Posted Aug 16, 2007 12:51 UTC (Thu) by corbet (editor, #1)
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Because there is a gap between when a page is dirtied and when it gets to the I/O scheduler. Pages are not scheduled for writeback immediately upon dirtying - that's the page cache in action. So, by the time the I/O scheduler gets involved, it's too late to keep more pages from being dirtied.