Database Reference
In-Depth Information
To understand why Oracle logs the entire block to the redo stream, consider what happens during a restore and
recovery. First, the backup files from the hot backup are restored. As explained earlier, these backup files contain
corrupt blocks, owing to the split-block issue. But, it doesn't matter, because once Oracle recovers the data files, for
any block that was modified during the hot backup, Oracle has an image copy of the block as it was before it was
modified. Oracle uses the copy of the block it has in the redo stream as a starting point for the recovery (of that block).
This process is illustrated in Figure
3-4
.
Figure 3-4.
Restore and recovery of a split block
In this way, it doesn't matter if there are corrupt blocks in the hot backup files. Oracle always starts the recovery
process for a block from a copy of the block (as it was before it was modified) in the redo stream.
Understanding the Need for Redo Generated During Backup
What happens if you experience a failure soon after you make a hot backup? Oracle knows when a tablespace was put
in backup mode (begin backup system SCN written to the redo stream), and Oracle knows when the tablespace was
taken out of backup mode (end-of-backup marker written to the redo stream). Oracle requires every archive redo log
generated during that time frame to successfully recover the data files.
