Re: PGXLOG variable worthwhile?

I don't see the gain of having a file called pg_xlog vs. using GUC.

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scott.marlowe wrote:
> On Wed, 25 Sep 2002, Curt Sampson wrote:
>
> > On Tue, 24 Sep 2002, Jan Wieck wrote:
> >
> > > And AFAICS it is scary only because screwing that up will simply corrupt
> > > your database. Thus, a simple random number (okay, and a timestamp of
> > > initdb) in two files, one in $PGDATA and one in $PGXLOG would be a
> > > totally sufficient safety mechanism to prevent starting with the wrong
> > > XLOG directory.
> >
> > But still, why set up a situation where your database might not
> > start? Why not set it up so that if you get just *one* environment
> > or command-line variable right, you can't set another inconsistently
> > and screw up your start anyway? Why store configuration information
> > outside of the database data directory in a form that's not easily
> > backed up, and not easily found by other utilities?
> >
> > It's almost like people *don't* want to put this in the config file
> > or something....
>
> Curt, did you see my post about this earlier? I'll repeat it now, just in
> case anyone else missed it.
>
> Problem:
> - People need to move the pg_xlog directory around on heavily
> loaded systems to improve performance
>
> Constraints:
> - Windows can't reliably use links to do this.
> - If the pg_xlog directory is moved wrong or referenced incorrectly, data
> corruption may occur. This makes using a switch or environmental var
> dangerous
>
> I consider using a GUC in the postgresql.conf file to be better than any
> other option listed so far, but it is still a dangerous place for it to
> be.
>
> So, the way I think that would work best would be:
>
> If there's a directory called pg_xlog in the $PGDATA directory, then use
> that.
>
> If there's a file called pg_xlog in the $PGDATA directory, then it will
> contain the path to the real pg_xlog directory.
>
> If you want to move the pg_xlog directory, you called a custom script
> called "mvpgxlog" or something like it that:
>
> 1: Checks to make sure the database is shut down
> 2: Checks to make sure the destination path has enough free space for the
> xlogs
> 3: If these are both true (and whatever logic we need here for safety)
> then copy the current pg_xlog directory contents to the new pg_xlog (even
> if we are already using an alternative location, this should work), set
> proper permissions, rename / move the pg_xlog file / directorry, then
> edit/create the $PGDATA/pg_xlog file to point to the new directory.
>
> This method has several advantages, and no real disadvantages I can think
> of. The advantages are:
>
> - It makes it easy to move the pg_xlog directory.
> - It works equally well for Windows and Unix.
> - Gets rid of another GUC setting people can scram their database with.
> - It is easy to backup your pg_xlog setting.
> - If painted green it should not rust.
>
> How's that sound for a general theory of operation?
>
>
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