Date: Mon, 27 May 2019 19:02:25 -0400
From: Noah Misch <noah@leadboat.com>
To: Kyotaro HORIGUCHI <horiguchi.kyotaro@lab.ntt.co.jp>
Cc: pgsql-hackers@postgresql.org, 9erthalion6@gmail.com,
	andrew.dunstan@2ndquadrant.com, hlinnaka@iki.fi,
	robertmhaas@gmail.com, michael@paquier.xyz
Subject: Re: [HACKERS] WAL logging problem in 9.4.3?
Message-ID: <20190527230225.GA59385@gust.leadboat.com>
References: <20190520.155430.215084510.horiguchi.kyotaro@lab.ntt.co.jp>
 <20190521.212948.34357392.horiguchi.kyotaro@lab.ntt.co.jp>
 <20190525023332.GE1624191@rfd.leadboat.com>
 <20190527.140826.258215605.horiguchi.kyotaro@lab.ntt.co.jp>
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On Mon, May 27, 2019 at 02:08:26PM +0900, Kyotaro HORIGUCHI wrote:
> At Fri, 24 May 2019 19:33:32 -0700, Noah Misch <noah@leadboat.com> wrote in <20190525023332.GE1624191@rfd.leadboat.com>
> > On Mon, May 20, 2019 at 03:54:30PM +0900, Kyotaro HORIGUCHI wrote:
> > > Following this direction, the attached PoC works *at least for*
> > > the wal_optimization TAP tests, but doing pending flush not in
> > > smgr but in relcache.
> > 
> > This task, syncing files created in the current transaction, is not the kind
> > of task normally assigned to a cache.  We already have a module, storage.c,
> > that maintains state about files created in the current transaction.  Why did
> > you use relcache instead of storage.c?
> 
> The reason was at-commit sync needs buffer flush beforehand. But
> FlushRelationBufferWithoutRelCache() in v11 can do
> that. storage.c is reasonable as the place.

Okay.  I do want this to work in 9.5 and later, but I'm not aware of a reason
relcache.c would be a better code location in older branches.  Unless you
think of a reason to prefer relcache.c, please use storage.c.

> > On Tue, May 21, 2019 at 09:29:48PM +0900, Kyotaro HORIGUCHI wrote:
> > > This is a tidier version of the patch.
> > 
> > > - Move the substantial work to table/index AMs.
> > > 
> > >   Each AM can decide whether to support WAL skip or not.
> > >   Currently heap and nbtree support it.
> > 
> > Why would an AM find it important to disable WAL skip?
> 
> The reason is currently it's AM's responsibility to decide
> whether to skip WAL or not.

I see.  Skipping the sync would be a mere optimization; no AM would require it
for correctness.  An AM might want RelationNeedsWAL() to keep returning true
despite the sync happening, perhaps because it persists data somewhere other
than the forks of pg_class.relfilenode.  Since the index and table APIs
already assume one relfilenode captures all persistent data, I'm not seeing a
use case for an AM overriding this behavior.  Let's take away the AM's
responsibility for this decision, making the system simpler.  A future patch
could let AM code decide, if someone find a real-world use case for
AM-specific logic around when to skip WAL.