Oracle FAQ Your Portal to the Oracle Knowledge Grid

Home -> Community -> Mailing Lists -> Oracle-L -> RE: How big is an SCN ? (was I/O activity during HOT backups)

RE: How big is an SCN ? (was I/O activity during HOT backups)

From: Rajesh Dayal <>
Date: Wed, 21 Jun 2000 15:00:30 +0400
Message-Id: <>

Curtsy Rama Velpuri (My most favorite one):

        SCNs are guaranteed to be unique to a DB and increases with time but may not be sequential. SCN values never get reset to zero unless the database is recreated.

        You don't need to worry about running out of SCNs- even if you were to commit 16,000 transactions per second, it would take more than 500 years to run out of SCN numbers.

Hope This Helps

-----Original Message-----
From: []On Behalf Of Oracle DBA - Oweson Flynn
Sent: Wednesday, June 21, 2000 1:46 PM
To: Multiple recipients of list ORACLE-L Subject: How big is an SCN ? (was I/O activity during HOT backups)

Hi Gang,

I was giving the 8i Backup & Recovery course last week for Oracle (great way
of getting all the latest training ...), and I explained to the students how
the hot backups worked. I said, as has been stated, that when you put the
tablespace (or datafile) into hot back-up mode, the SCN in the header is frozen.

On of my students then asked what is the maximum size of an SCN - just how
big can it get? I popped off to the WWS (WorldWide Support) desk, and asked
them to investigate. The answer I got back was 'bigger than you'll ever need'. I wasn't happy with that, and wanted to know (in bytes) what the size of the SCN was.

I was then told that there are different SCNs in the Header (of the file)
and in each of the Data Blocks, and these SCNsare of different sizes! Apparently the 'Header' SCN is made up of two parts, each of which seemed to
consist of 8 hexidecimal digits. The 'Block' SCNs only had the 8 hexidecimal digits.

Can anyone confirm this / shed more light on this matter?

Secondly, according to the course notes, you can do 'hot' backups using RMan, which will only log changed blocks - resulting in smaller and faster
backups. I saw that some of the RMan procedures are now integrated into the
Oracle kernel (with 8i, not 8 - which means they can be accessed with the
instance started but not even mounted). Are these the routines used? How
does Oracle (and RMan) keep track of changed blocks, and what happens if the
instance crashes while the RMan has an incremental backup running?

There were no explantations in the notes at all! We had to assume (as an object of faith) that the instance would recover fine - this is VERY different if the instance crashes doing a 'normal' hot backup! I would sleep
better (and would be able to recommend using RMan with good concience) if I
understood how it worked ...

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= Oweson Flynn
Senior Oracle DBA

Tel:    978-9826
Cell:   082-600-7-006
Fax:    978-1568

Author: Oracle DBA - Oweson Flynn

Fat City Network Services    -- (858) 538-5051  FAX: (858) 538-5051
San Diego, California        -- Public Internet access / Mailing Lists
To REMOVE yourself from this mailing list, send an E-Mail message
to: (note EXACT spelling of 'ListGuru') and in
the message BODY, include a line containing: UNSUB ORACLE-L
(or the name of mailing list you want to be removed from).  You may
Received on Wed Jun 21 2000 - 06:00:30 CDT

Original text of this message