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Re: Insert are going slower ...

From: Josh Berkus <josh(at)agliodbs(dot)com>
To: Hervé Piedvache <herve(at)elma(dot)fr>,pgsql-performance(at)postgresql(dot)org
Cc: Shridhar Daithankar <shridhar(at)frodo(dot)hserus(dot)net>, Hervé Piedvache <footcow(at)noos(dot)fr>
Subject: Re: Insert are going slower ...
Date: 2004-07-18 17:23:21
Message-ID: 200407181023.22004.josh@agliodbs.com (view raw or flat)
Thread:
Lists: pgsql-performance
Herve'

> Hum ... it's only for speed aspect ... I was using postgresql with this
> option since 7.01 ... and for me fsync=on was so slow ...
> Is it really no time consuming for the system to bring it ON now with
> v7.4.3 ??

Well, I wouldn't do it until you've figured out the current performance 
problem.

The issue with having fsync=off is that, if someone yanks the power cord on 
your server, there is a significant chance that you will have to restore the 
database from backup becuase it will be corrupted.   But clearly you've been 
living with that risk for some time.

It *is* true that there is significantly less performance difference between 
7.4 with fsync off and on than there was between 7.1 with fsync off and on.  
But there is still a difference.   In 7.0 and 7.1 (I think), when you turned 
fsync off it turned WAL off completely, resulting in a substantial difference 
in disk activity.   Now, it just stops checkpointing WAL but WAL is still 
recording -- meaning that disk activity decreases some but not a lot.   The 
difference is more noticable the more vulnerable to contention your disk 
system is.

The other reason not to think of fsync=off as a permanent performance tweak is 
that we're likely to remove the option sometime in the next 2 versions, since 
an increasing number of features depend on WAL behavior, and the option is 
largely a legacy of the 7.0 days, when WAL was sometimes buggy and needed to 
be turned off to get the database to start.

-- 
Josh Berkus
Aglio Database Solutions
San Francisco

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