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SubjectRe: [PATCH 0/3] blk-throttle: async write throttling
On Mon, Feb 28, 2011 at 11:15:02AM +0100, Andrea Righi wrote:

> ~~~~
> - Consider to add the following new files in the blkio controller to allow the
> user to explicitly limit async writes as well as sync writes:
> blkio.throttle.async.write_bps_limit
> blkio.throttle.async.write_iops_limit

I am kind of split on this.

- One way of thinking is that represent
the limits on requeuest queue of the IO which is actually passing
through queue now. So we should not mix the two and keep async limits
separately. This will also tell the customer explicitly that async
throttling does not mean the same thing as throttling happens before
entering the page cache and there can be/will be IO spikes later
at the request queue.

Also creating the separate files leaves the door open for future
extension of implementing async control when async IO is actually
submitted to request queue. (Though I think that will be hard as
making sure all the filesystems, writeback logic, device mapper
drivers are aware of throttling and will take steps to ensure faster
groups are not stuck behind slower groups).

So keeping async accounting separate will help differentiating that
async control is not same as sync control. There are fundamental

- On the other hand, it makes life a bit simple for user as they don't
have to specify the async limits separately and there is one aggregate
limit for sync and async (assuming we fix the throttling state issues
so that throttling logic can handle both bio and task throttling out
of single limit).

Any thoughts?


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