都丢失或损坏不可用的时候，还可以使用 RDB 来进行快速的数据恢复。
标签: redis
