Re: sorted writes for checkpoints

On 29.10.2010 06:00, Jeff Janes wrote:
> One of the items on the Wiki ToDo list is sorted writes for
> checkpoints. The consensus seemed to be that this should be done by
> adding hook(s) into the main code, and then a contrib module to work
> with those hooks. Is there an existing contrib module that one could
> best look to for inspiration on how to go about doing this? I have
> the sorted checkpoint working under a guc, but don't know where to
> start on converting it to a contrib module instead.

I don't think it's a good idea to have this as a hook. Bgwriter
shouldn't need to load external code, and checkpoint robustness should
dependend on user-written code. IIRC Tom Lane didn't even like pallocing
the memory for the list of dirty pages at checkpoint time because that
might cause an out-of-memory error. Calling a function in a contrib
module is much much worse.

Simon's argument in the thread that the todo item points to
(http://archives.postgresql.org/pgsql-patches/2008-07/msg00123.php) is
basically that we don't know what the best algorithm is yet and
benchmarking is a lot of work, so let's just let people do whatever they
feel like until we settle on the best approach. I think we need to bite
the bullet and do some benchmarking, and commit one carefully vetted
patch to the backend.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com