Exp导出可以检测physical corruption 和logical corruption
1. Export only reads:
- user data below the high water mark;
- parts of the data dictionary, while looking up information concerning
the objects being exported.
2. Only a Conventional path export is able to detect logical corrupt blocks.
When using a Direct path export , the data is read from disk directly
into the export session's program global area (PGA): the rows are
transferred directly to the Export session's private buffer. This means
that the SQL command-processing layer (evaluation buffer) is bypassed,
because the data is already in the format that Export expects.
Therefore, do not specify the export parameter DIRECT=Y but use DIRECT=N
which is also the default value for the DIRECT parameter.
3. Export does not detect all corruptions:
- does not detect disk corruptions above the high water mark;
- does not detect corruptions in indexes, or in free or temporary extents;
- does not detect all corruptions in the data dictionary (the export
utility does not read all of the data dictionary, so there could still
be undetected corruptions in the SYSTEM tablespace).
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