Proposed WAL changes

I have just sent to the pgsql-patches list a rather large set of
proposed diffs for the WAL code.  These changes:

* Store two past checkpoint locations, not just one, in pg_control. On startup, we fall back to the older checkpoint if
thenewer one is unreadable.  Also, a physical copy of the newest checkpoint record is kept in pg_control for possible
usein disaster recovery (ie, complete loss of pg_xlog).  Also add a version number for pg_control itself.  Remove
archdirfrom pg_control; it ought to be a GUC parameter, not a special case (not that it's implemented yet anyway).
 

* Suppress successive checkpoint records when nothing has been entered in the WAL log since the last one.  This is not
somuch to avoid I/O as to make it actually useful to keep track of the last two checkpoints.  If the things are right
nextto each other then there's not a lot of redundancy gained...
 

* Change CRC scheme to a true 64-bit CRC, not a pair of 32-bit CRCs on alternate bytes.

* Fix XLOG record length handling so that it will work at BLCKSZ = 32k.

* Change XID allocation to work more like OID allocation, so that we can flush XID alloc info to the log before there
isany chance an XID will appear in heap files.
 

* Fix a number of minor bugs, such as off-by-one logic for XLOG file wraparound at the 4 gig mark.

* Add documentation and clean up some coding infelicities; move file format declarations out to include files where
plannedcontrib utilities can get at them.
 


Before committing this stuff, I intend to prepare a contrib utility that
can be used to reset pg_control and pg_xlog.  This is mainly for
disaster recovery purposes, but as a side benefit it will allow people
to update 7.1beta installations to this new code without doing initdb.
I need to update contrib/pg_controldata, too.
        regards, tom lane