Tuning for PostGreSQL Database

Jim C. Nasby jim at nasby.net
Mon Jul 21 13:24:02 PDT 2003

On Mon, Jul 21, 2003 at 01:16:45PM -0700, Sean Chittenden wrote:
> > > > > Softupdates on, async off.  Softupdates is just a better
> > > > > async.
> > > > 
> > > > postgresql fsync's all its files before returning from a commit
> > > > in order to ensure durability, right?  Does softupdates
> > > > interfere with the functioning of sync(2)/fsync(2)?
> > > 
> > > It can, yes, but that's the risk of soft updates.  From tuning(7):
> > > 
> > >      Softupdates drastically improves meta-data performance,
> > >      mainly file creation and deletion.  We recommend enabling
> > >      softupdates on most file systems; however, there are two
> > >      limitations to softupdates that you should be aware of when
> > >      determining whether to use it on a file system.  First,
> > >      softupdates guarantees file system consistency in the case of
> > >      a crash but could very easily be several seconds (even a
> > >      minute!) behind on pending write to the physical disk.  If
> > >      you crash you may lose more work than otherwise.  Secondly,
> > >      softupdates delays the freeing of file system blocks.  If you
> > >      have a file system (such as the root file system) which is
> > >      close to full, doing a major update of it, e.g. ``make
> > >      installworld'', can run it out of space and cause the update
> > >      to fail.  For this reason, softupdates will not be enabled on
> > >      the root file system during a typical install.
> >  
> > So are you saying that softupdates on whatever partition contains
> > the database transaction logs is dangerous?
> Well, think about it.  What happens when you pull the plug while the
> OS is sitting on a pile of data that it's waiting to flush to disk?
> The problem isn't so much with the WAL logs, so much as it is with the
> actual files that contain the databases/table's data.  Any kind of
> volatle backed buffer increases the danger of data loss.
If you pull the plug you're going to lose in-flight transactions no
matter what (or at least you better). Once you commit, it should hit
disk, at least in the WAL. Shouldn't sync/fsync ensure this even with

Of course the *real* issue is ensuring that no matter what, the WAL hits
the disk before the main table data does, so that you can do proper
recovery (though I'm not as clear on how this works with MVCC). If the
issue is only a matter of delayed writes across the board, I don't see
that it should really matter... it's just as if the plug got pulled a
little earlier.
Jim C. Nasby, Database Consultant                  jim at nasby.net
Member: Triangle Fraternity, Sports Car Club of America
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