> +               Block device I/O bandwidth controller

How can this work?  You will limit the number of available buffer heads 
per second?

Unfortunaely, the problem is the fs above the block device.  If the
block device is artificially slowed then the fs will still happily allow
a process to fill buffers forever until memory is full, while the block
device continues to trickle the buffers away.

What one wants is for the fs buffering to be linked to the underlying
block device i/o speed. One wants the rate at which fs buffers are
filled to be no more than (modulu brief spurts) the rate  at which the
device operates.

That way networked block devices have a chance of having some memory
left to send the dirty buffers out to the net with. B/w limiting the
device itself doesn't seem to me to do any good.

Peter
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Peter,

I see your message only now, it seems you didn't add me in to or cc.

Anyway, I totally agree with you, but it seems there's a
misunderstanding here. The block device i/o bw controller *does*
throttling slowing down applications' requests and not the dispatching
of the already submitted i/o requests.

IMHO, for the same reason you pointed, delaying the dispatching of i/o
requests simply leads to an excessive page cache and buffers
consumption, because userspace apps dirty ratio is actually never
limited.

As reported in the io-throttle documentation:

"This controller allows to limit the I/O bandwidth of specific block
devices for specific process containers (cgroups) imposing additional
delays on I/O requests for those processes that exceed the limits
defined in the control group filesystem."

Do you think we can use a better wording to describe this concept?

-Andrea
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