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

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.

-- 
Robert Haas
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