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git-zjx avatar git-zjx commented on September 24, 2024

当内存不够用了，要将脏页写到磁盘，会有一个数据页淘汰机制（最久不使用），假设淘汰的是脏页，则此时脏页所对应的redo log的位置是随机的，当有多个不同的脏页需要刷，则对应的redo log可能在不同的位置，这样就需要把redo log的多个不同位置刷掉，这样对于redo log的处理不是就会很麻烦吗？

其实由于淘汰的时候，刷脏页过程不用动redo log文件的。这个有个额外的保证，是redo log在“重放”的时候，如果一个数据页已经是刷过的，会识别出来并跳过。

from git-zjx.github.io.

git-zjx avatar git-zjx commented on September 24, 2024

redo log是怎么记录对应脏页是否已经flush了？如果断电了重启导致内存丢失，前面几章说通过redo log进行数据恢复那redo log又怎么去释放空间？

不用记，重启了就从checkpoint 的位置往后扫。 如果已经之前刷过盘的, 不会重复应用redo log

from git-zjx.github.io.

git-zjx avatar git-zjx commented on September 24, 2024

innodb是如何知道一个页是不是脏页的？

每个数据页头部有LSN，8字节，每次修改都会变大。对比这个LSN跟checkpoint 的LSN，比checkpoint小的一定是干净页

from git-zjx.github.io.

git-zjx avatar git-zjx commented on September 24, 2024

一个内存配置为 128GB、innodb_io_capacity 设置为 20000 的大规格实例，正常会建议你将 redo log 设置成 4 个 1GB 的文件。但如果你在配置的时候不慎将 redo log 设置成了 1 个 100M 的文件，会发生什么情况呢？又为什么会出现这样的情况呢？

每次事务提交都要写 redo log，如果设置太小，很快就会被写满，这个“环”将很快被写满，write pos 一直追着 CP，这时候系统不得不停止所有更新，去推进 checkpoint，看到的现象就是磁盘压力很小，但是数据库出现间歇性的性能下跌

from git-zjx.github.io.

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