Database Reference
In-Depth Information
Figure 3-1. Hot backup split- (or fractured-) block issue
Looking at Figure 3-1 , the block copied to disk at time 3 is corrupt, as far as Oracle is concerned. The first half
of the block is from time 1, and the latter half is copied at time 3. When you make a hot backup, you're guaranteeing
block-level corruption in the backups of the data files.
To understand how Oracle resolves the split-block issue, first consider a database operating in its normal mode
(not in backup mode). The redo information that is written to the online redo logs is only what Oracle needs, to
reapply transactions. The redo stream doesn't contain entire blocks of data. Oracle only records a change vector in
the redo stream that specifies which block changed and how it was changed. Figure 3-2 shows Oracle operating under
normal conditions.
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