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Re: Configuration Recommendations

From: Shaun Thomas <sthomas(at)peak6(dot)com>
To: John Lister <john(dot)lister(at)kickstone(dot)co(dot)uk>
Cc: <pgsql-performance(at)postgresql(dot)org>
Subject: Re: Configuration Recommendations
Date: 2012-04-25 21:29:16
Message-ID: 4F986CAC.4060904@peak6.com (view raw or flat)
Thread:
Lists: pgsql-performance
On 04/25/2012 02:46 AM, John Lister wrote:

> Hi, I'd be grateful if you could share any XFS performance tweaks as I'm
> not entirely sure I'm getting the most out of my setup and any
> additional guidance would be very helpful.

Ok, I'll give this with a huge caveat: these settings came from lots of 
testing, both load and pgbench based. I'll explain as much as I can.

For initializing the XFS filesystem, you can take advantage of a few 
settings that are pretty handy.

* -d agcount=256 - Higher amount of allocation groups works better with 
multi-CPU systems. We used 256, but you'll want to do tests to confirm 
this. The point is that you can have several threads writing to the 
filesystem simultaneously.

* -l lazy-count=1 - Log data is written more efficiently. Gives a 
measurable performance boost. Newer versions set this, but CentOS 5 has 
the default to 0. I'm not sure about CentOS 6. Just enable it. :)

* -l version=2 - Forces the most recent version of the logging 
algorithm; allows a larger log buffer on mount. Since you're using 
CentOS, the default value is still probably 1, which you don't want.

And then there are the mount options. These actually seemed to make more 
of an impact in our testing:

* allocsize=256m - Database files are up to 1GB in size. To prevent 
fragmentation, always pre-allocate in 256MB chunks. In recent 3.0+ 
kernels, this setting will result in phantom storage allocation as each 
file is initially allocated with 256MB until all references have exited 
memory. Due to aggressive Linux inode cache behavior, this may not 
happen for several hours. On 3.0 kernels, this setting should be 
removed. I think the 2.6.35 kernel had this backported, so *TEST THIS 
SETTING BEFORE USING IT!*

* logbufs=8 - Forces more of the log buffer to remain in RAM, improving 
file deletion performance. Good for temporary files. XFS often gets 
knocked for file deletion performance, and this brings it way up. Not 
really an issue with PG usage, but handy anyway. See logbsize.

* logbsize=256k - Larger log buffers keep track of modified files in 
memory for better performance. See logbufs.

* noatime - Negates touching the disk for file accesses. Reduces disk IO.

* attr2 - Opportunistic improvement in the way inline extended 
attributes are stored on-disk. Not strictly necessary, but handy.


I'm hoping someone else will pipe in, because these settings are pretty 
"old" and based on a CentOS 5.5 setup. I haven't done any metrics on the 
newer kernels, but I have followed enough to know allocsize is dangerous 
on new systems.

Your mileage may vary. :)

-- 
Shaun Thomas
OptionsHouse | 141 W. Jackson Blvd. | Suite 500 | Chicago IL, 60604
312-444-8534
sthomas(at)peak6(dot)com

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