Re: redo logs in noarchivelog mode
- From: Hans Forbrich <fuzzy.graybeard@xxxxxxxxx>
- To: oracle-l@xxxxxxxxxxxxx
- Date: Wed, 28 Mar 2012 09:31:20 -0600
Redo logs are mainly used for 'online recovery'. (Ignoring Log Miner
and other uses.)
Common misconception is that the redo is only written at commit. In
fact, it is written pretty much when it is generated (at worst within 3
seconds), so a lot of state information is continuously available. (Not
counting exceptions.) Often forgotten is that it includes the changes
to the undo/rollback information as well.
If the database crashes for any reason, on restart the system will look
for the last known stable state of each file and apply any redo since
that time, if it s available, and then rolling back any uncommitted
transactions at the time of failure. Thus it brings the database back
to a stable state.
The redo logs are used as a kind of circular 'buffer' - assign several
[relatively small] files to be used as log files, write to one log file,
close it off and switch to the 'next' one, rinse and repeat until all of
the preassigned files are used and then go back to the first one.
Multiplex these files for safety (multiplex = software controlled
mirroring, slightly safer than hardware mirroring)
Archiving adds the ability to take that redo from 'online' to 'nearline'
(on a separate disk or machine) or 'offline' (tape, offsite, etc.) but
the recovery process stays pretty much the same - go back to a stable
image of the database from before the crash, apply all the redo - even
if that means getting near/off-line logs) and then rollback uncommitted
transactions.
Note that if you have a small database (small is a relative term -
non-SAN disk is cheap, so a couple 10's of GB) and if you have
sufficient redo groups to keep all the online redo logs with overwriting
since the last complete backup, you can simulate complete point in time
recovery without needing archive logs. Shutdown the database, make a
complete copy of all database and a *separate* redo log files. Just
make sure you understand the context of the redo log files - they can
ONLY be restored long with the *previous* day's data files, or you will
clobber the recovery.
HTH
/Hans
On 28/03/2012 8:37 AM, Richard, Carmelinda wrote:
> This may be a stupid question, but I was installing a new database for the
> first time, and I started wondering, what is the useful purpose of redo logs
> if a database is in noarchivelog mode? Only if the database crashes and
> transactions were written to the redo logs before they make it into the
> database files?
>
> Thanks,
> Carmelinda
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