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Re: postgres config

From: "Kasia Tuszynska" <ktuszynska(at)esri(dot)com>
To: "Shreya Bhargava" <shreya_bhargav(at)yahoo(dot)com>
Cc: <pgsql-novice(at)postgresql(dot)org>
Subject: Re: postgres config
Date: 2007-07-12 20:16:21
Message-ID: D7BFFE348C53EF4E8AA0698B1E395FA90912CFF6@flybywire.esri.com (view raw or flat)
Thread:
Lists: pgsql-novice
Hi Shreya,

I too am a novice, and have done a bit of forum surfing, below is some
interesting information regarding configuration.

Sincerely,

Kasia

 

1.) shared_buffers I see lot of reference to making
this the size of available ram (for the DB).  However,
I also read to make it the size of pgdata directory.
 
You generally want shared_buffers to be no more than 10% of available
ram. Postgres expects the OS to do it's own caching. 
 
2.) effective_cache_size - from what I read this is
the 'total' allowed memory for postgresql to use
correct? So, if I am willing to allow 1GB of memory
should I make this 1GB?
 
This is the effective amount of caching between the actual postgres
buffers, and the OS buffers. If you are dedicating this machine to
postgres, I would set it to something like 3.5G. If it is a mixed
machine, then you have to think about it.
 
This does not change how postgres uses RAM, it changes how postgres
estimates whether an Index scan will be cheaper than a Sequential scan,
based on the likelihood that the data you want will already be cached in
Ram.
3.) max_connections, been trying to figure 'how' to
determine this #.  I've read this is buffer_size+500k
per a connection. ie.  128mb(buffer) + 500kb = 128.5mb per connection?
 
Max connections is just how many concurrent connections you want to
allow. If you can get away with lower, do so.  Mostly this is to prevent
connections * work_mem to get bigger than your real working memory and
causing you to swap.
 
I was curious about 'sort_mem' I can't find reference
of it in the 8.0.3 documentation, has it been removed?
 
 
sort_mem changed to work_mem in 8.0, same thing with vacuum_mem ->
maintenance_work_mem.
 
work_mem and max_stack_depth set to 4096
maintenance_work_mem set to 64mb
 
 
Depends how much space you want to give per connection. 4M is pretty
small for a machine with 4G of RAM, but if your DB is only 85M it might
be plenty. work_mem is how much memory a sort/hash/etc will use before
it spills to disk. So look at your queries. If you tend to sort most of
your 85M db in a single query, you might want to make it a little bit
more. But if all of your queries are very selective, 4M could be plenty.
 
I would make maintenance_work_mem more like 512M. It is only used for
CREATE INDEX, VACUUM, etc. Things that are not generally done by more
than one process at a time. And it's nice for them to have plenty of
room to run fast.

 

Hi,

I am new to postgresql and trying to figure out postgres.config file.
There are few variables that I need an understanding about. 

shared_buffers,
temp_buffers,
work_mem
maintenance_work_mem

Can anyone please explain when and how these are used? I am trying to
understand the importance of these 4 while creating an index on a table.
Any help will be appreciated.

Thanks,
Shreya

  

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