Date: Thu, 25 Nov 2021 11:37:37 -0300
From: Alvaro Herrera <alvherre@alvh.no-ip.org>
To: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Cc: b1000101@pm.me, pgsql-docs@lists.postgresql.org
Subject: Re: max_slot_wal_keep_size unit is not specified
Message-ID: <202111251437.bfxei7ahcj6i@alvherre.pgsql>
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary="p5syqirl7mydkqrq"
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <20211124.095013.1928208020303363469.horikyota.ntt@gmail.com>
Archived-At: 
 <https://www.postgresql.org/message-id/202111251437.bfxei7ahcj6i%40alvherre.pgsql>
Precedence: bulk


--p5syqirl7mydkqrq
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
Content-Transfer-Encoding: 8bit

On 2021-Nov-24, Kyotaro Horiguchi wrote:

> At Tue, 23 Nov 2021 11:21:05 -0300, Alvaro Herrera <alvherre@alvh.no-ip.org> wrote in 

> > I wonder why did we make it round down rather than up.  Does this mean
> > that if you have max_slot_wal_keep_size=8MB and wal segments of 16 MB,
> > the slot gets invalidated with more than zero reserved segments?
> 
> That rounding behavior is the way we are generally doing on similar
> variables. It is based on the behavior of ConvertToXSegs(). So
> max_wal_size for example is rounded-up the same way.

After thinking on it some more, I think rounding down is correct.  If
somebody sets max_slot_wal_keep_size=40MB, then by keeping the limit at
32 MB (round down) we're honoring that request better than if we made it
48 MB (round up).  But in any case, I think it doesn't matter much:
users need a *rough* size limit, not a super-precise one.  After all,
this is only verified at checkpoint time.  We're probably dealing with a
few hundreds of megabytes at least, so 16 MB one way or the other don't
change things much.

Here's a proposed patch.

-- 
Álvaro Herrera           39°49'30"S 73°17'W  —  https://www.EnterpriseDB.com/
"Las mujeres son como hondas:  mientras más resistencia tienen,
 más lejos puedes llegar con ellas"  (Jonas Nightingale, Leap of Faith)

--p5syqirl7mydkqrq
Content-Type: text/x-diff; charset=utf-8
Content-Disposition: attachment;
	filename="0001-Document-units-for-max_slot_wal_keep_size.patch"