Re: Compression of full-page-writes

On 9 December 2014 at 04:21, Andres Freund <andres@2ndquadrant.com> wrote:
> On 2014-12-08 14:09:19 -0500, Robert Haas wrote:
>> > records, just fpis. There is no evidence that we even want to compress
>> > other record types, nor that our compression mechanism is effective at
>> > doing so. Simple => keep name as compress_full_page_writes
>>
>> Quite right.
>
> I don't really agree with this. There's lots of records which can be
> quite big where compression could help a fair bit. Most prominently
> HEAP2_MULTI_INSERT + INIT_PAGE. During initial COPY that's the biggest
> chunk of WAL. And these are big and repetitive enough that compression
> is very likely to be beneficial.

Yes, you're right there. I was forgetting those aren't FPIs. However
they are close enough that it wouldn't necessarily effect the naming
of a parameter that controls such compression.

> I still think that just compressing the whole record if it's above a
> certain size is going to be better than compressing individual
> parts.

I think its OK to think it, but we should measure it.

For now then, I remove my objection to a commit of this patch based
upon parameter naming/rethinking. We have a fine tradition of changing
the names after the release is mostly wrapped, so lets pick a name in
a few months time when the dust has settled on what's in.

-- Simon Riggs                   http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training & Services