Re: Redesigning checkpoint_segments

On 02/23/2015 01:01 PM, Andres Freund wrote:
> On 2015-02-22 21:24:56 -0500, Robert Haas wrote:
>> On Sat, Feb 21, 2015 at 11:29 PM, Petr Jelinek <petr@2ndquadrant.com> wrote:
>>> I am wondering a bit about interaction with wal_keep_segments.
>>> One thing is that wal_keep_segments is still specified in number of segments
>>> and not size units, maybe it would be worth to change it also?
>>> And the other thing is that, if set, the wal_keep_segments is the real
>>> max_wal_size from the user perspective (not from perspective of the
>>> algorithm in this patch, but user does not really care about that) which is
>>> somewhat weird given the naming.
>>
>> It seems like wal_keep_segments is more closely related to
>> wal_*min*_size.  The idea of both settings is that each is a minimum
>> amount of WAL we want to keep around for some purpose.  But they're
>> not quite the same, I guess, because wal_min_size just forces us to
>> keep that many files around - they can be overwritten whenever.
>> wal_keep_segments is an amount of actual WAL data we want to keep
>> around.
>>
>> Would it make sense to require that wal_keep_segments <= wal_min_size?
>
> I don't think so. Right now checkpoint_segments is a useful tool to
> relatively effectively control the amount of WAL that needs to be
> replayed in the event of a crash. wal_keep_segments in contrast doesn't
> have much to do with the normal working of the system, except that it
> delays recycling of WAL segments a bit.
>
> With a condition like above, how would you set up things that you have
> 50k segments around for replication (say a good days worth), but that
> your will never have to replay more than ~800 segments (i.e. something
> like checkpoint_segments = 800)?

Right. While wal_keep_segments and wal_min_size both set a kind of a 
minimum on the amount of WAL that's kept in pg_xlog, they are different 
things, and a rule that one must be less than or greater than the other 
doesn't make sense.

Everyone seems to be happy with the names and behaviour of the GUCs, so 
committed.

- Heikki