[pgpool-hackers: 3] Re: Fix error on the lsn math

[Guillaume Lelarge]

On Mon, 2011-11-28 at 07:56 +0900, Tatsuo Ishii wrote:
> Guillaume,
> 
> About your commit in the subject.
> 
> Before it used 0xff000000 and changed by me according to the
> suggestion:
> 
> From: Anton Yuzhaninov
> To: pgpool-hackers at pgfoundry.org
> Date: Wed, 04 May 2011 19:56:54 +0400
> 
> > I have two questions/suggestions about replication lag check for
> > straming replication in pgpool-II.
> > 
> > 1. In text_to_lsn() there is formula
> > 
> > lsn = xlogid * 16 * 1024 * 1024 * 255 + xrecoff;
> > 
> > 16 * 1024 * 1024 * 255 = 4278190080 = 0xff000000
> > 
> > where this magic numbers come from?
> > 
> > I think multiplier should be
> > 0xffffffff - XLogSegSize.
> > 
> > In PostgreeSQL source this constant used in
> > src/include/access/xlog_internal.h
> > 
> > #define XLogSegsPerFile (((uint32) 0xffffffff) / XLogSegSize)
> > 
> > But in comment noted, that one segment at the end of each log file is
> > wasted
> > 
> > Each xlogid corresponds to (XLogSegsPerFile - 1) * XLogSegSize =
> > 0xffffffff - XLogSegSize bytes
> 
> I confirmed in the source code and decided to change "16 * 1024 * 1024
> * 255 = 4278190080 = 0xff000000" to 0xffffffff. Now you changed it
> to original constant. Do you have any point against his argument?

To be honest, I didn't know about Anton's point of view. I guess he's
wrong. See
http://eulerto.blogspot.com/2011/11/understanding-wal-nomenclature.html
for details. Many agree that we should use 0xff000000.


-- 
Guillaume
  http://blog.guillaume.lelarge.info
  http://www.dalibo.com