From c1d9df4fa227781b31be44a5a3024865a7f48049 Mon Sep 17 00:00:00 2001
From: Bruce Momjian <bruce@momjian.us>
Date: Tue, 14 Feb 2012 16:54:54 -0500
Subject: [PATCH] Document random page cost is only 4x seqeuntial, and not 40x.
---
doc/src/sgml/config.sgml | 20 ++++++++++++++++++++
1 file changed, 20 insertions(+)
diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 3a843217602..0ea9aebdb02 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -2604,6 +2604,26 @@ SET ENABLE_SEQSCAN TO OFF;
parameters.
</para>
+ <para>
+ Random access to mechanical disk storage is normally much more expensive
+ than four-times sequential access. However, a lower default is used
+ (4.0) because the majority of random accesses to disk, such as indexed
+ reads, are assumed to be in cache. The default value can be thought of
+ as modeling random access as 40 times slower than sequential, while
+ expecting 90% of random reads to be cached.
+ </para>
+
+ <para>
+ If you believe a 90% cache rate is an incorrect assumption
+ for your workload, you can increase random_page_cost to better
+ reflect the true cost of random storage reads. Correspondingly,
+ if your data is likely to be completely in cache, such as when
+ the database is smaller than the total server memory, decreasing
+ random_page_cost can be appropriate. Storage that has a low random
+ read cost relative to sequential, e.g. solid-state drives, might
+ also be better modeled with a lower value for random_page_cost.
+ </para>
+
<tip>
<para>
Although the system will let you set <varname>random_page_cost</> to
--
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