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RE: I/O activity during HOT backups

From: Jack Silvey <JSilvey_at_XOL.com>
Date: Mon, 19 Jun 2000 16:30:31 -0500
Message-Id: <10533.109789@fatcity.com>


We must also remember the split block syndrome as well.

When changes are made to blocks during hot backups, the entire block image is written to the redo logs, not just the redo chain.

Why, might you ask?

Oracle writes in blocks, as does the OS, but OS blocks and Oracle blocks are not necessarily the same. For instance, if you have a UNIX block of 4k, and you have an Oracle block of 8k, then one Oracle block equals two UNIX blocks, n'est pas?

Given that the Oracle data block is a multiple of operating system blocks, it
is possible for a hot backup to contain an inconsistent version of a given data block due to the OS method used to perform a file copy. There exists an
identifying mark at both the beginning and end of every Oracle block. Upon recovery, the identifying marks are compared - if they match the block it is

consistant otherwise the redo copy of the block is necessary to reconstruct the block to a consistant state.    

The parameter LOG_BLOCKS_DURING_BACKUP is set to TRUE (default) in order to have redo
available for reconstruction in the event of a split block. If this parameter
is true, then a complete image of every block is copied to the redo log prior
to any change. If Oracle block size equals the operating system block size,

this parameter can be set to FALSE thus reducing the amount of redo generated
during a hot backup.

Jack
ocp x 2
senior @ xol.com  

-----Original Message-----
From: Lisa_Koivu_at_gelco.com [mailto:Lisa_Koivu_at_gelco.com] Sent: Monday, June 19, 2000 4:04 PM
To: Multiple recipients of list ORACLE-L Subject: Re: I/O activity during HOT backups

Yup it's not unusual when I am wrong. I agree I am wrong. Stress level = 130%
today.

"Rachel Carmichael" <carmichr_at_hotmail.com> on 06/19/2000 03:19:05 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)

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
>
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>--
>Author:
> INET: Lisa_Koivu_at_gelco.com
>
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--
Author: Rachel Carmichael
  INET: carmichr_at_hotmail.com

Fat City Network Services    -- (858) 538-5051  FAX: (858) 538-5051
San Diego, California        -- Public Internet access / Mailing Lists
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-- 
Author: 
  INET: Lisa_Koivu_at_gelco.com

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Received on Mon Jun 19 2000 - 16:30:31 CDT

Original text of this message

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