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

On Tue, Mar 3, 2015 at 5:17 AM, Rahila Syed <rahilasyed90(at)gmail(dot)com> wrote:
> Hello,
>
>>When I test the WAL or replication related features, I usually run
>>"make installcheck" and pgbench against the master at the same time
>>after setting up the replication environment.
> I will conduct these tests before sending updated version.
>
>>Seems this increases the header size of WAL record even if no backup block
>> image is included. Right?
> Yes, this increases the header size of WAL record by 1 byte for every block
> reference even if it has no backup block image.
>
>>Isn't it better to add the flag info about backup block image into
>> XLogRecordBlockImageHeader rather than XLogRecordBlockHeader
> Yes , this will make the code extensible,readable and will save couple of
> bytes per record.
> But the current approach is to provide a chunk ID identifying different
> xlog record fragments like main data , block references etc.
> Currently , block ID is used to identify record fragments which can be
> either XLR_BLOCK_ID_DATA_SHORT , XLR_BLOCK_ID_DATA_LONG or actual block ID.
> This can be replaced by chunk ID to separate it from block ID. Block ID can
> be used to number the block fragments whereas chunk ID can be used to
> distinguish between main data fragments and block references. Chunk ID of
> block references can contain information about presence of data, image ,
> hole and compression.
> Chunk ID for main data fragments remains as it is . This approach provides
> for readability and extensibility.

Already mentioned upthread, but I agree with Fujii-san here: adding
information related to the state of a block image in
XLogRecordBlockHeader makes little sense because we are not sure to
have a block image, perhaps there is only data associated to it, and
that we should control that exclusively in XLogRecordBlockImageHeader
and let the block ID alone for now. Hence we'd better have 1 extra
int8 in XLogRecordBlockImageHeader with now 2 flags:
- Is block compressed or not?
- Does block have a hole?
Perhaps this will not be considered as ugly, and this leaves plenty of
room for storing a version number for compression.
--
Michael