Ühel kenal päeval, K, 2006-03-08 kell 20:08, kirjutas Jim C. Nasby:
> But it will take a whole lot of those rewinds to equal the amount of
> time required by an additional pass through the data.
I guess that missing a sector read also implies a "rewind", i.e. if you
don't process the data read from a "tape" fast enough, you will have to
wait a whole disc revolution (~== "seek time" on modern disks) before
you get the next chunk of data.
> I'll venture a
> guess that as long as you've got enough memory to still read chunks back
> in 8k blocks that it won't be possible for a multi-pass sort to
> out-perform a one-pass sort. Especially if you also had the ability to
> do pre-fetching (not something to fuss with now, but certainly a
> possibility in the future).
>
> In any case, what we really need is at least good models backed by good
> drive performance data.
And filesystem performance data, as postgres uses OS-s native
filesystems.
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Hannu
