Re: XLog changes for 9.3

Heikki Linnakangas <heikki.linnakangas@enterprisedb.com> writes:
> When I worked on the XLogInsert scaling patch, it became apparent that 
> some changes to the WAL format would make it a lot easier. So for 9.3, 
> I'd like to do some refactoring:

> 1. Use a 64-bit integer instead of the two-variable log/seg 
> representation, for identifying a WAL segment. This has no user-visible 
> effect, but makes the code a bit simpler.

> 2. Don't waste the last WAL segment in each logical 4GB file. Currently, 
> we skip the WAL segment ending with "FF". The comments claim that 
> wasting the last segment "ensures that we don't have problems 
> representing last-byte-position-plus-1", but in my experience, it just 
> makes things more complicated.

I think that's actually an indivisible part of point #1.  The issue in
the 32+32 representation is that you'd overflow the low-order half when
trying to represent last-byte-of-file-plus-1, and have to do something
with propagating that to the high half.  In a 64-bit continuous
addressing scheme the problem goes away, and it would just get more
complicated not less to preserve the "hole".
        regards, tom lane