Re: PGXLOG variable worthwhile? - Mailing list pgsql-hackers

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?