Re: Simplifying replication

On Thu, Oct 21, 2010 at 9:09 PM, Josh Berkus <josh(at)agliodbs(dot)com> 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.
>
> Speaking of which, what's the relationship between checkpoint_segments
> and wal_keep_segments?  PG seems perfectly willing to let me set the
> latter higher than the former, and it's not documented.

I think it's pretty well explained in the fine manual.

http://www.postgresql.org/docs/current/static/runtime-config-wal.html#GUC-WAL-KEEP-SEGMENTS

> If checkpoint_segments were a hard limit, then we could let admins set
> wal_keep_segments to -1, knowing that they'd set checkpoint_segments to
> the max space they had available.

This assumes that more checkpoint segments is always better, which
isn't true. I might have 100 GB of disk space free, but not want to
replay WAL for 4 days if I have a crash.

I do think that the current default of checkpoint_segments=3 is
pathologically insane, but that's another can of worms.

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