Re: VACUUMs and WAL

Hannu Krosing wrote:
> On Tue, 2008-10-28 at 10:10 +0000, Simon Riggs wrote:
>> On Tue, 2008-10-28 at 11:45 +0200, Hannu Krosing wrote:
>>> On Tue, 2008-10-28 at 08:49 +0000, Simon Riggs wrote:
>>>> Looking at a VACUUM's WAL records makes me think twice about the way we
>>>> issue a VACUUM.
>>>>
>>>> 1. First we scan the heap, issuing a HEAP2 clean record for every block
>>>> that needs cleaning.
>>> IIRC the first heap pass just collects info and does nothing else.
>>> Is this just an empty/do-nothing WAL record ?
>> 8.3 changed that; it used to work that way. I guess I never looked at
>> the amount of WAL being generated.
>
> I can't see how it is safe to do anything more than just lookups on
> first pass.

What's done in the first pass is the same HOT pruning that is done
opportunistically on other page accesses as well. IIRC it's required for
correctness, though I can't remember what exactly the issue was.

I don't think the extra WAL volume is a problem; VACUUM doesn't generate
much WAL, anyway. As for the extra data page writes it causes; yeah,
that might cause some I/O that could be avoided, but remember that the
first pass often dirties buffers anyway to set hint bits.

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