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

On 2015-02-24 16:03:41 +0900, Michael Paquier wrote:
> Looking at this code, I think that it is really confusing to move the data
> related to the status of the backup block out of XLogRecordBlockImageHeader
> to the chunk ID itself that may *not* include a backup block at all as it
> is conditioned by the presence of BKPBLOCK_HAS_IMAGE.

What's the problem here? We could actually now easily remove
BKPBLOCK_HAS_IMAGE and replace it by a chunk id.

> the idea of having the backup block data in its dedicated header with bits
> stolen from the existing fields, perhaps by rewriting it to something like
> that:
> typedef struct XLogRecordBlockImageHeader {
> uint32 length:15,
>      hole_length:15,
>      is_compressed:1,
>      is_hole:1;
> } XLogRecordBlockImageHeader;
> Now perhaps I am missing something and this is really "ugly" ;)

I think it's fantastically ugly. We'll also likely want different
compression formats and stuff in the not too far away future. This will
just end up being a pain.

Greetings,

Andres Freund

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