RE: [HACKERS] What I'm working on

On Sun, 23 Aug 1998, Stupor Genius wrote:

> > There *has* to be some overhead, performance wise, in the database
> > having to keep track of row-spanning, and being able to reduce that, IMHO,
> > is what I see being able to change the blocksize as doing...
>
> If both features were present, I would say to increase the blocksize of
> the db to the max possible. This would reduce the number of tuples that
> are spanned. Each span would require another tuple fetch, so that could
> get expensive with each successive span or if every tuple spanned.
>
> But if we stick with 8k blocksizes, people with tuples between 8 and 16k
> would get absolutely killed performance-wise. Would make sense for them
> to go to 16k blocks where the reading of the extra bytes per block would
> be minimal, if anything, compared to the fetching/processing of the next
> span(s) to assemble the whole tuple.
>
> In summary, the capability to span would be the next resort after someone
> has maxed out their blocksize. Each OS would have a different blocksize
> max...an AIX driver breaks when going past 16k...don't know about others.

Oh...I like this :) that would give us something that the "big
guys" don't also, no? Bruce?

Can someone clarify something for me? If, for example, we have
the blocksize set to 16k, but the file system size is 8k, would the OS do
both reads at the same time in order to get the full 16k? I hope someone
can follow this through (unless I'm actually clear), but if we left the
tuples size at 8k fixed, and had that 16k tuple span two rows, do we send
a request to the OS for the one block, then, once we get that back,
determine that we need the next and request that?

Damn, not clear at all...if I'm thinking right, by increasing the
blocksize to 16k, postgres does one read request, while the OS does two.
If we don't, postgres does two read requests while the OS still does two.

Does that make sense?

Marc G. Fournier
Systems Administrator @ hub.org
primary: scrappy(at)hub(dot)org secondary: scrappy(at){freebsd|postgresql}.org