Re: [REVIEW] Re: Compression of full-page-writes

On 2014-11-27 13:00:57 +0900, Michael Paquier wrote:
> On Wed, Nov 26, 2014 at 8:27 PM, Syed, Rahila <Rahila(dot)Syed(at)nttdata(dot)com> wrote:
> > Don't we need to initialize doPageCompression similar to doPageWrites in InitXLOGAccess?
> Yep, you're right. I missed this code path.
>
> > Also , in the earlier patches compression was set 'on' even when fpw GUC is 'off'. This was to facilitate compression of FPW which are forcibly written even when fpw GUC is turned off.
> > doPageCompression in this patch is set to true only if value of fpw GUC is 'compress'. I think its better to compress forcibly written full page writes.
> Meh? (stealing a famous quote).

> This is backward-incompatible in the fact that forcibly-written FPWs
> would be compressed all the time, even if FPW is set to off. The
> documentation of the previous patches also mentioned that images are
> compressed only if this parameter value is switched to compress.

err, "backward incompatible"? I think it's quite useful to allow
compressing newpage et. al records even if FPWs aren't required for the
hardware.

One thing Heikki brought up somewhere, which I thought to be a good
point, was that it might be worthwile to forget about compressing FDWs
themselves, and instead compress entire records when they're large. I
think that might just end up being rather beneficial, both from a code
simplicity and from the achievable compression ratio.

Greetings,

Andres Freund

--
Andres Freund http://www.2ndQuadrant.com/
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