docs - correct some effective_cache_size guc sample settings
<topic id="effective_cache_size">
<title>effective_cache_size</title>
<body>
- <p>Sets the assumption about the effective size of the disk cache that is available to a
- single query for the Postgres Planner. This is factored into estimates of
- the cost of using an index; a higher value makes it more likely index scans will be used, a
- lower value makes it more likely sequential scans will be used. This parameter has no effect
- on the size of shared memory allocated by a Greenplum server instance, nor does it reserve
+ <p>Sets the assumption about the effective size of the disk cache that is available
+ to a single query for the Postgres Planner. This is factored into estimates of the
+ cost of using an index; a higher value makes it more likely index scans will be used,
+ a lower value makes it more likely sequential scans will be used. When setting this
+ parameter, you should consider both Greenplum Database's shared buffers and the
+ portion of the kernel's disk cache that will be used for data files (though some data
+ might exist in both places). Take also into account the expected number of concurrent
+ queries on different tables, since they will have to share the available space. This
+ parameter has no effect on the size of shared memory allocated by a Greenplum server
+ instance, nor does it reserve
kernel disk cache; it is used only for estimation purposes.</p>
- <p>Set this parameter to a number of 32K blocks (for example, <codeph>512</codeph> for 16MB),
- or specify the size of the effective cache (for example, <codeph>'32MB'</codeph> for 1024
- blocks). The <codeph>gpconfig</codeph> utility and <codeph>SHOW</codeph> command display the
- effective cache size value in units such as 'MB' or 'kB'.</p>
+ <p>Set this parameter to a number of
+ <codeph><xref href="#backslash_quote">block_size</xref></codeph> blocks
+ (default 32K) with no units; for example, <codeph>262144</codeph> for 8GB. You can
+ also directly specify the size of the effective cache; for example,
+ <codeph>'1GB'</codeph> specifies a size of 32768 blocks. The
+ <codeph>gpconfig</codeph> utility and <codeph>SHOW</codeph> command display the
+ effective cache size value in units such as 'GB', 'MB', or 'kB'.</p>
<table id="effective_cache_size_table">
<tgroup cols="3">
<colspec colnum="1" colname="col1" colwidth="1*"/>
</thead>
<tbody>
<row>
- <entry colname="col1">floating point</entry>
- <entry colname="col2">512 (16GB)</entry>
+ <entry colname="col1">1 - INT_MAX <i>or</i> number and unit</entry>
+ <entry colname="col2">524288 (16GB)</entry>
<entry colname="col3">master<p>session</p><p>reload</p></entry>
</row>
</tbody>
