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Subject - Operating Systems CS 4001(Same as CB/IT 4001)
Branch - Computer Science Engineering
Semester - 4th Semester
UNIT 4: Input/Output (I/O) Systems and Storage Management
This unit covers the basic concepts of the I/O system, mass storage structure, disk scheduling algorithms, swap space management, and RAID (Redundant Array of Independent Disks). Understanding these concepts is important for managing how data is transferred to and from storage devices, optimizing disk usage, and ensuring data integrity.
4.1 I/O System 🖥️
The I/O System is responsible for managing input and output operations in a computer system. It involves the interaction between the computer’s CPU and the external devices (e.g., keyboards, mouse, printers, storage devices). The I/O system handles data transfer, device management, and ensures that input/output operations are performed efficiently.
Key components of I/O systems:
- I/O Devices: These are hardware components like disk drives, keyboards, displays, and printers.
- Device Controllers: These are specialized hardware that manage the communication between the CPU and I/O devices.
- Device Drivers: Software that acts as an interface between the device controller and the operating system.
- Buffering: The temporary storage of data while it is being transferred between devices and the CPU.
4.2 Mass Storage Structure 💾
Mass storage refers to devices used for storing large amounts of data for long periods, such as hard drives, SSDs, and tape drives. The mass storage system is crucial for holding system files, application data, and user data.
4.2.1 Overview
Mass storage systems are designed to handle high capacity, high-speed data storage. These systems are responsible for long-term data retention and easy retrieval of data, even when the system is powered off.
Key features of mass storage:
- Capacity: Mass storage devices can hold terabytes to petabytes of data.
- Reliability: They are built to ensure long-term data preservation.
- Performance: Data can be retrieved quickly, though speed varies based on the type of device (HDD, SSD, tape drive).
4.2.2 Disk Structure 🖴
The disk structure refers to the physical layout of data on a disk. It is important for the efficient management of data on a disk. The key components include:
- Tracks: Circular paths on the disk where data is stored.
- Sectors: Small segments of a track where data is stored. Each sector typically holds 512 bytes of data.
- Cylinders: The set of tracks located at the same position on multiple platters (disk surfaces).
The disk is organized into a series of tracks and sectors, allowing data to be written and read efficiently.
4.2.3 Disk Attachment 💻
Disk attachment refers to how the disk is connected to the computer system. There are several methods of attaching a disk to a system:
- Direct Attached Storage (DAS): The disk is directly connected to a computer, usually via a cable or interface like SATA, SCSI, or IDE.
- Network Attached Storage (NAS): A storage device connected to a network and accessible by multiple systems over the network.
- Storage Area Network (SAN): A specialized, high-performance network designed to provide access to large-scale storage devices.
4.3 Disk Scheduling Algorithms 📅
Disk scheduling algorithms are used to manage the order in which disk requests are serviced. The goal is to minimize the seek time (the time it takes for the disk head to move to the correct position) and maximize efficiency.
4.3.1 FCFS (First Come, First Served) ⏳
In the FCFS algorithm, disk requests are processed in the order they arrive. While simple, this approach can be inefficient as it may lead to long seek times if the requests are spread out across the disk.
Advantages:
- Simple and easy to implement.
Disadvantages:
- May result in high waiting times if requests are not in a sequence that minimizes head movement.
4.3.2 SSTF (Shortest Seek Time First) ⏱️
The SSTF algorithm selects the disk request that is closest to the current position of the disk head. This minimizes the seek time by always choosing the shortest distance to the next request.
Advantages:
- Reduces average seek time.
- More efficient than FCFS.
Disadvantages:
- Can lead to starvation (some requests may not be serviced if closer requests keep arriving).
4.3.3 SCAN 🔄
The SCAN algorithm moves the disk arm in one direction (either inward or outward) and serves all the requests along the way until the end of the disk. After reaching the end, it reverses direction and services the requests in the opposite direction.
Advantages:
- Reduces starvation.
- Provides more uniform wait times than FCFS and SSTF.
Disadvantages:
- May have long wait times for requests at the opposite end of the direction the head is moving.
4.3.4 LOOK 🔍
The LOOK algorithm is similar to SCAN, but the disk arm stops once there are no further requests in the current direction, instead of going all the way to the end of the disk. This makes it more efficient than SCAN, especially when requests are clustered.
Advantages:
- More efficient than SCAN.
- Reduces unnecessary movement of the disk arm.
Disadvantages:
- Similar to SCAN, it can still have long wait times for requests at the opposite end.
4.4 Swap Space Management 🔄
Swap space is a portion of the disk used to simulate additional RAM when the physical memory (RAM) is full. The operating system swaps out data from RAM to the swap space, making room for more data in RAM.
Key features of swap space management:
- Virtual Memory: Swap space is a key component of virtual memory, allowing systems to handle more data than the available physical memory.
- Efficiency: Efficient swap space management helps prevent performance degradation when the system is low on RAM.
- Page Replacement Algorithms: These algorithms determine which data should be swapped in and out of the swap space.
4.5 RAID (Redundant Array of Independent Disks) 💾🔐
RAID is a data storage technology that combines multiple physical disk drives into one or more logical units to improve performance, reliability, and data redundancy.
RAID Levels:
- RAID 0: Striping – Data is split across multiple disks for performance improvement. No redundancy, so data loss can occur if one disk fails.
- RAID 1: Mirroring – Data is duplicated on two or more disks, providing redundancy and reliability. However, storage capacity is halved.
- RAID 5: Striping with Parity – Data and parity (error-checking) information are distributed across multiple disks. Offers both performance and redundancy. At least three disks are required.
- RAID 10: Combination of RAID 1 and RAID 0, offering both mirroring and striping, providing both high performance and data redundancy.
Advantages of RAID:
- Improved Performance: Especially for read/write operations, as data is distributed across multiple disks.
- Data Redundancy: Offers protection against data loss by mirroring or using parity.
Disadvantages of RAID:
- Cost: Additional disks are needed to implement RAID, leading to higher costs.
- Complexity: Some RAID configurations can be complex to set up and maintain.
Conclusion 🎯
Understanding I/O systems and mass storage management is essential for optimizing computer systems' performance, reliability, and data integrity. Disk scheduling algorithms improve efficiency in handling disk requests, while RAID configurations ensure data redundancy and fault tolerance. Efficient management of swap space allows systems to handle more data, and knowing disk structures aids in better data organization.
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