今天第二次遇到Redis “MISCONF Redis is configured to save RDB snapshots, but is currently not able to persist on disk”的问题。这个错误信息是Redis客户端工具在保存数据时候抛出的异常信息。

网上查了一下,很多人都是建议“config set stop-writes-on-bgsave-error no”。这样做其实是不好的,这仅仅是让程序忽略了这个异常,使得程序能够继续往下运行,但实际上数据还是会存储到硬盘失败!

上一次遇到这个问题是因为一个程序的Bug造成系统内存被耗尽了,后来修复了那个Bug问题就解决了。今天出现问题时查看系统内存还有2GB左右,“感觉好像不是内存的缘故”(后面发现还是因为内存的缘故)。

由于Redis是daemon模式运行的,没法看到详细的日志。修改配置文件设置logfile参数为文件(默认是stdout,建议以后安装完毕就修改这个参数为文件,不然会丢掉很多重要信息),重启Redis,查看日志,看到程序启动时就有一行警告提示:

“WARNING overcommit_memory is set to 0! Background save may fail under low memory condition. To fix this issue add 'vm.overcommit_memory = 1' to /etc/sysctl.conf and then reboot or run the command 'sysctl vm.overcommit_memory=1' for this to take effect.”(警告:过量使用内存设置为0!在低内存环境下,后台保存可能失败。为了修正这个问题,请在/etc/sysctl.conf 添加一项 'vm.overcommit_memory = 1' ,然后重启(或者运行命令'sysctl vm.overcommit_memory=1' )使其生效。)

当时没明白意思,就忽略了。再启动Redis客户端,程序保存数据时继续报“MISCONF Redis is configured to save RDB snapshots, but is currently not able to persist on disk”异常,再查看Redis日志,看到有这样的错误提示“Can’t save in background: fork: Cannot allocate memory”,这个提示很明显"Fork进程时内存不够用了!"(还是内存的问题)。

通过谷歌查询“Can’t save in background: fork: Cannot allocate memory”这个提示,找到了解决方法:

  1. // 原文:http://pydelion.com/2013/05/27/redis-cant-save-in-background-fork-cannot-allocate-memory/
  2. If you get this error
  3. Can't save in background: fork: Cannot allocate memory
  4. it means that your current database is bigger than memory you have. To fix the issue enable vm.overcommit_memory:
  5. sysctl vm.overcommit_memory=1
  6. To have if after reboot add this line to /etc/sysctl.cnf:
  7. vm.overcommit_memory=1

修改vm.overcommit_memory=1后问题果然解决了。

为什么系统明明还剩2GB的内存,Redis会说内存不够呢?

网上查了一下,有人也遇到类似的问题,并且给出了很好的分析(详见:http://www.linuxidc.com/Linux/2012-07/66079.htm),简单地说:Redis在保存数据到硬盘时为了避免主进程假死,需要Fork一份主进程,然后在Fork进程内完成数据保存到硬盘的操作,如果主进程使用了4GB的内存,Fork子进程的时候需要额外的4GB,此时内存就不够了,Fork失败,进而数据保存硬盘也失败了。

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