Re: Redesigning checkpoint_segments

On 23/02/15 03:24, 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.

Err yes of course, min not max :)

>
> Would it make sense to require that wal_keep_segments <= wal_min_size?
>

It would to me, the patch as it stands is confusing in a sense that you 
can set min and max but then wal_keep_segments somewhat overrides those.

And BTW this brings another point, I actually don't see check for 
min_wal_size <= max_wal_size anywhere in the patch.

--  Petr Jelinek                  http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training &
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