See fractured block
A type of media corruption that can occur when database writer is writing a block at the same time an operating system utility is reading the block for backup. The block that the operating system reads can be split, that is, the top of the block is written at one point in time while the bottom of the block is written at another point in time. If you restore a file containing a fractured block and Oracle reads the block, then the block is considered a corrupt block.
The potential for fractured blocks necessitates putting tablespaces in backup mode before user-managed online backups. A database in backup mode writes whole Oracle data blocks to the redo log, so that if a block is split during the backup, you can repair it by using redo. Recovery Manager does not experience this problem because the server process performing the backup or copy reads each block to determine whether it is split and re-reads the block until it gets a consistent version.
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