Re: [HACKERS] WAL logging problem in 9.4.3?

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(at)leadboat(dot)com> wrote in <20190525023332(dot)GE1624191(at)rfd(dot)leadboat(dot)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.