Bulk loading improves the performance of a session that inserts a large amount of data to the target database.
When bulk loading, the Integration Service invokes the database bulk utility and bypasses the database log, which speeds performance. Without writing to the database log, however, the target database cannot perform rollback. As a result, you may not be able to perform recovery
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Bulk loading uses oracle SQL Loader utility to load data. Bulk loader bypasses DBMS system( redolog and undolog), it directly writes to data file. Transaction can't be rolled back in the case of bulk loading. However, bulk loading is very fast as compare to normal loading.
the above said is true, also, in taget if you are set as primary key then you cannot use bulk mode.
Bulk mode is trying to update the records in bulk way which should not be any constraints.