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Date: Mon, 6 Jun 2016 11:04:21 +0800
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权叔真是惜字如金啊:-)

On 2016年06月04日 10:50, Quan Zongliang wrote:
> 有人问我现在 checkpoint_segments 怎么计算，所以写了一篇这个分想给大家。
> 希望我拙劣的文笔能够把问题说清楚。
>
> 原文链接：http://my.oschina.net/quanzl/blog/686932
>
> 1、GUC参数最大最小值的设置是一个开区间，我们看实数的设置
>
> if (newval->realval < conf->min || newval->realval > conf->max)
>
>
> 2、checkpoint_completion_target 取值范围 (0.0, 1.0)
>
>     {
>         {"checkpoint_completion_target", PGC_SIGHUP, WAL_CHECKPOINTS,
>             gettext_noop("Time spent flushing dirty buffers during
> checkpoint, as fraction of checkpoint interval."),
>             NULL
>         },
>         &CheckPointCompletionTarget,
>         0.5, 0.0, 1.0,
>         NULL, NULL, NULL
>     },
>
>
> 3、新参数 min_wal_size、max_wal_size
> 以前叫做 checkpoint_segments 已经消失，原意是 Maximum number of log
> file segments between automatic WAL checkpoints
>
>
> 4、max_wal_size 的定义和计算方式
>
>     {
>         {"max_wal_size", PGC_SIGHUP, WAL_CHECKPOINTS,
>             gettext_noop("Sets the WAL size that triggers a
> checkpoint."),
>             NULL,
>             GUC_UNIT_XSEGS
>         },
>         &max_wal_size,
>         64, 2, INT_MAX,
>         NULL, assign_max_wal_size, NULL
>     },
> 公式
>
>     {"TB", GUC_UNIT_XSEGS, (1024 * 1024 * 1024) / (XLOG_SEG_SIZE /
> 1024)},
>     {"GB", GUC_UNIT_XSEGS, (1024 * 1024) / (XLOG_SEG_SIZE / 1024)},
>     {"MB", GUC_UNIT_XSEGS, -(XLOG_SEG_SIZE / (1024 * 1024))},
>     {"kB", GUC_UNIT_XSEGS, -(XLOG_SEG_SIZE / 1024)},
> 结合 GUC代码知道，最后得到的值已经是段数
>
>
> 5、每次 checkpoint 涉及的最大段数，原来是由我们通过参数来直接指定的，
> 而现在
>
>     target = (double) max_wal_size / (2.0 + CheckPointCompletionTarget);
>
>     /* round down */
>     CheckPointSegments = (int) target;
>
>     if (CheckPointSegments < 1)
>         CheckPointSegments = 1;
> 我们知道 CheckPointCompletionTarget 取值范围是 (0.0, 1.0)，所以最终
> CheckPointSegments得到的值范围是 max_wal_size 的 1/3 ～ 1/2，最小为1。
>
>
> --------------------------------------------
> 权宗亮
>
>

-- 
----------------------
张文升 | PostgreSQL DBA 
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