Re: Online Backup

The party line: "Oracle generates more redo during hot backup mode than during normal operations. That is why Oracle recommends that hot backups be taken during non-peak hours."

Reality: If backups are taken one tablespace at a time, as they are supposed to be, the redo generation rate can be kept to a minimum, because the extra redo is limited to redo generated from changes to blocks in the tablespace being backed up. The reason for the extra redo is "full block image logging," which allows any split blocks in the backups to be resolved by applying full block images from redo during recovery. The full blocks are not generated for every block change, but only for the first block change to a particular block after the tablespace is placed in backup mode. A full block image must also be logged for any block after it is written from cache to disk during backup mode.

The reality of the impact of full block logging is minimal, but it depends on the environment. I can only see it being a serious issue for systems with very slow I/O subsystems during peak hours.

Oracle8 has a new mechanism in addition to hot backup mode, called RMAN. RMAN is able to backup directly to a media management product such as Legato or Veritas without using hot backup mode or full block image logging. It also supports true incremental backup, unlike the old hot backup method. Using RMAN to back up data eliminates the slight performance impact attributed to hot backup mode. RMAN backups have an impact similar to normal filesystem backups on a server.

It should make no difference that the application is SAP, and if SAP's documentation really contraindicates hot backups, then I would love to see the exact wording. One of my hobbies is collecting erroneous Oracle-related books, articles and documentation. No, I don't get out much :-)

--
Jeremiah

Roger Hunwicks wrote:
>
> We are considering using Oracle for a 24*7 OLTP application. The other
> main contender is probably DB2.
>
> One of my colleagues has raised the following concern:
>
> >From the work I have been involved in with SAP, I have major concerns
> >over the practicality of using Oracle for true 24x7 operation. Oracle's
> >On-line backup has a very detrimental impact it has on OLTP performance
> >and has not been proposed by some SAP hardware vendors. Complex
> >alternatives have been proposed based on having triple mirrored copies
> >of the full production database and splitting one of these copies off a
> >to provide a stable copy copied to tape. The time to split the mirror
> >copies and re-sync (resilver) the mirror varies dramatically according
> >to the disc sub system vendor.
>
> Is this a recognised issue? And if it is, is it an issue on all
> databases, or only on particularly large ones? Can anyone give me an
> idea of how much performance degradation we should expect while running
> an on-line backup?
>
> TIA
>
> Roger
Received on Thu Oct 21 1999 - 23:37:53 CDT