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Re: [HACKERS] fsync method checking

From: Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us>
To: markw(at)osdl(dot)org
Cc: manfred(at)colorfullife(dot)com, tgl(at)sss(dot)pgh(dot)pa(dot)us, josh(at)agliodbs(dot)com,Q(at)ping(dot)be, 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 18:52:56
Message-ID: 200403251852.i2PIquv29582@candle.pha.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-performance
markw(at)osdl(dot)org wrote:
> > 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
> 
> We have 2 Adaptec 2200s controllers, without the battery backed add-on,
> connected to four 10-disk arrays in those systems.  I can't think of
> anything off hand that would skew the results.
> 
> The pg_xlog directory is not on a separate drive.  I haven't found the
> best way to lay out of the drives on those systems yet, so I just have
> everything on a 28 drive lvm2 volume.

We don't actually extend the WAL file during writes (preallocated), and
the access/modification timestamp is only in seconds, so I wonder of the
OS only updates the inode once a second.  What else would change in the
inode more frequently than once a second?

-- 
  Bruce Momjian                        |  http://candle.pha.pa.us
  pgman(at)candle(dot)pha(dot)pa(dot)us               |  (610) 359-1001
  +  If your life is a hard drive,     |  13 Roberts Road
  +  Christ can be your backup.        |  Newtown Square, Pennsylvania 19073

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