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

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
http://wiki.postgresql.org/wiki/Reliable_Writes 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 www.2ndQuadrant.us