James's Blog

Oracle: what happened during hot backup

1. Oracle checkpoints the tablespace, flushing all changes from shared memory to disk.
2. The SCN markers for each datafile in that tablespace are "frozen" at their current values. Even though further updates will be sent to the datafiles, the SCN markers will not be updated until the tablespace is taken out of backup mode.
3. Oracle switches to logging full images of changed database blocks to the redologs. Instead of recording how it changed a particular block (the change vector), it will log the entire image of the block after the change. This is why the redologs grow at a much faster rate while hot backups are going on.

posted by James Pei @ 5:48 PM