Re: I/O activity during HOT backups

Now we know what is the book that Racheal was discrete not to mention.

May this should not be your favourite book any more.

Djordje

-----Original Message-----
From: William Beilstein <BeilstWH_at_obg.com> To: Multiple recipients of list ORACLE-L <ORACLE-L_at_fatcity.com> Date: Tuesday, June 20, 2000 9:23 AM
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:
>> INET: LBhat_at_LEVI.com
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>
>--
>Author: Eric Lansu
> INET: eric.lansu_at_quicknet.nl
>
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>--
>Author: William Beilstein
> INET: BeilstWH_at_obg.com
>
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Received on Wed Jun 21 2000 - 18:59:34 CDT