Re: Hot Backups

From: Don Vick <dvick_at_lanier.com>
Date: 1995/04/19
Message-ID: <D7AFun.BvL_at_lanier.com>#1/1

In article <798282774snz_at_jlcomp.demon.co.uk>, Jonathan Lewis <Jonathan_at_jlcomp.demon.co.uk> wrote:
>In article <D79F3K.887_at_eskimo.com> lparsons_at_eskimo.com "Lee Parsons" writes:
>
>: In article <D78GMr.1yL_at_lanier.com>, Don Vick <dvick_at_lanier.com> wrote:
>: >When a tablespace is in backup status, Oracle saves all
>: >update activity in the redo logs and catches up the data files after the
>: >backup is done. Redo logs cannot be archived if they contain active data,
>: >so you run the risk of running out of online redo logs and stalling your
>: >system (i.e., blocking udpates). (I may have some details wrong here, but
>: >that is the general idea. :-)
 

>: Not true. ...
>
>Lee is correct: there is no problem with redo logs being overwritten
>or archived if they contain 'active data' even when in hot backup.
>A simple test of this is to start two sessions (as sys or system),
>do an update in the first, switch the relevant tablespace to BEGIN BACKUP
>then 'alter system switch logfile;' N+1 times in the other (where N is
>the number of redo log groups) in the other. The system does not lock up.
>(remember to END BACKUP after the test).
>
>
>This is the second time in a few weeks that this suggestion has come
>up: is it possible that it is a mis-interpration of what happens if
>you re-use redo logs so fast that a database checkpoint has not completed
>by the time you cycle back to the redo log that kicked it off ? In this
>case the database _does_ stop all update activity until the checkpoint
>is complete... This event is then recorded in the alert.log with text
>like: 'unable to advance to next log file. checkpoint not complete.'
>

Very astute, Jonathan. I heard in DBA class that when a tablespace is put in backup mode, Oracle "stops taking checkpoints on that tablespace". I took this to mean that no changed data blocks would be written to the file. Lee's explanation, confirmed in the manual, makes a lot more sense: the checkpoint information in the file's header is not updated during backup mode, but data blocks are written to the file.

But this leaves me wondering why Oracle would recommend against putting multiple tablespaces in backup mode at once. The increased data volume on the redo's doesn't seem to be much of a problem. Anyone know why they say this?

BTW, thanks to Lee and Jonathan for correcting my ignorance :-)

Don

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Donald E. Vick (dvick_at_lanier.com, dvick_at_crl.com) Voice: (404) 493-2194 Fax: (404) 493-2399 Received on Wed Apr 19 1995 - 00:00:00 CEST