[Disksim-users] Problems with rotational latenxy

[Sivasubramanian R]

Hi,
I'm trying to simulate a single disk (HDD) using disksim 4.0.
* The I/O subsystem architecture is very simple : 
driver->bus->controller->bus->disk.
* I just need to simulate 1ms latency overall from the time the request 
came (read from trace) to when it reaches the disk.
* The input that I'm trying to simulate is synchronous writes (for which 
I have an idea of how the output will look like). I need to verify that 
the disksim is
    configured so testing it with this input. I have also disabled disk 
block cache for this reason.
* Average Rotational latency of the disk is 11ms.
* I'm measuring access time for each request and plotting it against 
sector number. The output looks very similar to what I'm expecting 
except for one thing :
   -> the access times for initial few writes is low. Normally, this is 
fine as the disk starts at sector 0 and input requests are also in 
increasing order of sector.
   -> however, due to latency I have introduced in the controller (bulk 
sector transfer time : 1.0), it should be something like this :
        at time 0ms, request for sector 0 comes, it writes and now head 
points to sector 1 (I write only one sector per request)
        after 1ms, request for sector 1 comes. Now by this time, the 
disk head should have moved past sector 1 and hence should complete a 
full rotation
        before it comes to sector 1.
     So the first few sector requests should incur high latency. yes? 
However I do not find this happening. Does the disk start rotating at 
time 0 (when request
     is at the highest level i.e read from file) or does it start when 
the request actually reaches the disk? If its the latter, is there 
anyway to move the head
     past a few sector based on the rotational latency?

I'm attaching both the expected and actual latency graphs.
Thanks a lot!

Best
Siva


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