Re: Re: [COMMITTERS] pgsql: Make CheckRequiredParameterValues() depend upon correct

On Thu, 2010-04-29 at 13:01 -0500, Kevin Grittner wrote:
> Simon Riggs  wrote:
>  
> > I've asked for evidence that recovery is any slower as a result of
> > HS.
>  
> Can you quantify the impact on the number of bytes written to WAL
> when switching from the archiving level to the hot standby level?

Sure, done that a few times.

Extra WAL data is written for these actions, listed in order of
increasing size

* commit records contain a variable length list of
relcacheinvalidations, mostly applies only to DDL
* one extra WAL record in most VACUUMs, fairly small, optimised away in
some cases
* a transaction issues more than 64 subtransactions it will issue a
record approx 256 bytes plus header
* one extra WAL record every checkpoint, containing a full current
snapshot's worth of running xids 100-400 bytes typically, could go up
from there to 4000 bytes in very extreme write workloads that also have
many, many subtransactions

-- Simon Riggs           www.2ndQuadrant.com