Disk Scheduling And Storage Management : MCQs & PYQs with Explanation

Option C

To calculate the maximum disk size, we use the formula:
Disk Size = Cylinders × Heads × Sectors per Track × Bytes per Sector.
Here, the important detail is that each head has two tracks, so the effective number of heads becomes 16*2 = 32. This increases the total storage capacity accordingly.
Substituting the values, we get, 1024*32*63*512 = 1,056,964,608 bytes. This represents the total number of bytes that can be stored on the disk based on the given configuration.
Now converting bytes into megabytes using decimal notation (1 MB = 106 bytes), we get approximately 1056 MB.

Option C

Disk scheduling algorithms manage the order in which disk I/O requests are processed. Common methods are FCFS (FIFO), SSTF, SCAN, LOOK, etc. The SCAN (also called Elevator algorithm) which moves the disk arm across the disk servicing requests in one direction.
In Batch Processing, jobs are processed in the order they arrive or according to some simple sequence. A common scheduling method is FIFO (First In First Out), where the first job to arrive is the first to be processed.
Time-sharing systems use Round Robin Scheduling to give each process a fair share of CPU.
Interrupts are handled in the reverse order of occurrence, often following a LIFO (Last In First Out) scheme where the most recent interrupt is serviced first.

Option B

In disk storage, data is organized into structures like tracks, sectors, and cylinders. The Sector is the smallest unit of data that can be physically read from or written to a disk.

• A block is a logical unit used by the operating system and may consist of one or more sectors. 
• A track is a circular path on the disk surface.
• A cylinder is a collection of tracks aligned vertically across platters.

Option A

Disk scheduling algorithms decide the order in which I/O requests are serviced to minimize disk head movement. Two common algorithms are:
SSTF (Shortest Seek Time First):

• Selects the request closest to the current head position.
• Reduces total head movement but can cause starvation for far-away requests.

SCAN (Elevator Algorithm):

• The disk head moves in one direction, servicing all requests until it reaches the end, then reverses direction.
• Provides more uniform wait times and avoids starvation.

Option D

RAID (Redundant Array of Independent Disks) is a storage technology that combines multiple physical disks into a single logical unit to improve performance and reliability (fault tolerance). Key characteristics of RAID include:

• Data Striping: Distributing data across multiple disks to improve read/write performance.
• Redundancy/Parity: Using extra disk space to store parity or mirrored data for fault tolerance.
• Logical View: The OS treats the RAID array as a single logical drive, even though it consists of multiple physical drives.

Option D

In RAID 5, data and parity information are distributed across all drives. The usable storage capacity is calculated using the formula:
RAID 5 Capacity = (N-1)*Size of Smallest Drive

• N is total number of drives.

In the given question, there are a total of 10 drives (Nine 1 TB drives and one 500 GB drive), and since RAID 5 always considers the smallest disk size, all drives are effectively treated as 500 GB each for capacity calculation.

• (10-1)*500 GB = 9*500 GB = 4500 GB = 4.5 TB

So,

• RAID 5 always considers the smallest disk size.
• One disk space is used for parity.
• Remaining disks store actual data.
   

Option A

Different Operating System techniques are used for different management tasks such as CPU scheduling, disk scheduling, page replacement, and deadlock handling.

Option C

Seek Time is the time required to move the disks read/write head to the correct track where the required data is located.
When a disk operation is requested, the read/write head may initially be positioned on a different track. It must move inward or outward across the disk surface until it reaches the required track. The time taken for this movement is called Seek Time.

Option A

The rotational speed of the hard disk is 6000 RPM, which means the disk completes 6000 rotations in one minute. To find the time for one complete rotation, we divide 60 seconds by 6000, giving 60/6000=0.01 seconds per rotation.
The average rotational latency is the time the system waits for the required sector to come under the read/write head. On average, this waiting time is half of one full rotation.
So, the average latency time is 0.01/2=0.005 seconds, which can also be written as 5*10^-3 seconds.

Option B

In SSTF (Shortest Seek Time First), at every step we choose the request which is closest to the current head position.
Given:

• Requests: 98, 183, 37, 122, 14, 124, 65, 67
• Initial head: 53

At each step, keep track of:

• Current head position.
• Remaining requests.
• Distance to each pending request.
• Choose the one with minimum distance.

Step 1: Head at 53
Pending: {98, 183, 37, 122, 14, 124, 65, 67}
Distances from 53:

• 65 → |65‑53| = 12
• 37 → |37‑53| = 16
• 67 → |67‑53| = 14
• 98 → |98‑53| = 45
• 14 → |14‑53| = 39
• 122 → |122‑53| = 69
• 124 → |124‑53| = 71
• 183 → |183‑53| = 130

Closest: 65 (Distance 12).
Movement: 53 → 65
Remaining: 98, 183, 37, 122, 14, 124, 67
Step 2: Head at 65
Pending: {98, 183, 37, 122, 14, 124, 67}
Distances from 65:

• 67 → 2, 37 → 28, 98 → 33, 122 → 57, 124 → 59, 14 → 51, 183 → 118

Closest: 67 (Distance 2).
Movement: 65 → 67
Remaining: 98, 183, 37, 122, 14, 124
Step 3: Head at 67
Pending: {98, 183, 37, 122, 14, 124}
Distances from 67:

• 37 → 30, 98 → 31, 122 → 55, 124 → 57, 14 → 53, 183 → 116

Closest: 37 (Distance 30).
Movement: 67 → 37
Remaining: 98, 183, 122, 14, 124
Step 4: Head at 37
Pending: {98, 183, 122, 14, 124}
Distances from 37:

• 14 → 23, 98 → 61, 122 → 85, 124 → 87, 183 → 146

Closest: 14 (Distance 23).
Movement: 37 → 14
Remaining: 98, 183, 122, 124
Step 5: Head at 14
Pending: {98, 183, 122, 124}
Distances from 14:

• 98 → 84, 122 → 108, 124 → 110, 183 → 169

Closest: 98 (Distance 84).
Movement: 14 → 98
Remaining: 183, 122, 124
Step 6: Head at 98
Pending: {183, 122, 124}
Distances from 98:

• 122 → 24, 124 → 26, 183 → 85

Closest: 122 (Distance 24).
Movement: 98 → 122
Remaining: 183, 124
Step 7: Head at 122
Pending: {183, 124}
Distances from 122:

• 124 → 2, 183 → 61

Closest: 124 (Distance 2).
Movement: 122 → 124
Remaining: 183
Step 8: Head at 124
Pending: {183}
Distances from 124:

• 183 → 59

Movement: 124 → 183
Total Head Movement: 
12+2+30+23+84+24+2+59 = 236 Cylinders.