A resident memory page with modifications that must be preserved in its backing storage before its page frame can be reused. In the simple model here, a write sets its dirty flag, and successful page writeback clears it. A clean memory page with a recoverable backing copy can be discarded without an outgoing data write.[ref]
Beyond Physical Memory: Policies. Section 22.9, p. 13: modified/dirty bit and the outgoing I/O difference between dirty and clean eviction
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