Re: Multiple freelist groups to reduce BBW in non-RAC systems?

From: Don Burleson <don_at_burleson.cc>
Date: 18 Jan 2003 11:15:47 -0800
Message-ID: <998d28f7.0301181115.4af44ee0@posting.google.com>

Hi Jonathan,

Wow! You sure know how to write a detailed answer! Thanks!

However, I keep on hearing about this PCTFREE-PCTUSED "gap", and it doesn't make sense to me!

I asked Steve Adams about this and he wrote "A small PCTUSED to PCTFREE gap is a problem because most deletes and many inserts involve freelist changes. "

Initially when I heard "gap", I took it to mean that 100-(pctfree+pctused)=0, but I must be mistaken because this zero-gap setting would not cause high "freelist changes". For example:

Pctfree = 95 - Un-link at 95% full
Pctused = 5 - Re-link when block is 95% empty

But . . . . . .

I could not be that a high "gap" refers to the absolute difference between
PCTUSED and PCTFREE? Nah, that does not work either:

(PCTFREE 5 PCTUSED 95) - Of course, excessive freelist re-links happen when PCTUSED is high, and frequent freelist un-links happen when PCTFREE is low.
(PCTFREE 95 PCTUSED 5) - This high "gap" in the other direction implies much less freelist movement!

So, neither of my definitions of "gap" make sense . . . .

What is this "gap"? Received on Sat Jan 18 2003 - 13:15:47 CST