Messages in this thread | | | Date | Mon, 7 Dec 2009 20:32:54 +0100 | From | Jens Axboe <> | Subject | Re: CCISS performance drop in buffered disk reads in newer kernels |
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On Mon, Dec 07 2009, Miller, Mike (OS Dev) wrote: > > > (max_hw_sectors_kb is 512 on my 2.6.25.20 setup and 1024 on > > 2.6.30.9 > > > but it seems that it's read-only) > > > > The *_hw_* values are the driver exported hardware limits, so > > they are always read-only. > > Ahhh, I didn't know that. There is also an nr_requests attribute which > to me implies limiting requests somewhere. The value of nr_request is > 128 but the max commands to the cciss controllers exceed that value. > What is nr_request supposed to do?
It controls what the block layer queue depth may be. As a rule of thumb, it should be twice the hardware queue depth. A value of 128 means you can have at most 128 reads and 128 writes queued in the IO scheduler. In practice it's a bit more due to request allocation batching.
-- Jens Axboe
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