RE: I/O activity during HOT backups

Don't want to be rude, but you should change your favorite Oracle reference book ;-)
Apart the facts that :
- I find this book of minor ( if no) interest ( this is my very *personal* opinion )
- I've always found the Oracle doc very complete and reliable ( don't laught ! )
- There is a good book on this subject ( Velpuri's Backup & Recovery Handbook )
- Both sources agree with the following

there has been a long thread about this subject in this group during the last weeks, and some of the most famous & knowledgeable DBAs posting here gave detailled, clear and convergent explanations of what's happening during a hot backup.

When one doesn't know very well something he has to explain, it's a commonly used way to talk about "... a complex combination of rollback segments, buffers, redo logs, and archive logs ..." , I do it sometimes too ;-)

So, once again, don't take it personally, but I'm just tired of all of these books that pretend teaching you
"Everything-you've-always-wanted-to-know-about-Oracle" or "Become a Senior DBA in 21 days on Windoze NT", but add nothing but mistakes to the good old Oracle doc.  

> ----------
> From: William Beilstein[SMTP:BeilstWH_at_obg.com]
> Reply To: ORACLE-L_at_fatcity.com
> Sent: Tuesday, June 20, 2000 15:57
> To: Multiple recipients of list ORACLE-L
> Subject: Re: I/O activity during HOT backups
>
> The following is a quote from my favorite Oracle reference boot titled
> "Oracle Unleashed, Edition 2" by SAMS Publishing.
>
> When you place a tablespace in backup mode, the Oracle instance notes that
> a backup is being performed and internally compensates for it. As you
> know, it is impossible to make an authentic copy of a database file that
> is being written to. Upon receipt of the command to begin the backup,
> however, Oracle ceases to make direct changes to the database file. It
> uses a complex combination of rollback segments, buffers, redo logs, and
> archive logs to store the data until the end backup command is received
> and the database files are brought back to sync. .... What you should
> understand is that the trade-off for taking a hot backup is increased use
> of rollback segments, redo logs, archive logs, and internal buffers within
> the SGA.
>
> You must be running the database in Archive mode to perform a hot backup.
> Archive logs assure data consistency when backing up only pieces of a
> database at a time.
>
> >>> "Eric Lansu" <eric.lansu_at_quicknet.nl> 06/20/00 09:11AM >>>
> Am I wrong, or am I wrong?
>
> But as far as I know you have to do a log-switch BEFORE starting the
> HOT-backup, and one AFTER the HOT-backup. The tape containing the
> database-files should also contain the archive-log-files created between
> the
> log-switches. This way you can roll-forward (recover) the database to the
> point-in-time after completion of the backup, and thus bringing it all in
> a
> consistent state. ( yes, yes, not from a book.... )
>
> Still the question remains; how can the OS make a copy of a datafile if
> Oracle is still writing in it? Better, how does Oracle know what to update
> after recovery?
>
> Eric Lansu
>
> ----- Original Message -----
> To: "Multiple recipients of list ORACLE-L" <ORACLE-L_at_fatcity.com>
> Sent: Tuesday, 20 June 2000 11:03
>
>
> > Wow! your explanation is wonderful.
> >
> > Thanks Rachel.
> >
> >
> > Bhat
> >
> > -----Original Message-----
> > Sent: Tuesday, June 20, 2000 4:19 AM
> > To: Multiple recipients of list ORACLE-L
> >
> >
> > Lisa,
> >
> > I'm sorry but you are wrong. Writes to the datafiles continue during hot
> > backup.....
> >
> > take a hypothetical situation:
> >
> > you have 3 redo logs
> > you have put every tablespace in your database into hot backup mode
> > you do a LARGE dataload (enough to cycle through all your redo logs
> several
> > times)
> >
> >
> > so... if Oracle does NOT write to the datafiles, then the changes you
> have
> > been making to the blocks get overwritten in the redo logs after the
> logs
> > are archived. Once you take the tablespaces out of backup mode, given
> your
> > thinking, Oracle would have to then write all the blocks to the database
> > files at once. But where would it get them from? The archived redo logs
> are
> > NOT re-read, nor are the redo logs.
> >
> > So...... writing continues to the database files.
> >
> > Rachel
> >
> > >From: Lisa_Koivu_at_gelco.com
> > >Reply-To: ORACLE-L_at_fatcity.com
> > >To: Multiple recipients of list ORACLE-L <ORACLE-L_at_fatcity.com>
> > >Subject: Re: I/O activity during HOT backups
> > >Date: Mon, 19 Jun 2000 11:50:37 -0800
> > >
> > >No! The command below stops all writes to the datafiles in the
> tablespace
> > >for
> > >the duration of the backup, to ensure consistency.
> > >
> > >The i/o overload I see during backups is the data being copied out to
> our
> > >backup
> > >server. And it is usually very high: like 80% of all current
> activity.
> > >
> > >Lisa
> > >
> > >
> > >
> > >
> > >
> > >Dan.Hubler_at_midata.com on 06/19/2000 01:18:14 PM
> > >
> > >Please respond to ORACLE-L_at_fatcity.com
> > >
> > >To: Multiple recipients of list ORACLE-L <ORACLE-L_at_fatcity.com>
> > >cc: (bcc: Lisa Koivu/GELCO)
> > >
> > >
> > >
> > >
> > >
> > >Please settle a discussion amongst our DBA team:
> > >
> > >Is there ANY I/O that takes place to the database files (*.DBF)
> > >during a HOT backup? (That is, ALTER TABLESPACE BEGIN BACKUP).
> > >
> > >If not, how does the process work?
> > >
> > >Thanks.
> > >
> > >
> > >
> > >
> > >???
> > >
> > >
> > >--
> > >Author:
> > > INET: Dan.Hubler_at_midata.com
> > >
> > >Fat City Network Services -- (858) 538-5051 FAX: (858) 538-5051
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> > >
> > >
> > >
> > >
> > >
> > >--
> > >Author:
> > > INET: Lisa_Koivu_at_gelco.com
> > >
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> > --
> > Author: Rachel Carmichael
> > INET: carmichr_at_hotmail.com
> >
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> > Author:
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> --
> Author: Eric Lansu
> INET: eric.lansu_at_quicknet.nl
>
> Fat City Network Services -- (858) 538-5051 FAX: (858) 538-5051
> San Diego, California -- Public Internet access / Mailing Lists
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> --
> Author: William Beilstein
> INET: BeilstWH_at_obg.com
>
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Received on Tue Jun 20 2000 - 09:36:51 CDT