On Mon, May 27, 2019 at 02:08:26PM +0900, Kyotaro HORIGUCHI wrote: > At Fri, 24 May 2019 19:33:32 -0700, Noah Misch <n...@leadboat.com> wrote in > <20190525023332.ge1624...@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.
