Обсуждение: AW: RE: xlog loose ends, continued

> > > Anyway I like idea of StartUpID in page headers - this will help
> > 
> > Can you please describe StartUpID for me ? 
> > Ideal would be a stamp that has the last (smallest open) XID, or something else 
> > that has more or less timestamp characteristics (without the actual need of wallclock)
> > in regard to the WAL.
> 
> StartUpID counts database startups and so has timestamp characteristics.
> Actually, idea is to use SUI in future to allow reusing XIDs after startup: seeing
> old SUI in data pages we'll know that all transaction on this page was committed
> "long ago" (ie visible from MVCC POV). This requires UNDO, of course.

First thanx for the description, but db startup would only count to 5-7 per year :-),
is that sufficient ? It hardly sounds like anything useful to include in page header.
What about the xlog id, that is also used for xlog file name, but I still think a xid would 
be the best candidate.

Andreas

> > StartUpID counts database startups and so has timestamp characteristics.
> > Actually, idea is to use SUI in future to allow reusing XIDs after startup: seeing
> > old SUI in data pages we'll know that all transaction on this page was committed
> > "long ago" (ie visible from MVCC POV). This requires UNDO, of course.
> 
> First thanx for the description, but db startup would only count to 5-7 per year :-),
> is that sufficient ? It hardly sounds like anything useful to include in page header.

It will be sufficient if DB will not use all 2^32 XIDs without shutdown.
Removing pg_log *segments* for old XIDs is another story.

> What about the xlog id, that is also used for xlog file name, but I still think a xid would 
> be the best candidate.

logid would be ok too, xid is not - we have to shorten xids lifetime in near future.

Vadim