Re: Publish checkpoint timing and sync files summary data to pg_stat_bgwriter

Jeff Janes wrote:
> I'm finding the backend_writes column pretty unfortunate.  The only
> use I know of for it is to determine if the bgwriter is lagging
> behind.  Yet it doesn't serve even this purpose because it lumps
> together the backend writes due to lagging background writes, and the
> backend writes "by design" due to the use buffer access strategy
> during bulk inserts.
>
> If I'm running a tightly controlled benchmark on an otherwise unused
> server and I know that no BAS is being used, then I can meaningfully
> use backend_writes.  That is a pretty limiting limit.
>   

I don't think it's quite that bad in general; this presumes a moderate 
amount of BAS writes relative to other activity.  But I understand your 
concern better now.  I don't think the sorts of workloads you seem to 
have a lot of were considered very carefully before here.

> I think we should either create a separate column to segregate BAS
> backend_writes, or just don't report them at all and report only the
> non-BAS ones into pg_stat_bgwriter.
>   

We can't not report them.  One of the goals of pg_stat_bgwriter is to 
account for all writes out of the buffer cache.  If there enough BAS 
writes on your system that them being lumped together is a serious 
problem, having them go missing altogether would be even worse.  And any 
whacking around of pg_stat_bgwriter might as well fix that too.

Do you think you could put together a quick test case that's similar to 
the ones you're seeing unfair accounting for here?  This isn't quite 
buggy behavior, but a solid example I could test against showing it's a 
sketchy approach would be enough for me to incorporate a fix for it into 
this suggested redesign.