CLOG contention, part 2

Recent results from Robert show clog contention is still an issue.

In various discussions Tom noted that pages prior to RecentXmin are
readonly and we might find a way to make use of that fact in providing
different mechanisms or resources.

I've taken that idea and used it to build a second Clog cache, known
as ClogHistory which allows access to the read-only tail of pages in
the clog. Once a page has been written to for the last time, it will
be accessed via the ClogHistory Slru in preference to the normal Clog
Slru. This separates historical accesses by readers from current write
access by committers. Historical access doesn't force dirty writes,
nor are commits made to wait when historical access occurs.

The patch is very simple because all the writes still continue through
the normal route, so is suitable for 9.2.

I'm no longer working on "clog partitioning" patch for this release.

--
 Simon Riggs                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services