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Diffstat (limited to 'doc/src')
| -rw-r--r-- | doc/src/sgml/config.sgml | 36 |
1 files changed, 18 insertions, 18 deletions
diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml index 0a2a8b49fdb..06d1e4403b5 100644 --- a/doc/src/sgml/config.sgml +++ b/doc/src/sgml/config.sgml @@ -5924,24 +5924,24 @@ ANY <replaceable class="parameter">num_sync</replaceable> ( <replaceable class=" </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, - e.g., <literal>1.1</literal>. + Random access to durable 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 storage, + such as indexed reads, are assumed to be in cache. Also, the + latency of network-attached storage tends to reduce the relative + overhead of random access. + </para> + + <para> + If you believe caching is less frequent than the default + value reflects, and network latency is minimal, you can increase + random_page_cost to better reflect the true cost of random storage + reads. Storage that has a higher random read cost relative to + sequential, like magnetic disks, might also be better modeled with + a higher value for random_page_cost. Correspondingly, if your data + is likely to be completely in cache, such as when the database + is smaller than the total server memory, or network latency is + high, decreasing random_page_cost might be appropriate. </para> <tip> |