Re: Not to use SCAN in 11gR2 RAC
- From: Yong Huang <yong321@xxxxxxxxx>
- To: Robert Bialek <bialekr@xxxxxxxxx>
- Date: Mon, 19 Oct 2009 16:44:30 -0700 (PDT)
Thank you and all others. For now, I'll use SCAN, at least during
installation.
Here's some more thought for whoever likes to research. The extra
SCAN listeners won't use much CPU or memory. But there's another
aspect of overhead. They hand off connections to the regular listeners.
If I connect to the regular listener in a dedicated connection, the
listener fork's (clone's) and exec's $ORACLE_HOME/bin/oracle. Do you
think the SCAN listener does the same if the least loaded node happens
to be the current node? No. It's still the regular listener that does
that. This can be verified with simple strace. On a 2-node RAC, 50%
of the time this extra hand-off could be made unnecessary. If the
database has frequent client connections and disconnections, this
overhead may be non-negligible.
I don't know if the SCAN listeners have load stats of all the nodes.
If yes, the probability of node-to-node redirection is the same as
using the traditional VIPs in the connection strings, even if you have
more than 3 nodes. (Am I right?) And the regular listeners shouldn't
need to store each node's load stats. There's another factor of
overhead if the regular listener is not modified to skip checking
load stats. (It's probably not, based on the fact that it still has
pmon's service update as the SCAN listener does.)
One interesting observation is that, if you run strace on either the
SCAN or regular listener, as soon as you Control-C to stop strace,
the listener hangs when you tnsping or sqlplus to it. (Just kill it
and it'll be respawned immediately.)
Something else is different in 11gR2. Both listeners open and then
close their log files frequently, unlike in previous versions where
the listener process holds the log file open.
Yong Huang
--- On Sun, 10/18/09, Robert Bialek <bialekr@xxxxxxxxx> wrote:
> Yes, SCAN is mandatory during
> software installation and optional
> afterward. If you DO NOT plan to
> use it AND not like to create A records in DNS for the
> installation
> simply create the scan name with
> one IP in /etc/hosts on every node in your cluster. The
> installation
> will work, but CLUVFY run
> automatically by OUI at the end of installation will
> complain !!
>
> SCAN is very useful for "dynamic" grids. If you like to use
> the "old"
> method:
>
> 1. Do not use the SCAN at your clients, leaving the
> resources running
>
> or
>
> 2. Stop and disable the SCAN VIP(s) and Listener(s):
>
> srvctl stop scan_listener
> srvctl stop scan
> sudo srvctl disable scan
> srvctl disable scan_listener
>
> If you decide later to use SCAN you can easily activate it
> or recreate /
> change the amount of SCAN VIPs and Listeners.
>
> Robert
>
>
>
> Yong Huang wrote:
> > Can anybody think of any risk or loss of benefit we
> haven't thought of? Is there any way to completely bypass
> it, not even bothering our DNS admin to create an entry (one
> or three A records)? If not during installation, can I stop
> the SCAN listener and shutdown its network interface (if I
> can) and remove it from OCR?
--- On Sun, 10/18/09, Robert Bialek <bialekr@xxxxxxxxx> wrote:
> From: Robert Bialek <bialekr@xxxxxxxxx>
> Subject: Re: Not to use SCAN in 11gR2 RAC
> To: yong321@xxxxxxxxx
> Cc: oracle-l@xxxxxxxxxxxxx
> Date: Sunday, October 18, 2009, 10:13 AM
> Yes, SCAN is mandatory during
> software installation and optional
> afterward. If you DO NOT plan to
> use it AND not like to create A records in DNS for the
> installation
> simply create the scan name with
> one IP in /etc/hosts on every node in your cluster. The
> installation
> will work, but CLUVFY run
> automatically by OUI at the end of installation will
> complain !!
>
> SCAN is very useful for "dynamic" grids. If you like to use
> the "old"
> method:
>
> 1. Do not use the SCAN at your clients, leaving the
> resources running
>
> or
>
> 2. Stop and disable the SCAN VIP(s) and Listener(s):
>
> srvctl stop scan_listener
> srvctl stop scan
> sudo srvctl disable scan
> srvctl disable scan_listener
>
> If you decide later to use SCAN you can easily activate it
> or recreate /
> change the amount of SCAN VIPs and Listeners.
>
> Robert
>
>
>
> Yong Huang wrote:
> > Can anybody think of any risk or loss of benefit we
> haven't thought of? Is there any way to completely bypass
> it, not even bothering our DNS admin to create an entry (one
> or three A records)? If not during installation, can I stop
> the SCAN listener and shutdown its network interface (if I
> can) and remove it from OCR?
> >
>
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