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Subject: Re: Relation between parameters in postgresql.conf file!!
From: Rui DeSousa <rui@crazybean.net>
In-Reply-To: <1541140765942-0.post@n3.nabble.com>
Date: Fri, 2 Nov 2018 10:56:03 -0400
Cc: pgsql-admin@postgresql.org
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Message-Id: <CB2CFFB1-0FAD-4967-8FD8-C372A3C4351C@crazybean.net>
References: <1541140765942-0.post@n3.nabble.com>
To: pavan95 <pavan.postgresdba@gmail.com>
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> On Nov 2, 2018, at 2:39 AM, pavan95 <pavan.postgresdba@gmail.com> =
wrote:
>=20
>=20
> 1. vacuum_cost_page_hit
> 2. vacuum_cost_page_miss
> 3. vacuum_cost_page_dirty
>=20
>=20

These are tunable parameters to control auto vacuum.  Look at =
vacuum_cost_limit and then consider how the above parameters would =
affect it.

> 1. autovacuum_vacuum_scale_factor=3D0.2(20%)
> 2. autovacuum_analyze_scale_factor=3D0.1(10%)
>=20

Those are just the factors used to determine when auto vacuum or analyze =
should vacuum/analyze a table.=20

> =46rom the below description:
>=20
>> vacuum_cost_page_hit (integer)
>> The estimated cost for vacuuming a buffer found in the shared buffer =
cache.
> It represents the cost to lock the buffer pool, lookup the shared hash =
table
> and scan the content of the page. The default value is one
> 1. Does the vacuum operate on the buffer pool(memory)? Assuming it is
> operated on the data files!!!
> 2. What does the sentence "lock the buffer pool, lookup the shared =
hash
> table and scan the content of the page" refers to??
>=20

To do anything with the block in the datafile it first needs to be =
loaded into the buffer pool thus all associated overhead of that =
operation.

>> vacuum_cost_page_dirty (integer)
>> The estimated cost charged when vacuum modifies a block that was =
previously
> clean. It represents the extra I/O required to flush the dirty block =
out to
> disk again. The default value is 20.
> What does actually happen here?? Why does this generate extra I/O?? =20=

>=20
> What is the meaning of the sentence "It represents the extra I/O =
required to
> flush the dirty block out to disk again"?? Why does vacuum flush the =
dirty
> block to disk when writer process does that?
>=20

Again it=E2=80=99s the same as the other tunable parameters.. it just =
states the cost of modifying a clean block.