While reading through src/backend/storage/buffer/README, I noticed that
the following text seems to no longer be correct:

        As of 7.1, the only operation that removes tuples or compacts
        free space is (oldstyle) VACUUM.  It does not have to implement
        rule #5 directly, because it instead acquires exclusive lock at
        the relation level, which ensures indirectly that no one else is
        accessing pages of the relation at all.
        
        To implement concurrent VACUUM we will need to make it obey rule
        #5 fully. To do this, we'll create a new buffer manager
        operation LockBufferForCleanup() that gets an exclusive lock and
        then checks to see if the shared pin count is currently 1.  If
        not, it releases the exclusive lock (but not the caller's pin)
        and waits until signaled by another backend, whereupon it tries
        again.  The signal will occur when UnpinBuffer decrements the
        shared pin count to 1.  As indicated above, this operation might
        have to wait a good while before it acquires lock, but that
        shouldn't matter much for concurrent VACUUM.  The current
        implementation only supports a single waiter for pin-count-1 on
        any particular shared buffer. This is enough for VACUUM's use,
        since we don't allow multiple VACUUMs concurrently on a single
        relation anyway.
        
Could someone who's aware of the bufmgr changes made for 7.2's lazy
VACUUM comment on how this should be updated?

-Neil



Fri, 31 Oct 2003 17:15:55 -0500
Neil Conway <neilc@samurai.com>
out of date bufmgr README
Neil Conway <neilc@samurai.com> writes:
> Could someone who's aware of the bufmgr changes made for 7.2's lazy
> VACUUM comment on how this should be updated?

The only problem with it is that the future tense should be replaced by
the present tense ;-).  Done --- thanks for noting the problem.

			regards, tom lane

Fri, 31 Oct 2003 17:49:09 -0500
Tom Lane <tgl@sss.pgh.pa.us>
Re: out of date bufmgr README 