From: | Petr Jelinek <petr(at)2ndquadrant(dot)com> |
---|---|
To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
Cc: | Heikki Linnakangas <hlinnakangas(at)vmware(dot)com>, Josh Berkus <josh(at)agliodbs(dot)com>, Venkata Balaji N <nag1010(at)gmail(dot)com>, Peter Eisentraut <peter_e(at)gmx(dot)net>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: Redesigning checkpoint_segments |
Date: | 2015-02-23 03:01:46 |
Message-ID: | 54EA981A.7000507@2ndquadrant.com |
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Lists: | pgsql-hackers |
On 23/02/15 03:24, Robert Haas wrote:
> On Sat, Feb 21, 2015 at 11:29 PM, Petr Jelinek <petr(at)2ndquadrant(dot)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 & Services
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