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Rollbackshrinks

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oracle foreign key primary key constraints performance tuning MTS IOT 9i block size backup rman corrupted column drop rename recovery controlfile backup clone architecture database archives export …

oracle foreign key primary key constraints performance tuning MTS IOT 9i block size backup rman corrupted column drop rename recovery controlfile backup clone architecture database archives export dump dmp duplicate rows extents segments fragmentation hot cold blobs migration tablespace locally managed redo undo new features rollback ora-1555 shrink free space user password link TNS tnsnames.ora listener java shutdown sequence

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  • 1. When do Rollback Segments Shrink? Administration TipsWhen do Rollback Segments shrink?Rollback segments shrink whenever a transaction crosses an extent boundary, providedthat OPTIMAL has been set. Crossing the extent boundary is known as a wrap (erroneously,many think a wrap only occurs when the segment starts re-using its first extent. It doesnt.Any time a new extent starts to be used, thats a wrap).If OPTIMAL has not been set, shrinks will not occur, however many wraps you have.If you think about it, shrinking as a transaction seeks to cross the extent boundary is aludicrous time to do the deed. Theres your transaction, wanting to get on and write itsrollback, and suddenly, it has to take time out waiting for the rollback segment to decidewhich extents are to be de-allocated. Shrinkage also forces DBWR to flush rollback blocksin the Buffer Cache back down to disk -and any time anything induces additional I/O,performance suffers.Therefore, my strong advice has always been: First, size your segments properly in the firstplace, so that they neither grow nor shrink. Second, if they do inadvertently balloon insize (perhaps because of a blocking transaction, which theres very little you can do toprevent happening), perform a shrink manually, at a time and place of your choosing,instead of relying on the automatic OPTIMAL method firing off in the middle oftransactions.To induce a manual shrink, issue the command:ALTER ROLLBACK SEGMENT BLAH SHRINK TO 50M;(...or whatever size is appropriate for you). The segment might not get all the way downto the specified size (it will never drop extents that contain active rollback -i.e., livetransactions- for example), but it will get as close as it can. If that command fires at nightwhen few people are using the system, nobody will notice the performance hit it induces.Copyright © Howard Rogers 2001 10/18/2001 Page 1 of 1