RE: Re[2]: I/O activity during HOT backups

I fully agree with this explanation expect that the redo increases by 100%. Redo increase depend on the changes made to the block.When a block is copied to the backup and somebody else make a change to a part of that block oracle has to write the complete block again because the block is fractured and
oracle cannot construct the entire block in the advent of failure, so whenever a
block gets changed during the hot backup oracle has to write the entire block to
keep the consistency of the block. So the redo can be increased by 100% or can be
by 1000% depending on the type of activity goinig on. That's why it is recommended
to keep the file in the hotbackup mode for the shortest possible time.That is one
tablespace at a time.

Cheers

-----Original Message-----
From: dgoulet_at_vicr.com [mailto:dgoulet_at_vicr.com] Sent: Tuesday, June 20, 2000 11:25 AM
To: Multiple recipients of list ORACLE-L Subject: Re[2]: I/O activity during HOT backups

Well, I will disagree with all of you to a point.

When a tablespace is put into hot backup mode with the "begin backup" command
the file headers of all datafiles comprising the tablespace have their SCN's locked (if you want to confirm that, take a very quite DB, alter one tablespace
and take a look at the datafile date/time stamps. They will be different from
all other datafiles in the database, very current, and equal to that of the control file.). If you remember the SCN is Oracle's internal clock used for insuring a consistent state within the database. This SCN lock gets released &
the SCN updated to the current one by the "end backup" command. Now during the
intervening time period, dbwr continues to write to the file as normal, but the
use of redo does increase by 100% since both before and after images of the data
blocks are being saved. Now, depending on what redo files your saving it may
well be a good idea to switch logfiles before starting a backup. In my case we
save all of them for one week longer than we save the datafile backups as a safety net.

Now, if you need to restore from this hot backup the locked SCN's will guarantee
you an inconsistent database, especially if you follow the Oracle book and backup the control file LAST (it's SCN never gets locked). This is why you always have to roll a hot backup forward, at least until the datafile SCN's match that of the control file.

Does the IO load on the computer increase, you bet. But if you've wisely chosen
the time of your backup or the size of your backplane/io ports it should not be
noticeable. Anyway, what's the bother. IO loading is a SA problem (or so my SA
tells me).

Dick Goulet

____________________Reply Separator____________________
Author: "William Beilstein" <BeilstWH_at_obg.com>
Date:       6/20/00 5:57 AM

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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> >
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> >
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> >Author:
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> --
> Author: Rachel Carmichael
> INET: carmichr_at_hotmail.com
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-- 
Author: Eric Lansu
  INET: eric.lansu_at_quicknet.nl 

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