it goes like this; there is a key function that is used to submit IO
and DM and co need to change the IO a bit and then cause it to be submitted again
XFS -> function -> dm layer -> function -> dm layer -> function -> raid layer -> function -> scsi layer -> driver
(well theoretical example but you get the idea I hope)
in the new situation, instead of calling the function again, you return "do again" so it goes like
XFS -> function -> dm layer -|
function -> dm layer -|
function -> raid layer -|
function -> scsi layer -> driver
so the max depth is 4 (well real world is slightly more complex) independent on how often you layer this. In the old situation you could layer a lot and go really big (11 in the example, but just layer more and it goes higher)
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