Re: Block write statistics WIP

On 19.05.2013 11:15, Greg Smith wrote:
> I've thought of two paths to get a block write count out of that so far:
>
> -Provide a function to find the Relation from the RelFileNode. There is
> a warning about the perils of assuming you can map that way from a
> buftag value in buf_internals.h though:
>
> "Note: the BufferTag data must be sufficient to determine where to write
> the block, without reference to pg_class or pg_tablespace entries. It's
> possible that the backend flushing the buffer doesn't even believe the
> relation is visible yet (its xact may have started before the xact that
> created the rel). The storage manager must be able to cope anyway."
>
> -Modify every caller of MarkDirty* to include a relation when that
> information is available. There are about 200 callers of those functions
> around, so that won't be fun. I noted that many of them are in the XLog
> replay functions, which won't have the relation--but those aren't
> important to count anyway.
>
> Neither of these options feels very good to me, so selecting between the
> two feels like picking the lesser of two evils. I'm fine with chugging
> through all of the callers to modify MarkDirty*, but I didn't want to do
> that only to have the whole approach rejected as wrong. That's why I
> stopped here to get some feedback.

Adding a Relation argument to all the Mark* calls seems fine to me. I 
don't find it ugly at all. The biggest objection would be that if there 
are extensions out there that call those functions, they would need to 
be changed, but I think that's fine.

> The way that MarkDirty requires this specific underlying storage manager
> behavior to work properly strikes me as as a bit of a layering violation
> too. I'd like the read and write paths to have a similar API, but here
> they don't even operate on the same type of inputs. Addressing that is
> probably harder than just throwing a hack on the existing code though.

To be honest, I don't understand what you mean by that. ?

- Heikki