| From: | Andres Freund <andres(at)anarazel(dot)de> |
|---|---|
| To: | Jim Nasby <Jim(dot)Nasby(at)BlueTreble(dot)com> |
| Cc: | Robert Haas <robertmhaas(at)gmail(dot)com>, Magnus Hagander <magnus(at)hagander(dot)net>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Avoid full page images in streaming replication? |
| Date: | 2015-10-22 22:52:06 |
| Message-ID: | 20151022225206.GL14196@alap3.anarazel.de |
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| Lists: | pgsql-hackers |
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
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