Re: Block device throttling [Re: Distributed storage.]

On Mon, Aug 13, 2007 at 06:04:06AM -0700, Daniel Phillips (phillips@phunq.net) wrote:

Perhaps you never worried about the resources that the device mapper 
mapping function allocates to handle each bio and so did not consider 
this hole significant.  These resources can be significant, as is the 
case with ddsnap.  It is essential to close that window through with 
the virtual device's queue limit may be violated.  Not doing so will 
allow deadlock.

This is not a bug, this is special kind of calculation - total limit is
number of physical devices multiplied by theirs limits. It was done 
_on purpose_ to allow different device to have different limits (for 
example in distributed storage project it is possible to have both remote 
and local node in the same device, but local device should not have _any_
limit at all, but network one should).

Virtual device essentially has _no_ limit. And that as done on purpose.

-- 
	Evgeniy Polyakov