==>The only part of the data file that is "un-writable" is the datafile header block, which gets frozen ==>until the hot backup of that tablespace completes.
Logically correct, but internally it does update the file header block. Here is how it works: The file header contains several structures which keeps track of hot backup activity and the structures relevant to hot backup are:
1. Checkpoint counter and status
2. Online checkpoint structure
3. Backup checkpoint structure
While the tablespaces are NOT in backup mode, then both online and backup checkpoint structure are updated, when the tablespaces are in backup mode, only the backup checkpoint structure is updated and the online checkpoint structure is frozen. Checkpoint counter is updated regardless of whether the tablespaces are in backup mode or not. Also, the status of the file is updated to 0x01 (hot backup in progress). Using the above structures, status of the file from the file header and the checkpoint structures from the control files are used to drive the media/instance recovery.
File header dumps before the hot backup command: Notice both online and backup checkpoint structures are kept current.
status:0x4 root dba:0x00000000 chkpt cnt: 984 ctl cnt:983
begin-hot-backup file size: 12928
Checkpointed at scn: 0x0000.10e83b12 05/30/2001 12:15:27
thread:1 rba:(0x2de.2.10)
Backup Checkpointed at scn: 0x0000.10e83b12 05/30/2001 12:15:27
thread:1 rba:(0x2de.2.10)
Checkpoint cnt:984 scn: 0x0000.10e83b12 05/30/2001 12:15:27
File header dumps while in hot backup mode. Notice "checkpointed SCN" structure is not updated whereas the "backup checkpointed" structure is updaed.
status:0x1 root dba:0x00000000 chkpt cnt: 988 ctl cnt:987
begin-hot-backup file size: 12928
Checkpointed at scn: 0x0000.10e83b14 05/30/2001 12:29:08
thread:1 rba:(0x2de.3.10)
Backup Checkpointed at scn: 0x0000.10e83b17 05/30/2001 12:37:20
thread:1 rba:(0x2e1.2.10)
Checkpoint cnt:988 scn: 0x0000.10e83b14 05/30/2001 12:29:08
Hope this helps!
Thanks
Riyaj "Re-yas" Shamsudeen
Certified Oracle DBA
i2 technologies www.i2.com
| [EMAIL PROTECTED]
Sent by: [EMAIL PROTECTED] 06/26/01 08:46 AM
|
To: Multiple recipients of list ORACLE-L <[EMAIL PROTECTED]> cc: Subject: Re[2]: Common Oracle RDBMS Misconceptions |
Joe & company,
Jeremiah has it exactly correct. The only part of the data file that is
"un-writable" is the datafile header block, which gets frozen until the hot
backup of that tablespace completes. Look at it this way, when the "start
backup" command gets issued Oracle is in fact freezing that datafile(s) at that
point in time and assuming that all changes to the datafile(s) have not been
written. When you restore the file, you then apply redo from your archive logs
(the reason you MUST be in archive mode) from the start of the backup till the
end of the recovery just as if those changes to the file(s) had never been made
in the first place. In fact Oracle knows the changes were made during the
backup, it just has no idea if the change was written to the file before or
after your backup software copied that portion of the file to tape or wherever.
Therefore, simple solution assume it was after. In practice the process is very
simple, does slow the database down a tad, and works as advertised. (Been
there, done that several times.)
BTW: IMHO, don't waste your money on any of the SAMS books. They are full of
similar misconceptions.
Dick Goulet
____________________Reply Separator____________________
Author: "Joseph S. Testa" <[EMAIL PROTECTED]>
Date: 6/26/2001 4:00 AM
Well i dont know about everyone else, but i knew thats how the hot
backup worked, but then again, i've not attended oracle education
classes either, just some hard core reading and have gotten all of my
backup/recovery concepts from Rama Velpuri's book. An excellent book if
you dont have it.
joe
> On Jun 26, 2001 at 01:05:59AM, novicedba wrote:
> > Hi everyone,
> > I visited Jeremiah Wilton's web page http://www.speakeasy.net/~jwilton
> > I was shocked to read Hot backup mode explained
> > If this is true then I may be a victim of a disease called
> > 'Common Oracle RDBMS Misconceptions' . Somebody help me!! (Jim carrey-MASK
style)
> > Please help me. If some one has few more articles like this enlighten me
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