Question

How does disk mirroring help improve reliability? Give a quantitative example.

Short answer

Disk mirroring improves reliability by providing a real-time backup of data. If one disk fails, the system can automatically switch to the mirror disk without any data loss or downtime. For instance, with a disk MTBF of 100,000 hours, disk mirroring would double the MTBF to 200,000 hours.

Step-by-step solution

Defining Disk Mirroring

Disk Mirroring is a part of RAID (Redundant Array of Independent Disks) where data is copied exactly on two or more disks. This redundancy increases the reliability as the data is not lost if one disk fails.

Explaining Reliability

Reliability, in computer science, refers to the ability of a system to run without failure over a specific period. In the context of data storage and disk mirroring, it refers to the robustness of data storage and the ability to avoid data loss.

Quantitative Example

Consider a disk drive which has a Mean Time Between Failures (MTBF) of 100,000 hours. With disk mirroring, even if one disk fails, the other disk can prevent data loss until the faulty disk is replaced. This would effectively double the MTBF to 200,000 hours and halves the Annual Failure Rate (AFR), thus improving the reliability.

Key concepts

RAID

RAID, which stands for Redundant Array of Independent Disks, is a technology used to increase the reliability, performance, and capacity of data storage devices. It works by combining multiple disk drives into a single logical unit, where data is distributed across the drives in a manner that allows for redundancy and quick access.
RAID can be implemented in different levels, each offering a distinct combination of reliability, performance, and storage capacity. Some common RAID levels are:

• RAID 0: Known for boosting performance by striping data across multiple disks, but it does not provide redundancy.
• RAID 1: Focuses on reliability by mirroring data on two or more disks.
• RAID 5: Offers a balance of performance, capacity, and reliability by distributing parity information across all disks.

Each level is suitable for different needs and applications, depending on the priority between performance, capacity, and reliability.

Reliability in Computer Science

Reliability in computer science involves ensuring that systems function correctly under defined conditions for a specified period. It's a key factor, particularly in data storage, where losing information can have severe consequences.
Reliable systems are engineered to handle errors gracefully and continue operating even in the presence of hardware failures. For data storage solutions like RAID systems, reliability means ensuring data can be retrieved accurately even in the event of disk failures, power outages, or other operational disruptions.
By implementing disk mirroring (as in RAID 1), systems can enhance reliability significantly because an exact copy of data is always available. This approach helps to ensure that data remains safe and accessible, regardless of individual disk failures.

Data Redundancy

Data redundancy is a crucial concept in enhancing the reliability of data storage systems. It involves storing additional copies of data, so that if one set is lost or corrupted, the other can be used to restore it.
Redundancy can be achieved in several ways, with disk mirroring being one of the most straightforward approaches. By creating exact copies of data on separate disks, any failure in one disk will not lead to data loss, as it can be retrieved from the mirrored disk.
This principle of making "backup" copies is fundamental in ensuring that data is always protected against unforeseen failures, helping sustain continuous data availability, a vital aspect in critical systems and applications.

Mean Time Between Failures (MTBF)

Mean Time Between Failures (MTBF) is a reliability metric that quantifies the average time a system or component operates before encountering a failure. It's essential for assessing the reliability of disk drives and other hardware components.
A high MTBF value implies that a system is likely to perform longer without failure. In the context of disk mirroring and RAID, the MTBF of a single disk might be doubled or improved significantly by adding redundancy, thereby extending the time before a failure impacts the overall system.
For instance, if a disk's MTBF is 100,000 hours, implementing a mirrored setup may effectively increase this to 200,000 hours because, while one disk is being repaired or replaced, the other continues operating without data loss. Thus, understanding and improving MTBF through techniques like RAID can vastly enhance system reliability.