Re: Avoid full page images in streaming replication?

On 2015-10-22 17:47:01 -0500, Jim Nasby wrote:
> On 10/22/15 5:11 PM, Robert Haas wrote:
> >It's true that if the standby didn't have the master's FPIs, it could
> >generate its own in some side location that behaves like a separate
> >WAL stream or a double-write buffer.  But that would be a heck of a
> >lot of work to implement for an uncertain benefit.
> 
> If the receiver didn't write the WAL before processing it then it can just
> stick the page image into the WAL it's writing for itself. Probably not good
> for syncrep, but I don't think you'd want this on for syncrep anyway.

To me this sounds like a recipe for disaster (i.e. complex bugs). WAL
(and thus CRC checksums) differing between nodes. Ugh.

> But yes, this is all very hand-wavy without any actual data on what
> percentage of the WAL stream is FPIs. Looks like pageinspect doesn't work
> for WAL... does anyone have a script/tool that breaks out what percentage of
> a WAL file is FPIs?

pg_xlogdump --stats