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help / color / mirror / Atom feedFrom: Quan Zongliang <zongliang.quan@postgresdata.com>
To: pgsql-zh-general@postgresql.org <pgsql-zh-general@postgresql.org>
Subject: [博文] 参数max_wal_size与min_wal_size的计算与影响
Date: Sat, 4 Jun 2016 10:50:15 +0800
Message-ID: <e0990ee6-6efa-8ca0-4bdc-9052add439e7@postgresdata.com> (raw)
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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
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To: pgsql-zh-general@postgresql.org
Cc: zongliang.quan@postgresdata.com
Subject: Re: [博文] 参数max_wal_size与min_wal_size的计算与影响
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