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

On Wed, Nov 26, 2014 at 11:00 PM, Michael Paquier
<michael(dot)paquier(at)gmail(dot)com> 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.

If we have a separate GUC to determine whether to do compression of
full page writes, then it seems like that parameter ought to apply
regardless of WHY we are doing full page writes, which might be either
that full_pages_writes=on in general, or that we've temporarily turned
them on for the duration of a full backup.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company