Re: Confused about the buffer pool size

<table border="0" cellpadding="0" cellspacing="0"><tr><td style="font: inherit;" valign="top">Hello Heikki:<br /><br
/>Thisis what the documentation says (see below).<br /><br />But it does not tell my anything about what the actual
buffersize is.<br />How do I know what the real buffer size is? I am using 8.4.4 and I am running only one query at a
time.<br/><br />Cheers,<br /><br />MMK.<br /><br />Sets the planner's assumption about the effective size of the disk
cachethat is available to a single query. This is factored into estimates of the cost of using an index; a higher value
makesit more likely index scans will be used, a lower value makes it more likely sequential scans will be used. When
settingthis parameter you should consider both <span class="PRODUCTNAME">PostgreSQL</span>'s shared buffers and the
portionof the kernel's disk cache that will be used for <span class="PRODUCTNAME">PostgreSQL</span> data files. Also,
takeinto account the expected number of concurrent queries on different tables, since they will have to share the
availablespace. This parameter has no effect on the size of shared memory allocated by <span
class="PRODUCTNAME">PostgreSQL</span>,nor does it reserve kernel disk cache; it is used only for estimation purposes.
Thedefault is 128 megabytes (<tt class="LITERAL">128MB</tt>). <br /><br /><br /><br />--- On <b>Tue, 5/25/10, Heikki
Linnakangas<i><heikki.linnakangas@enterprisedb.com></i></b> wrote:<br /><blockquote style="border-left: 2px solid
rgb(16,16, 255); margin-left: 5px; padding-left: 5px;"><br />From: Heikki Linnakangas
<heikki.linnakangas@enterprisedb.com><br/>Subject: Re: [HACKERS] Confused about the buffer pool size<br />To:
"MMK"<bomuvi@yahoo.com><br />Cc: "PostgreSQL-development" <pgsql-hackers@postgresql.org><br />Date:
Tuesday,May 25, 2010, 11:36 AM<br /><br /><div class="plainMail">On 25/05/10 19:49, MMK wrote:<br />> Hello All:<br
/>>In the code (costsize.c), I see that effective_cache_size is set to DEFAULT_EFFECTIVE_CACHE_SIZE.<br />> This
isdefined as follows in cost.h<br />> #define DEFAULT_EFFECTIVE_CACHE_SIZE 16384<br />> But when I say<br />>
showshared_buffers in psql I get,<br />> shared_buffers ---------------- 28MB<br />> In postgresql.conf file, the
followinglines appear<br />> shared_buffers = 28MB                   # min 128kB           # (change requires
restart)#temp_buffers= 8MB                     # min 800kB<br />> <br />> So I am assuming that the buffer pool
sizeis 28MB = 28 * 128 = 3584 8K pages.<br />> So should effective_cache_size be set to 3584 rather than the
16384?<br/><br />No. Please see the manual for what effective_cache_size means:<br /><br /><a
href="http://www.postgresql.org/docs/8.4/interactive/runtime-config-query.html#GUC-EFFECTIVE-CACHE-SIZE"
target="_blank">http://www.postgresql.org/docs/8.4/interactive/runtime-config-query.html#GUC-EFFECTIVE-CACHE-SIZE</a><br
/><br/>--   Heikki Linnakangas<br />  EnterpriseDB   <a href="http://www.enterprisedb.com"
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