Re: Migration study, step 1: bulk write performance

On Tue, Mar 21, 2006 at 01:10:32PM +0100, Steinar H. Gunderson wrote:
> On Tue, Mar 21, 2006 at 06:01:58AM -0600, Jim C. Nasby wrote:
> > Are you sure? Metadate changes are probably a lot more common on the WAL
> > partition. In any case, I don't see why there should be a difference.
> > The real issue is: is related filesystem metadata sync'd as part of a
> > file being fsync'd?
> 
> I've been told on this list that PostgreSQL actually takes care to fill a new
> WAL file with zeroes etc. when initializing it; dig a few months back and I'm
> sure it's there.

That's fine and all, but does no good if the filesystem doesn't know
that the file exists on a crash. The same concern is also true on the
data partition, although it's less likely to be a problem because if you
happen to crash soon after a DDL operation it's likely that you haven't
had a checkpoint yet, so the operation will likely be repeated during
WAL replay. But depending on that is a race condition.

Basically, you need to know for certain that if PostgreSQL creates a
file and then fsync's it that that file is safely on disk, and that the
filesystem knows how to find it (ie: the metadata is also on disk in
some fashion).
-- 
Jim C. Nasby, Sr. Engineering Consultant      jnasby@pervasive.com
Pervasive Software      http://pervasive.com    work: 512-231-6117
vcard: http://jim.nasby.net/pervasive.vcf       cell: 512-569-9461