Aug 22, 2022
Archived Redo Log- Files
The Oracle database can run in one of two modes: ARCHIVELOG mode and NOARCHIVELOG mode. The difference between these two modes is simply what happens to a redo log file when Oracle goes to reuse it. “Will we keep a copy of that redo or should Oracle just overwrite it, losing it forever?” is an important question to answer. Unless you keep this file, you can’t recover data from a backup to that point in time.
Say you take a backup once a week on Saturday. Now, on Friday afternoon, after you have generated hundreds of redo logs over the week, your hard disk fails. If you have not been running in ARCHIVELOG mode, the only choices you have right now are as follows:
•\ Drop the tablespace(s) associated with the failed disk. Any tablespace that had a file on that disk must be dropped, including the contents of that tablespace. If the SYSTEM tablespace (Oracle’s data dictionary) or some other important system-related tablespace like your UNDO tablespace is affected, you can’t do this. You will have to use the next option instead.
•\ Restore last Saturday’s data and lose all of the work you did that week.
Neither option is very appealing. Both imply that you lose data. If you had been executing in ARCHIVELOG mode, on the other hand, you simply would have found another disk and restored the affected files from Saturday’s backup onto it. Then, you would have applied the archived redo logs and, ultimately, the online redo logs to them (in effect replaying the week’s worth of transactions in fast-forward mode). You lose nothing. The data is restored to the point of the failure.
People frequently tell me they don’t need ARCHIVELOG mode for their production systems. I have yet to meet anyone who was correct in that statement. I believe that a system is not a production system unless it is in ARCHIVELOG mode. A database that is not in ARCHIVELOG mode will, someday, lose data. It is inevitable; you will lose data (not might, but will) if your database is not in ARCHIVELOG mode.
“We are using RAID-5, so we are totally protected” is a common excuse. I’ve seen cases where, due to a manufacturing error, all disks in a RAID set froze, all at about the same time. I’ve seen cases where the hardware controller introduced corruption into the datafiles, so people safely protected corrupt data with their RAID devices. RAID also does not do anything to protect you from operator error, one of the most common causes of data loss. RAID does not mean the data is safe, it might be more available, it might be safer, but data solely on a RAID device will be lost someday; it is a matter of time.
“If we had the backups from before the hardware or operator error and the archives were not affected, we could have recovered.” The bottom line is that there is no excuse for not being in ARCHIVELOG mode on a system where the data is of any value. Performance is no excuse; properly configured archiving adds little to no overhead. This, and the fact that a fast system that loses data is useless, means that even if archiving added 100 percent overhead, you still need to do it. A feature is overhead if you can remove it and lose nothing important; overhead is like icing on the cake. Preserving your data and making sure you don’t lose your data isn’t overhead—it’s the DBA’s primary job!
Only a test or maybe a development system should execute in NOARCHIVELOG mode. Most development systems should be run in ARCHIVELOG mode for two reasons:
•\ This is how you will process the data in production; you want development to act and react as your production system would.
•\ In many cases, the developers pull their code out of the data dictionary, modify it, and compile it back into the database. The development database holds the current version of the code. If the development database suffers a disk failure in the afternoon, what happens to all of the code you compiled and recompiled all morning? It’s lost.
Don’t let anyone talk you out of being in ARCHIVELOG mode. You spent a long time developing your application, so you want people to trust it. Losing their data will not instill confidence in your system.
Note There are some cases in which a large DW could justify being in NOARCHIVELOG mode—if it made judicious use of READ ONLY tablespaces and was willing to fully rebuild any READ WRITE tablespace that suffered a failure by reloading the data.
More Details