8/6/2019 Oracle - hot bkp archives

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Hot Backups and Archivelog Mode Backup and Recovery Tips

Copyright Howard Rogers 2001 19/10/2001 Page 1 of 2

Must I take Hot Backups in archivelog mode?

What does archivelog mode mean?

Merely that an Online Redo Log which has a status of ACTIVE may not be overwritten, andthat an Online Redo Log will remain in an ACTIVE state unless and until it has been copied.

Thats all it means. It doesnt even arrange for the copy to be taken (thats the job of

ARCH, and is provided for only when the init.ora is adjusted to include the line

LOG_ARCHIVE_START=TRUE) which is why your database will eventually hang ifall you

do is put it into archivelog mode (eventually, all your Online Redo Logs will fill, loop back

on top of themselves, and the first one will still be sitting there with an ACTIVE status,

because ARCH has not been switched on and taken its copy).

So whether you are in archivelog mode or not makes precisely zilch difference to whether

or not you have to take hot or cold backups. The choice is still yours, and either would be

acceptable.

The real point, of course, is that hot backups are simply not possible unless the database is

in archivelog mode and ARCH has been switched on. Hot copies of datafiles will be

internally inconsistent, and unusable but the application of redo will make them

consistent and useable. So hot backups depend upon the supply of all redo since the time

the backup started, if they are to be rendered usable and that rather suggests that

archives (which represent an uninterrupted redo stream reaching back in time much

further than the online logs alone could manage) need to be available to make hot backups

useful in the event of disaster.

So only in archivelog mode can you perform hot backups. But the syllogism is not

commutative in plain English, the fact that hot backups require you to be in archivelog

mode does not imply that archivelog mode requires you to take hot backups.

Put yet another way: archivelog mode guarantees that youll be able to perform complete

database recovery, with zero data loss (because youll be able to supply all required redo).

But that can be achieved just as well by restoring from a cold backup as it can from a hot

one: its the application of redo that recovers data, not the type of data file backup that

its being applied to.

Id go so far as to suggest that the best possible backup strategy, combining ease of backup

with total data recoverability, would be a closed (cold) database backup taken whilst in

archivelog mode. Cold backups are dead easy to perform (COPY *.* springs to mind as the

script required!), and the presence of archives merely means that a restore from that

backup can be rolled forward right to the time of failure hence no data loss.

8/6/2019 Oracle - hot bkp archives

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Hot Backups and Archivelog Mode Backup and Recovery Tips

Copyright Howard Rogers 2001 19/10/2001 Page 2 of 2