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Re: [HACKERS] fsync method checking

From: Manfred Spraul <manfred(at)colorfullife(dot)com>
To: Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us>
Cc: Mark Kirkwood <markir(at)paradise(dot)net(dot)nz>,pgsql-performance(at)postgresql(dot)org,PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: [HACKERS] fsync method checking
Date: 2003-12-12 20:54:34
Message-ID: 3FDA2B0A.1060709@colorfullife.com (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-performance
Bruce Momjian wrote:

>	write                  0.000360
>	write & fsync          0.001391
>	write, close & fsync   0.001308
>	open o_fsync, write    0.000924
>  
>
That's 1 milliseconds vs. 1.3 milliseconds. Neither value is realistic - 
I guess the hw cache on and the os doesn't issue cache flush commands. 
Realistic values are probably 5 ms vs 5.3 ms - 6%, not 30%. How large is 
the syscall latency with BSD/OS 4.3?

One advantage of a seperate write and fsync call is better performance 
for the writes that are triggered within AdvanceXLInsertBuffer: I'm not 
sure how often that's necessary, but it's a write while holding both the 
WALWriteLock and WALInsertLock. If every write contains an implicit 
sync, that call would be much more expensive than necessary.

--
    Manfred


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