RE: WAL & SHM principles

> 1) WAL
> We have buffer manager, ok. So why not to use WAL as part of 
> it and don't log INSERT/UPDATE/DELETE xlog records but directly
> changes into buffer pages ? When someone dirties page it has to
> inform bmgr about dirty region and bmgr would formulate xlog record.
> The record could be for example fixed bitmap where each bit corresponds
> to part of page (of size pgsize/no-of-bits) which was changed. These
> changed regions follows. Multiple writes (by multiple backends) can be
> coalesced together as long as their transactions overlaps and there is
> enough memory to keep changed buffer pages in memory.
> 
> Pros: upper layers can think thet buffers are always safe/logged and
>       there is no special handling for indices; very simple/fast redo
> Cons: can't implement undo - but in non-overwriting is not needed (?)

But needed if we want to get rid of vacuum and have savepoints.

Vadim