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RE: Tablespace Backup question

From: Rachel Carmichael <>
Date: Tue, 06 Jun 2000 23:01:20 GMT
Message-Id: <>

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!

>To: Multiple recipients of list ORACLE-L <>
>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
> > 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.
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