Re: pg_xlogdump --stats

On 2014-09-11 12:14:42 +0300, Heikki Linnakangas wrote:
> On 09/11/2014 11:43 AM, Abhijit Menon-Sen wrote:
> >At 2014-08-21 10:06:39 +0300, hlinnakangas(at)vmware(dot)com wrote:
> >>I think the names that rm_identify returns should match those that the
> >>rm_desc functions print.
> >
> >I had originally started off trying to keep the output in sync, but it
> >doesn't work very well. There are rm_desc functions that print things
> >like "truncate before" and "Create posting tree", and many decisions
> >are quite arbitrary ("freeze_page", "cleanup info", "multi-insert").
>
> It would be good to clean up those messages to be more sensible and
> consistent.

> >I think it's better the (grep-friendly) way it is. If anything, perhaps
> >rm_desc should output "${rm_identify}[: optional explanation]". That
> >would also make it very clear what should go in rm_identify and what
> >should go in rm_desc.

> Yeah, that makes sense too.

I'm not sure I agree here. From a theoretical POV sure, but wouldn't
that lead to even longer lines for xlogdump and other error messages for
some records?
We probably need to see how it plays out.

> >>The corresponding rm_identify output is:
> >>
> >>HOT_UPDATE+INIT
> >
> >The +INIT is admittedly a special case, and I would have no objection to
> >writing that as (INIT) or something instead.
>
> I don't care much, as long as it's consistent the rm_desc output.
>
> It's quite arbitrary that the INIT cases are considered different record
> types, with their own "identity" string, instead of being just a flag in the
> same record type. It's an implementation detail that that flag is stored in
> the xl_info, and that implementation detail is now exposed in the stats
> pg_xlogdump --stats output.

It's also actually quite good from a display purpose. +INIT will have
quite different wal logging characteristics :)

> There are similar cases in B-tree splits, for
> example, where a split where the new tuple goes to the left half is
> considered a different record type than one where it goes ot the right half.
> It might be interesting to count them separately in the stats, but then
> again it might just be confusing. The xl_info flags and the rm_desc output
> were never intended to be a useful categorization for statistics purposes,
> so it's not surprising that it's not too well suited for that. Nevertheless,
> the "pg_xlogdump --stats" is useful as it is, if you know the limitations.

I agree it's not perfect, but I think it's a huge step forward. We very
well might want to improve upon the stats granularity once in, but I
think it already gives a fair amount of direction where to look.

Greetings,

Andres Freund

--
Andres Freund http://www.2ndQuadrant.com/
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