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Re: What's the state of postgresql on ext4 now?

From: Greg Smith <greg(at)2ndQuadrant(dot)com>
To: pgsql-performance(at)postgresql(dot)org
Subject: Re: What's the state of postgresql on ext4 now?
Date: 2011-11-16 03:48:07
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-performance
On 11/14/2011 05:00 AM, Alexandru wrote:
> I know there were a lot of performance issues with ext4, but i don't 
> know the state of it now.
> I have a private openstreetmap server installed on a ubuntu 11.10 
> 64bit pc with both partitions (/ and /home) formated with ext4. My 
> problem is that the server works very slow.

The only performance issue with ext4 is that it is careful to flush data 
to disk when the database asks it to.  That is slow on most hard drives, 
and it wasn't done correctly by ext3 on older Linux kernels.  So to many 
people this looked like a performance problem, when it was actually a 
reliability improvement as far as PostgreSQL is concerned.  The reliable 
behavior just takes longer.  See for more information 
about why this is important.  Reports about ext4 being slower by sources 
like Phoronix were very misinformed about what was going on.

When you turn off the synchronous_commit parameter in the 
postgresql.conf, the database will stop asking the filesystem to ensure 
things are on disk this way.  You can lose some data in the event of a 
crash, but things will be faster.  Try your system with that 
configuration change.  If it gets rid of what you see as a performance 
problem, then the reliability changes made going from ext3 to ext4 are 
your problem.  If instead the server is still slow to you even with 
synchronous_commit disabled, whatever is happening is unlikely to be 
caused by the ext4 changes.  In just about every other way but commit 
performance, ext4 is faster than most other filesystems.

Greg Smith   2ndQuadrant US    greg(at)2ndQuadrant(dot)com   Baltimore, MD
PostgreSQL Training, Services, and 24x7 Support

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Subject: Re: What's the state of postgresql on ext4 now?
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Subject: Re: Slow queries / commits, mis-configuration or hardware issues?

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