diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 3a84321760..0ea9aebdb0 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -2604,6 +2604,26 @@ SET ENABLE_SEQSCAN TO OFF;
parameters.
+
+ 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.
+
+
+
+ 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.
+
+
Although the system will let you set random_page_cost> to