Re: Re: We have got a serious problem with pg_clog/WAL synchronization

Kenneth Marshall <ktm(at)is(dot)rice(dot)edu> writes:
> On Thu, Aug 12, 2004 at 09:58:56AM -0400, Tom Lane wrote:
>> How would a read-only action work to block out the checkpoint?

> The latch+version number is use by the checkpoint process. The
> other processes can do a read of the latch to determine if it has
> been set. This does not cause a cache invalidation hit. If the
> latch is set, the competing processes read until it has been
> cleared and the version updated. This makes the general case of
> no checkpoint not incur a write and the consequent cache-line
> invalidation and reload by all processors on an SMP system.

Except that reading the latch and finding it clear offers no guarantee
that a checkpoint isn't about to start. The problem is that we are
performing two separate actions (write a COMMIT xlog record and update
transaction status in clog) and we have to prevent a checkpoint from
starting in between those actions. I don't see that there's any way to
do that with a read-only latch.

regards, tom lane