redis的持久化——RDB（Redis DataBase）

小志的博客发布时间：2019-08-12 22:29:45 ，浏览量：1

本文参考自：尚硅谷redis视频

一、RDB是什么

1、在指定的时间间隔内将内存中的数据集快照写入磁盘，也就是行话讲的Snapshot快照，它恢复时是将快照文件直接读到内存里。 2、Redis会单独创建（fork）一个子进程来进行持久化，会先将数据写入到一个临时文件中，待持久化过程都结束了，再用这个临时文件替换上次持久化好的文件。整个过程中，主进程是不进行任何IO操作的，这就确保了极高的性能如果需要进行大规模数据的恢复，且对于数据恢复的完整性不是非常敏感，那RDB方式要比AOF方式更加的高效。RDB的缺点是最后一次持久化后的数据可能丢失。

二、Fork的作用

1、fork的作用是复制一个与当前进程一样的进程。新进程的所有数据（变量、环境变量、程序计数器等）数值都和原进程一致，但是是一个全新的进程，并作为原进程的子进程

三、RDB保存的是什么

1、rdb 保存的是dump.rdb文件

四、RDB配置位置（SNAPSHOTTING）

180 ################################ SNAPSHOTTING  ################################
 181 #
 182 # Save the DB on disk:
 183 #
 184 #   save  
 185 #
 186 #   Will save the DB if both the given number of seconds and the given
 187 #   number of write operations against the DB occurred.
 188 #
 189 #   In the example below the behaviour will be to save:
 190 #   after 900 sec (15 min) if at least 1 key changed
 191 #   after 300 sec (5 min) if at least 10 keys changed
 192 #   after 60 sec if at least 10000 keys changed
 193 #
 194 #   Note: you can disable saving completely by commenting out all "save" lines.
 195 #
 196 #   It is also possible to remove all the previously configured save
 197 #   points by adding a save directive with a single empty string argument
 198 #   like in the following example:
 199 #
 200 #   save ""
 201 
 202 save 900 1
 203 save 300 10
 204 save 60 10000
 205 
 206 # By default Redis will stop accepting writes if RDB snapshots are enabled
 207 # (at least one save point) and the latest background save failed.
 208 # This will make the user aware (in a hard way) that data is not persisting
 209 # on disk properly, otherwise chances are that no one will notice and some
 210 # disaster will happen.
 211 #
 212 # If the background saving process will start working again Redis will
 213 # automatically allow writes again.
 214 #
 215 # However if you have setup your proper monitoring of the Redis server
 216 # and persistence, you may want to disable this feature so that Redis will
 217 # continue to work as usual even if there are problems with disk,
 218 # permissions, and so forth.
 219 stop-writes-on-bgsave-error yes
 220 
 221 # Compress string objects using LZF when dump .rdb databases?
 222 # For default that's set to 'yes' as it's almost always a win.
 223 # If you want to save some CPU in the saving child set it to 'no' but
 224 # the dataset will likely be bigger if you have compressible values or keys.
 225 rdbcompression yes
 226 
 227 # Since version 5 of RDB a CRC64 checksum is placed at the end of the file.
 228 # This makes the format more resistant to corruption but there is a performance
 229 # hit to pay (around 10%) when saving and loading RDB files, so you can disable it
 230 # for maximum performances.
 231 #
 232 # RDB files created with checksum disabled have a checksum of zero that will
 233 # tell the loading code to skip the check.
 234 rdbchecksum yes
 235 
 236 # The filename where to dump the DB
 237 dbfilename dump.rdb
 238 
 239 # The working directory.
 240 #
 241 # The DB will be written inside this directory, with the filename specified
 242 # above using the 'dbfilename' configuration directive.
 243 #
 244 # The Append Only File will also be created inside this directory.
 245 #
 246 # Note that you must specify a directory here, not a file name.
 247 dir ./
 248

数据保存频率： 1、save 900 1 900秒后保存，至少有1个key被更改时才会触发 2、save 300 10 300秒后保存，至少有10个key被更改时才会触发 3、save 60 10000 60秒后保存，至少有10000个key被更改时才会触发
stop-writes-on-bgsave-error yes 最近一次save操作失败则停止写操作
rdbcompression yes 启用压缩
rdbchecksum yes 启用CRC64校验码，当然这个会影响一部份性能
dbfilename dump.rdb 指定存储数据的文件名
dir ./ 指定工作目录，rdb文件和aof文件都会存放在这个目录中，默认为当前目录

五、如何触发RDB快照

1、先修改redis.conf配置文件的快照选项，为了演示方便：设置每2分钟执行10次更改保存，并且设置保存的文件名称dump.rdb备份文件，如下图：在这里插入图片描述 2、执行10次以上的修改操作，2分钟后查看默认备份目录下会自动产生rdb格式的备份文件。如下图： 3、冷拷贝dump.rdb备份文件为dump_bk.rdb，如下图： 4、执行flushall清空数据库命令，在执行shutdown停止命令，在执行exit退出，重新启动redis，查看数据为空。原因：执行flushall命令，也会产生dump.rdb文件，但里面是空的，无意义。如下图：