Discussion: Week 8 - Disk Systems
Solutions
Disk Transfer Rate
Suppose a hard disk drive has the following characteristics:
- 2 platters (4 heads = 4 tracks/cylinder)
- 16383 cylinders
- 64 sectors/track
- 512 bytes/sector
- Rotational speed = 7200 rpm = rps
- Average seek time: 10ms
- Adjacent cylinder seek time: 1ms
- Head switch time: 0.1ms
How much data can this disk hold?
Calculate the sustained transfer rate (no initial seek, sequential
access across entire disk).
Now suppose that the last sector on each track is left blank, so in
essence we now have 63 sectors per track with a one-sector gap at the
end of the track. How much data can this new disk hold?
Calculate the sustained transfer rate for this 63-sector/track disk.
RAID
Here is a summary of the different RAID levels for your convenience:
- Data striped across n disks, no redundancy
- RAID 0 + mirrors (n extra disks)
- RAID 0 + error-correcting codes (log n extra disks)
- RAID 0 + parity (1 extra disk)
- RAID 3 with large strips (block-level parity, 1 extra disk)
- RAID 4 with parity strips distributed among all disks (1 extra disk)
- RAID 5 with dual parity (2 extra disks)
What RAID level would you use for the following applications and why?
- Streaming media internet server
- Bank record database server
- Small business video editing box
Suppose disk 2 goes down in a 5-disk RAID 3 array. The contents of the
remaining disks are:
X0(i) = 11001001
X1(i) = 10101010
X3(i) = 11111111
X4(i) = 00000000
Reconstruct X2(i).
Disk Scheduling
Schedule the following sequence of requests using FCFS, SSTF and SCAN:
(Assume the starting position is 100 in the "up" direction)
|
| FCFS
| SSTF
| SCAN
|
|---|
| 110 | | |
| | 5 | | |
| | 32 | | |
| | 205 | | |
| | 176 | | |
| | 3 | | |
| | 1 | | |
| | 57 | | |
| | 23 | | |
| | 48 | | |
|
|
CIS 415 Operating Systems - Spring 2003