Date: Wed, 25 Aug 2021 08:32:31 -0400
From: "alvherre@alvh.no-ip.org" <alvherre@alvh.no-ip.org>
To: "Bossart, Nathan" <bossartn@amazon.com>
Cc: Kyotaro Horiguchi <horikyota.ntt@gmail.com>,
	"pgsql-hackers@lists.postgresql.org" <pgsql-hackers@lists.postgresql.org>,
	"mengjuan.cmj@alibaba-inc.com" <mengjuan.cmj@alibaba-inc.com>,
	"Jakub.Wartak@tomtom.com" <Jakub.Wartak@tomtom.com>
Subject: Re: prevent immature WAL streaming
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On 2021-Aug-24, Bossart, Nathan wrote:

> If moving RegisterSegmentBoundary() is sufficient to prevent the flush
> pointer from advancing before we register the boundary, I bet we could
> also remove the WAL writer nudge.

Can you elaborate on this?  I'm not sure I see the connection.

> Another interesting thing I see is that the boundary stored in
> earliestSegBoundary is not necessarily the earliest one.  It's just
> the first one that has been registered.  I did this for simplicity for
> the .ready file fix, but I can see it causing problems here.

Hmm, is there really a problem here?  Surely the flush point cannot go
past whatever has been written.  If somebody is writing an earlier
section of WAL, then we cannot move the flush pointer to a later
position.  So it doesn't matter if the earliest point we have registered
is the true earliest -- we only care for it to be the earliest that is
past the flush point.

-- 
Álvaro Herrera              Valdivia, Chile  —  https://www.EnterpriseDB.com/