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.Syed@nttdata.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
compressionof 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
compressforcibly 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/PostgreSQL Development, 24x7 Support, Training &
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