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

On Tue, Dec 16, 2014 at 3:46 AM, Robert Haas <robertmhaas@gmail.com> wrote:
> On Sat, Dec 13, 2014 at 9:36 AM, Michael Paquier
> <michael.paquier@gmail.com> wrote:
>> Something to be aware of btw is that this patch introduces an
>> additional 8 bytes per block image in WAL as it contains additional
>> information to control the compression. In this case this is the
>> uint16 compress_len present in XLogRecordBlockImageHeader. In the case
>> of the measurements done, knowing that 63638 FPWs have been written,
>> there is a difference of a bit less than 500k in WAL between HEAD and
>> "FPW off" in favor of HEAD. The gain with compression is welcome,
>> still for the default there is a small price to track down if a block
>> is compressed or not. This patch still takes advantage of it by not
>> compressing the hole present in page and reducing CPU work a bit.
>
> That sounds like a pretty serious problem to me.
OK. If that's so much a problem, I'll switch back to the version using
1 bit in the block header to identify if a block is compressed or not.
This way, when switch will be off the record length will be the same
as HEAD.
-- 
Michael