The Linux I/O law of fatherhood

• Whenever a read bio is submitted to a block device - the active thread is accounted for it - regardless whether this is the originating process or pdflush.
• Whenever (virtual) memory pages are marked dirty - the active thread is accounted for this as a write request.
  • Why not during sending the bio to the block device?
  • Because then we'll always think it was done by pdflush.
• Whenever pages in the page-cache are truncated - the active thread is accounted for "cancelled write bytes" (i.e. "negative" I/O).

Originally written by guy keren