Re: Simplifying replication

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
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company