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Home -> Community -> Mailing Lists -> Oracle-L -> Re:RE: How big is an SCN ? (was I/O activity during HOT bac
Lets see, that would make the max SCN > 252,288,000,000,000
____________________Reply Separator____________________Subject: RE: How big is an SCN ? (was I/O activity during HOT backup Author: Rajesh Dayal <Rajesh_at_ohitelecom.com> Date: 6/21/00 4:08 AM
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
Rajesh
-----Original Message-----
- Oweson Flynn
Sent: Wednesday, June 21, 2000 1:46 PM
To: Multiple recipients of list ORACLE-L
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 ...
Regards
Oweson
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
Oweson Flynn
Senior Oracle DBA
Email: OracleDBA_at_flysaa.com
Tel: 978-9826 Cell: 082-600-7-006 Fax: 978-1568
-- Author: Oracle DBA - Oweson Flynn INET: OracleDBA_at_flysaa.com 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: ListGuru_at_fatcity.com (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 also send the HELP command for other information (like subscribing). -- Author: Rajesh Dayal INET: Rajesh_at_ohitelecom.com 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: ListGuru_at_fatcity.com (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 mayReceived on Wed Jun 21 2000 - 10:41:00 CDT
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