RE: Tablespace Backup question

Colin,

Thanks... I am going to borrow from your excellent reply if I may... and combine both into a column for the NYOUG newsletter I think

Not bad all from memory....

Rachel

>From: Colin.Shaw_at_phs.com
>To: ORACLE-L_at_fatcity.com
>CC: carmichr_at_hotmail.com
>Subject: RE: Tablespace Backup question
>Date: Tue, 6 Jun 2000 17:18:40 -0700
>
>Rachel:
>
>Looks fine to me! The only thing I'd change is: "your process starts to
>copy the file and Oracle writes to it in the middle of the copy". Because
>it's the disk blocks you're worried about, not the file as a whole.
>There's
>no problem if your backup program and Oracle are *always* at different
>points in the file. I'd change it to "... starts to copy a portion of an
>Oracle block which Oracle writes ...."
>
>Regards,
>
>Colin.
>
>-----Original Message-----
>From: Rachel Carmichael [mailto:carmichr_at_hotmail.com]
>Sent: Tuesday, June 06, 2000 5:05 PM
>To: Multiple recipients of list ORACLE-L
>Subject: RE: Tablespace Backup question
>
>
>We had this discussion at one point on (I think) the lazydba list.
>
>Think of it this way:
>
>you are in archive log mode. You alter a tablespace to begin backup. You
>walk off and forget to take it out of backup mode (okay, so you are a bad
>DBA, this is hypothetical).
>
>So as the users come in for the day and begin to do data entry, the data in
>that tablespace is changing. If Oracle kept it all cached in memory, then
>the promise of recoverability would not hold true. At some point all the
>log
>
>files would fill and need to be archived. But since the data blocks are
>still in memory....
>
>And remember that Oracle ALWAYS flushes the buffer on a commit... and will
>write data block buffers down to disk if it needs space in the buffer
>cache.
>
>All that holding true, it follows that oracle HAS to write blocks to disk
>even while in backup mode.
>
>What Oracle does do, that is different in backup mode, is write full
>database blocks to the log files, instead of just enough information to
>recover the transaction. This ensures that even if split blocks are
>written
>
>to the backup copy of the datafile (your process starts to copy the file
>and
>
>Oracle writes to it in the middle of the copy), you will have a consistent
>picture of that block in the log file.
>
>Geez, I remembered all that? Someone PLEASE tell me what parts I got wrong!
>
>Rachel
> >From: sean.hull_at_pobox.com
> >Reply-To: ORACLE-L_at_fatcity.com
> >To: Multiple recipients of list ORACLE-L <ORACLE-L_at_fatcity.com>
> >Subject: RE: Tablespace Backup question
> >Date: Tue, 06 Jun 2000 15:04:16 -0800
> >
> >
> >On Tue, 6 Jun 2000, Rachel Carmichael wrote:
> >
> > > I need to bring in the backup class notes from the internals classes.
> > >
> > > Colin is right.... and Oracle keeps both a header and footer section
> >where
> > > the information is used to reconcile which blocks need "recovery"
> > >
> > > this is from memory, I'd have to look up my notes to be certain
> >
> >Still seems a bit odd though. I guess Oracle has no where else to write
> >the changes, but keep them cached in memory... It might have to flush the
> >cache sooner or later. I guess we could verify this by putting a
> >tablespace in backup mode, and then using the unix command "lsof" to list
> >open files, and find out if the files were still open for writing.
> >
> >The more I think about it the more it makes sense.
> >
> >Thanks Rachel.
> >Sean
> >
> >--
> >Author:
> > INET: sean.hull_at_pobox.com
> >
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>--
>Author: Rachel Carmichael
> INET: carmichr_at_hotmail.com
>
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