Re: Are logs any use in a no logging database? (fwd)

Cheryl Kendricks wrote: > Cheryl Kendricks > Internet:cherylk@prod1.jcdc.doleta.gov OR cherylk@gwysmtp.jcdc.doleta.gov > DTSI, Inc. Voice: 1-800-598-5008 Fax: 512-393-7296 > Database Administrator - DOL Job Corps San Marcos, Texas > ------------------------------------------------------------------------> > On Wed, 22 May 1996 Spetc@aol.com wrote: > > > We do not use logging on our database. > > > > 1. We perform level 0,1 archives, but we don't backup the logs, do we really > > need to? Logical logs are used for two purposes. Fast recovery every time the engine is brought up from Offline mode to Quiescent, and after a restore from an archive. If your database does not use transaction logging, then when restoring from an archive you simply answer "no" to the question "Do you want to restore from logical logs?" (In ontape). The restore will bring your system back to the start time of the archive. Any work performed after that point will be lost. The logical logs are responsible for applying transactions that have happened since the last archive, which will bring your system back to the actual time when the problem happened (ie. a disk crash). The logs are also used when OnLine goes down (power outage) but you don't actually lose a disk. When OnLine starts back up, it will copy any "before images" found in the physical log back to the appropriate table on disk. At a system checkpoint, all modified pages are flushed back out, so if you were making changes to data since the last system checkpoint, your changes would be lost. If you work against a logged database, your transactions would be played back, retrieving most (if not all) of your transaction information, which means data loss would be very minimal. > > > > 2. What are the logs used for in a no logging database? For example, if I > > update every row in a table with 10 million rows with a single update > > statement and I interrupted it half way through would the first 5 million > > updates be rolled back? Would it use the logs for this purpose? > > In an un-logged database, the transaction logs do very little. They record certain SQL DDL commands (such as create table), but that's about it. Any SQL DML (insert, update,delete,etc) statements are not recorded in the transaction logs. If you are updating 10 million rows and it gets halfway through, there is no rollback capability in a non-logged database. Keaton Adams Informix Software