Message-ID: <4CC1C74E.8080409@agliodbs.com>
Date: Fri, 22 Oct 2010 10:18:06 -0700
From: Josh Berkus <josh@agliodbs.com>
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To: Fujii Masao <masao.fujii@gmail.com>
CC: Robert Haas <robertmhaas@gmail.com>, Bruce Momjian <bruce@momjian.us>,
	Greg Stark <gsstark@mit.edu>, pgsql-hackers@postgresql.org
Subject: Re: Simplifying replication
References: <201010220058.o9M0waa06053@momjian.us>
	<4CC0E465.4000700@agliodbs.com>
	<AANLkTineXrnRAkRV4Cs=rmSo0QkWkFRVYQpj0AWXGn2=@mail.gmail.com>
	<4CC0F10D.3000201@agliodbs.com>
	<AANLkTi=Nz06NGkUAEzJ7r=CMRsiUAJXRsWALqw-jZ62A@mail.gmail.com>
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> Please see
> http://archives.postgresql.org/pgsql-docs/2010-10/msg00038.php

Ye gods and little fishes!

You really want to talk arcane formulas.  I've re-read that
three times, and am still not sure that I could tell someone 
definitively how much disk space WAL needs for a given group of 
settings.  I'll also point out that that formula is not in our docs -- 
what's an appropriate location?

I think this needs to be corrected in 9.1, *even if it means breaking 
backwards compatibility*.

What would be sensible for DBAs is to have two settings:

max_wal_size
min_wal_size

These would be expresses in MB or GB and would be simple direct 
quantities, which our formulas would work backwards from.  max_wal_size 
would be a hard limit (i.e. Postgres would stop accepting writes if we 
hit it), and Admins would not be allowed to set min_wal_size to more 
than max_wal_size - 2.

Even better would be to replace min_wal_size with min_wal_time, which 
would set a time span for the oldest WAL segment to be kept (up to 
max_wal_size - 2).   Hmmm.  That doesn't seem that hard to implement. 
Is it?

(BTW, Robert, that e-mail is what I meant by "relationship")

-- 
                                   -- Josh Berkus
                                      PostgreSQL Experts Inc.
                                      http://www.pgexperts.com