Re: [tarantool-patches] Re: [PATCH 8/9] wal: pass struct instead of vclock to checkpoint methods

[Vladimir Davydov <vdavydov.dev@gmail.com>]

On Fri, Nov 30, 2018 at 12:00:14PM +0300, Konstantin Osipov wrote:
> * Vladimir Davydov <vdavydov.dev@gmail.com> [18/11/28 19:16]:
> > Currently, we only need to pass a vclock between TX and WAL during
> > checkpointing. However, in order to implement auto-checkpointing
> > triggered when WAL size exceeds a certain threshold, we will need to
> > pass some extra info so that we can properly reset the counter
> > accounting the WAL size in the WAL thread. To make it possible,let's
> > move wal_checkpoint struct, which is used internally by WAL to pass a
> > checkpoint vclock, to the header and require the caller to pass it to
> > wal_begin/commit_checkpoint instead of just a vclock.
> 
> wal is a singleton and there could be only one ongoing checkpoint. 
> 
> I am ok with this change but I don't understand the purpose of it:
> why not simply store wal_checkpoint in struct wal then you don't
> have to pass it around at all?

Yeah, we could store the vclock in the WAL thread, but IMO that would
look ugly, because the new member of the wal_writer struct would only be
used during checkpointing and would be meaningless for the rest of the
WAL operation.