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Re: [HACKERS] fsync method checking

From: Manfred Spraul <manfred(at)colorfullife(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: markw(at)osdl(dot)org, josh(at)agliodbs(dot)com, Q(at)ping(dot)be,pgman(at)candle(dot)pha(dot)pa(dot)us, markir(at)paradise(dot)net(dot)nz,pgsql-performance(at)postgresql(dot)org, pgsql-hackers(at)postgresql(dot)org
Subject: Re: [HACKERS] fsync method checking
Date: 2004-03-25 06:21:35
Message-ID: 40627A6F.9010202@colorfullife.com (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-performance
Tom Lane wrote:

>markw(at)osdl(dot)org writes:
>  
>
>>I could certainly do some testing if you want to see how DBT-2 does.
>>Just tell me what to do. ;)
>>    
>>
>
>Just do some runs that are identical except for the wal_sync_method
>setting.  Note that this should not have any impact on SELECT
>performance, only insert/update/delete performance.
>  
>
I've made a test run that compares fsync and fdatasync: The performance 
was identical:
- with fdatasync:

http://khack.osdl.org/stp/290607/

- with fsync:
http://khack.osdl.org/stp/290483/

I don't understand why. Mark - is there a battery backed write cache in 
the raid controller, or something similar that might skew the results? 
The test generates quite a lot of wal traffic - around 1.5 MB/sec. 
Perhaps the writes are so large that the added overhead of syncing the 
inode is not noticable?
Is the pg_xlog directory on a seperate drive?

Btw, it's possible to request such tests through the web-interface, see
http://www.osdl.org/lab_activities/kernel_testing/stp/script_param.html

--
    Manfred


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