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Re: Simplifying replication

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Simon Riggs <simon(at)2ndquadrant(dot)com>
Cc: Greg Stark <gsstark(at)mit(dot)edu>, Josh Berkus <josh(at)agliodbs(dot)com>, Bruce Momjian <bruce(at)momjian(dot)us>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Simplifying replication
Date: 2010-10-26 21:32:58
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Lists: pgsql-hackers
On Tue, Oct 26, 2010 at 8:27 AM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
> On Thu, 2010-10-21 at 20:57 -0400, Robert Haas wrote:
>> Very true.  But the lack of a -1 setting for wal_keep_segments means
>> that if you would like to take a backup without archiving, you must
>> set wal_keep_segments to a value greater than or equal to the rate at
>> which you generate WAL segments multiplied by the time it takes you to
>> run a backup.  If that doesn't qualify as requiring arcane knowledge,
>> I'm mystified as to what ever could.
> People are missing the point here:
> You have to put the WAL files *somewhere* while you do the base backup.
> PostgreSQL can't itself work out where that is, nor can it work out
> ahead of time how big it will need to be, since it is up to you how you
> do your base backup. Setting a parameter to -1 doesn't make the problem
> go away, it just pretends and hopes it doesn't exist, but screws you
> badly if you do hit the wall.

If you set wal_keep_segments=0, archive_mode=on, and
archive_command=<something>, you might run out of disk space.

If you set wal_keep_segments=-1, you might run out of disk space.

Are you any more screwed in the second case than you are in the first
case?  Why?

Robert Haas
The Enterprise PostgreSQL Company

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