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Re: Defaulting wal_sync_method to fdatasync on Linux for 9.1?

From: Scott Carey <scott(at)richrelevance(dot)com>
To: Greg Smith <greg(at)2ndquadrant(dot)com>
Cc: Marti Raudsepp <marti(at)juffo(dot)org>, "pgsql-performance(at)postgresql(dot)org"<pgsql-performance(at)postgresql(dot)org>
Subject: Re: Defaulting wal_sync_method to fdatasync on Linux for 9.1?
Date: 2010-11-17 23:20:15
Message-ID: A66BB3B4-4621-4BAF-A360-0F0CF9B0061E@richrelevance.com (view raw or flat)
Thread:
Lists: pgsql-performance
On Nov 17, 2010, at 1:24 PM, Greg Smith wrote:

> Scott Carey wrote:
>> Did you recompile your test on the RHEL6 system? 
> 
> On both systems I showed, I checked out a fresh copy of the PostgreSQL 
> 9.1 HEAD from the git repo, and compiled that on the server, to make 
> sure I was pulling in the appropriate kernel headers.  I wasn't aware of 
> exactly how the kernel sync stuff was refactored though, thanks for the 
> concise update on that.  

Thanks!

So this could be another bug in Linux.  Not entirely surprising.
Since fsync/fdatasync relative performance isn't similar to open_sync/open_datasync relative performance on this test there is probably a bug that either hurts fsync, or one that is preventing open_sync from dealing with metadata properly.   Luckily for the xlog, both of those can be avoided -- the real choice is fdatasync vs open_datasync.  And both work in newer kernels or break in certain older ones.


> I can do similar tests on a RHEL5 system, but 
> not on the same hardware.  Can only make my laptop boot so many 
> operating systems at a time usefully.

Yeah, I understand.  I might throw this at a RHEL5 system if I get a chance but I need one without a RAID card that is not in use.  Hopefully it doesn't turn out that fdatasync is write-cache safe but open_sync/open_datasync isn't on that platform.  It could impact the choice of a default value.

> 
> -- 
> Greg Smith   2ndQuadrant US    greg(at)2ndQuadrant(dot)com   Baltimore, MD
> PostgreSQL Training, Services and Support        www.2ndQuadrant.us
> "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books
> 


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