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
nothave to implement rule #5 directly, because it instead acquires exclusive lock at the relation level,
whichensures indirectly that no one else is accessing pages of the relation at all. To implement
concurrentVACUUM we will need to make it obey rule #5 fully. To do this, we'll create a new buffer manager
operationLockBufferForCleanup() that gets an exclusive lock and then checks to see if the shared pin count is
currently1. If not, it releases the exclusive lock (but not the caller's pin) and waits until signaled by
anotherbackend, whereupon it tries again. The signal will occur when UnpinBuffer decrements the shared pin
countto 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-1on any particular shared buffer. This is enough for VACUUM's use, since we don't allow multiple
VACUUMsconcurrently 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
