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
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