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Re: postgresql cluster on SAN

From: Chris Browne <cbbrowne(at)acm(dot)org>
To: pgsql-admin(at)postgresql(dot)org
Subject: Re: postgresql cluster on SAN
Date: 2005-09-21 13:06:44
Message-ID: 60vf0upo2j.fsf@dba2.int.libertyrms.com (view raw or flat)
Thread:
Lists: pgsql-admin
robertngo(at)perridot(dot)com (Robert Ngo) writes:

> Robert Ngo wrote:
>
>> Chris Browne wrote:
>>
>>> robertngo(at)perridot(dot)com (Robert Ngo) writes:
>>>
>>>
>>>> Can i create create a cluster of postgresql server by sharing the
>>>> database file on a SAN? I am also looking into slony but slony
>>>> replicate
>>>> data to each server and my database will potentially have terabytes of
>>>> data. I am thinking about a solution where a cluster of database server
>>>> will share database files on SAN, can this be done? I am also looking
>>>> for a load balancing salution for the postgresql database. Any
>>>> sugestion?
>>>>
>>>
>>>
>>> What you wish can NOT be done.
>>>
>>> There can only be ONE postmaster for each database cluster; you can
>>> NOT have multiple servers sharing a single cluster.
>>>
>> so there is no ways for the file storage to be shared between
>> postgres cluster? Then i will need to add a huge amount of storage
>> every time new server is added.
>>
> Is there any way i can reduce the storage requirement of the database
> server cluster?

Well, the base amount of storage for each instance is only about 26MB.

sh-3.00$ /opt/OXRS/dbs/pgsql8/bin/initdb -D .
Success. You can now start the database server using:

    /opt/OXRS/dbs/pgsql8/bin/postmaster -D .
or  
    /opt/OXRS/dbs/pgsql8/bin/pg_ctl -D . -l logfile start

sh-3.00$ du
176     ./global
4       ./pg_xlog/archive_status
16412   ./pg_xlog
12      ./pg_clog
12      ./pg_subtrans
4640    ./base/1
4704    ./base/17229
9348    ./base
4       ./pg_tblspc
25992   .
sh-3.00$ 

For any sort of interesting-sized database, this bit of overhead,
which is mostly WAL files, doesn't strike me as being terribly
material.  I can't see 26MB being considered "huge" in a SAN context.

You only need additional storage (beyond that) when your applications
are actually requesting to store data.  There is no magical way to
avoid that need for additional storage.
-- 
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