[Disksim-users] Synthetic sequential read performance

[Anjo Vahldiek]

Dear DiskSim users

Currently I'm facing an issue for the clean disksim 4.0 distribution 
using the Seagate Cheetah model provided on the CMU website in which 
sequential reads take longer than local reads. I enclosed the used 
parameter file. (changed the controller to type 3, type 1 shows the same 
result)

I tried to locate the problem using a synthetic seq. read workload and 
it seems that the following happens:
1. read access (starts at blkno)
  - seek to cylinder
  - rotate until first read block
  - start reading and transfering
  - finish request
2. read access (starts at blkno + 8)
  - misses the block blkno + 8 (first block)
  - reads the following 7
  - make one full rotation to read blkno + 8
3. read access (starts at blkno + 16)
  - rotate to blkno + 16
  - read 8 blocks and finish request
4. read access (starts at blkno + 24)
  - misses block blkno + 24 (first block)
  - same behaviour as 2. read access

Basically every second access introduces a full rotation, because it 
misses the first block. Unfortunately it does not read during the 
rotation into a disk buffer segment such that subsequent accesses would 
return the result immediately.

The avg. response time per request is about 4ms which results in a 
throughput of about 1MB/s. Usually I would expect for a 15k rpm disk 
70-80MB/s.

Hopefully one of you has seen this before and may be able to tell me how 
I have to configure or change the disk model parameters. Any idea would 
be very helpful.

Thank you very much in advance.

Anjo Vahldiek

-----
Max Planck Institute for Software Systems (MPI-SWS)
PhD-Student
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