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Home -> Community -> Mailing Lists -> Oracle-L -> 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
> 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=20
> >Reply-To: ORACLE-L_at_fatcity.com=20
> >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=20
> >
> >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=20
> >
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> >
> >
> >
> >
> >
> >
> >--
> >Author:
> > INET: Lisa_Koivu_at_gelco.com=20
> >
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> --
> Author: Rachel Carmichael
> INET: carmichr_at_hotmail.com=20
>
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> INET: LBhat_at_LEVI.com=20
>
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--=20
Author: Eric Lansu
INET: eric.lansu_at_quicknet.nl=20
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 Received on Tue Jun 20 2000 - 07:49:08 CDT
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