Question about 'split blocks'

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I couldnt understand this concept in the internal operation of hot backups. In the 'Oracle Backup and Recovery' book by Rama Velpuri, this is the explanation given for Oracle logging entire block images during hot backups...

" If the Oracle block size is a multiple of OS blocks, then depending on how the OS copies blocks, it is possible for a hot backup to contain an inconsistent version of a given data block. For example, if a block is updated on disk between the reads, the copy in the backup file could be useless since the front and back halves of a block may be written at different times."

Question: I can understand that the OS can copy OS blocks at different times and in the meantime, the Oracle block could have been changed. But since this change is in the redo, why should the hot backup ( being an OS copy ) contain an inconsistent version of the data block OR in what way will this cause a problem during recovery ?

Thank you. If possible, please send me a reply to pady_at_imncorp.com

Pady Srinivasan
Integrated Medical Networks
pady_at_imncorp.com

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