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Re: WAL & RC1 status

From: Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us>
To: pgsql-hackers(at)postgresql(dot)org
Subject: Re: WAL & RC1 status
Date: 2001-03-02 22:10:33
Message-ID: 200103022210.RAA13760@candle.pha.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
> It Seems to Me that after an orderly shutdown, the WAL files should be, 
> effectively, slag -- they should contain no deltas from the current 
> table contents.  In practice that means the only part of the format that 
> *should* matter is whatever it takes to discover that they really are 
> slag.

> 
> That *should* mean that, at worst, a change to the WAL file format should 
> only require doing an orderly shutdown, and then (perhaps) running a simple
> program to generate a new-format empty WAL.  It ought not to require an 
> initdb.  
> 
> Of course the details of the current implementation may interfere with
> that ideal, but it seems a worthy goal for the next beta, if it's not
> possible already.  Given the opportunity to change the current WAL format, 
> it ought to be possible to avoid even needing to run a program to generate 
> an empty WAL.

This was my question too.  If we are just changing WAL, why can't we
just have them stop the postmaster, install the new binaries, and
restart.

Tom told me on the phone that there was a magic number in the WAL log
file, and I see it now:

	#define XLOG_PAGE_MAGIC 0x17345168

Couldn't we just have our new beta ignore WAL pages with this entry,
knowing that startup/shutdown creates new WAL files anyway, 

Aside from inconveniencing the beta users, people can do testing easier
if we don't require a dump/reload for every WAL format change.

-- 
  Bruce Momjian                        |  http://candle.pha.pa.us
  pgman(at)candle(dot)pha(dot)pa(dot)us               |  (610) 853-3000
  +  If your life is a hard drive,     |  830 Blythe Avenue
  +  Christ can be your backup.        |  Drexel Hill, Pennsylvania 19026

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