RE: RE: wait events and v$session_event
- From: "Cary Millsap" <cary.millsap@xxxxxxxxxx>
- To: <oracle-l@xxxxxxxxxxxxx>
- Date: Tue, 11 May 2004 11:55:49 -0500
1. Here's an example of what I meant: RAC node B begins a query at =
12:00n,
which will end at 12:03p. At 12:01p, RAC node A updates a block that B =
won't
read until 12:02p. At 12:02p, B discovers that the current incarnation =
of
the block is newer than 12:00n, so it has to reconstruct the 12:00n =
image of
the block using undo. This undo (one or more undo blocks) was at least =
at
some point in node A's buffer cache. If it still is, then the block has =
to
travel the interconnect and endure the expensive cache coherency lock
management code path. (If it's not still in A's cache, then it's just a =
PIO
on node B, but then you have different problems.)
2. The confusion comes from calling timed events "waits." The duration =
of an
Oracle timed event like 'db file sequential read' is the duration of the =
OS
read() call that the Oracle kernel called to fetch the block. In the
queueing theory sense, it's not a "wait" at all; it's a service time. =
It's a
"wait" only from the perspective of how long the Oracle kernel has to =
"wait"
from the issuance of a gettimeofday call occurring right before the =
read()
to the issuance of another gettimeofday call occurring right after the
read().
Cary Millsap
Hotsos Enterprises, Ltd.
http://www.hotsos.com
* Nullius in verba *
Upcoming events:
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Boston
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-----Original Message-----
From: oracle-l-bounce@xxxxxxxxxxxxx =
[mailto:oracle-l-bounce@xxxxxxxxxxxxx]
On Behalf Of ryan.gaffuri@xxxxxxx
Sent: Tuesday, May 11, 2004 11:23 AM
To: oracle-l@xxxxxxxxxxxxx
Subject: Re: RE: wait events and v$session_event
Cary,
Could you be a little more specific, I don't quite understand this? I am
using one node right now for everything in my ETL process. However, we =
have
load_balancing turned on.=20
Also, why would I get file read waits if I just have one session =
running?=20
One note worth
mentioning here is that even the famous "I'll do TP on one RAC node and
query-only on the other" strategy is prone to lots of "global cache *"
activity.
Why? Because of the undo blocks that are used by both nodes.
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