Here's a more detailed summary of the Wikipedia article on **RAID (Redundant Array of Independent Disks)**.

### What is RAID?

RAID is a storage virtualization technology that combines multiple physical hard drives or SSDs into one logical storage unit. It was originally designed to improve:

* **Performance** (faster data access)
* **Reliability** (protecting against drive failures)
* **Availability** (keeping systems running even when a drive fails)

Originally, RAID stood for **Redundant Array of Inexpensive Disks**, but it is now commonly referred to as **Redundant Array of Independent Disks**. Data is distributed across drives using different techniques called **RAID levels**, each balancing speed, capacity, and fault tolerance differently. ([Harvey Mudd College][1])

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## History

RAID was introduced in **1987** by researchers at the University of California, Berkeley. At the time, many small, inexpensive disks could be combined to match or outperform a single large, expensive disk (known as a **SLED – Single Large Expensive Disk**). Over time, RAID became a standard feature in servers, enterprise storage systems, and some desktop computers. ([Harvey Mudd College][1])

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## How RAID Works

RAID organizes data using three main techniques:

### 1. Striping

* Data is divided into blocks.
* Blocks are spread across multiple drives.
* Increases read and write speed because several drives work simultaneously.

Example:

| Disk 1  | Disk 2  |
| ------- | ------- |
| Block 1 | Block 2 |
| Block 3 | Block 4 |
| Block 5 | Block 6 |

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### 2. Mirroring

* Every piece of data is copied onto another drive.
* If one drive fails, the copy remains available.

Example:

| Disk 1 | Disk 2 |
| ------ | ------ |
| File A | File A |
| File B | File B |

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### 3. Parity

Parity stores extra information that allows lost data to be reconstructed if a drive fails.

Instead of storing complete copies like mirroring, RAID calculates parity values from the data blocks.

Example:

| Disk 1 | Disk 2 | Disk 3 |
| ------ | ------ | ------ |
| Data A | Data B | Parity |

If Disk 1 fails, Data A can be rebuilt using Data B and the parity information.

Parity is much more storage-efficient than mirroring but requires additional calculations.

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# Standard RAID Levels

## RAID 0 – Striping

**Minimum disks:** 2

Features:

* Fastest RAID level
* No redundancy
* Uses 100% of storage capacity

Advantages:

* Excellent read/write performance
* Full disk capacity available

Disadvantages:

* If one disk fails, all data is lost

Typical use:

* Gaming
* Video editing
* Temporary data

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## RAID 1 – Mirroring

**Minimum disks:** 2

Features:

* Every disk has an identical copy.
* Very reliable.

Advantages:

* Survives one disk failure.
* Easy recovery.

Disadvantages:

* Only 50% of storage is usable.

Example:

Two 2 TB drives become **2 TB usable**, not 4 TB.

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## RAID 2

Uses bit-level striping with error-correcting codes.

It is rarely used today because modern drives already perform internal error correction.

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## RAID 3

Uses:

* Byte-level striping
* One dedicated parity disk

Advantages:

* High sequential throughput

Disadvantages:

* Dedicated parity disk becomes a bottleneck.

Rarely used today.

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## RAID 4

Uses:

* Block-level striping
* One dedicated parity disk

Improves random reads but still suffers from the parity-disk bottleneck.

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## RAID 5

**Minimum disks:** 3

Uses:

* Striping
* Distributed parity

Parity blocks are spread across all drives instead of using one dedicated parity disk.

Advantages:

* Good performance
* Efficient storage
* Can survive one drive failure

Storage formula:

**(Number of drives − 1) × Drive size**

Example:

Four 4 TB drives → **12 TB usable**

Disadvantages:

* Slow rebuild after a failure
* A second drive failure during rebuild causes complete data loss

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## RAID 6

Similar to RAID 5 but stores **two parity blocks**.

Advantages:

* Can survive two simultaneous disk failures.
* Better suited for very large storage arrays.

Disadvantages:

* More storage overhead
* Slightly slower writes due to additional parity calculations

Storage formula:

**(Number of drives − 2) × Drive size**

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## RAID 10 (1+0)

Combines:

* RAID 1 (mirroring)
* RAID 0 (striping)

Requires at least four disks.

Advantages:

* Excellent performance
* High fault tolerance
* Fast rebuilds

Disadvantages:

* Only 50% of storage is usable

Widely used for:

* Databases
* Virtual machines
* Enterprise servers

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# Nested RAID Levels

Some RAID systems combine multiple RAID levels, including:

* RAID 01
* RAID 10
* RAID 50
* RAID 60

These combinations aim to balance speed, storage efficiency, and fault tolerance for specific workloads. ([Harvey Mudd College][1])

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# RAID Implementations

### Hardware RAID

A dedicated RAID controller manages the array.

Advantages:

* Better performance
* Less CPU usage
* Often includes battery-backed cache

Disadvantages:

* More expensive
* Hardware failures may require a compatible controller

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### Software RAID

The operating system manages the RAID.

Advantages:

* No special hardware required
* Lower cost
* Flexible configuration

Disadvantages:

* Uses CPU resources
* Performance depends on the operating system

Examples include Linux `mdadm`, Windows Storage Spaces, and ZFS/Btrfs (which provide RAID-like capabilities).

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### Firmware ("Fake RAID")

Implemented in motherboard firmware with operating system drivers.

Advantages:

* Cheaper than dedicated hardware RAID

Disadvantages:

* Often offers little performance benefit over software RAID
* May have compatibility issues

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# Reliability and Weaknesses

Although RAID improves reliability, it has limitations:

### RAID is **not a backup**

RAID protects against hardware failures but does **not** protect against:

* Accidental deletion
* Malware or ransomware
* File corruption
* Fire, flood, or theft
* Human error

Regular backups are still essential.

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### Rebuild Time

When a failed drive is replaced, RAID rebuilds the lost data.

Modern large-capacity drives (e.g., 20 TB or more) can take many hours or even days to rebuild, during which the array is more vulnerable to additional failures. ([Harvey Mudd College][1])

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### Unrecoverable Read Errors (UREs)

During a rebuild, if another disk has an unreadable sector, the rebuild may fail—especially in RAID 5. This is one reason RAID 6 is often preferred for large arrays. ([Harvey Mudd College][1])

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## Choosing the Right RAID Level

| RAID Level |     Speed | Fault Tolerance | Storage Efficiency | Typical Use                        |
| ---------- | --------: | --------------: | -----------------: | ---------------------------------- |
| RAID 0     | Excellent |            None |               100% | High-performance workloads         |
| RAID 1     |      Good |            High |                50% | Critical personal or business data |
| RAID 5     |      Good |       One drive |               High | General-purpose servers            |
| RAID 6     |      Good |      Two drives |           Moderate | Large storage systems              |
| RAID 10    | Excellent |            High |                50% | Databases and virtualization       |

### Key Takeaways

* RAID combines multiple drives to improve **performance**, **availability**, and/or **fault tolerance**.
* Different RAID levels use **striping**, **mirroring**, and **parity** in different combinations.
* RAID 0 maximizes speed but provides no protection.
* RAID 1 focuses on data redundancy through mirroring.
* RAID 5 and RAID 6 use parity to balance capacity and reliability.
* RAID 10 delivers both high performance and strong redundancy but requires more disks and sacrifices half the raw storage capacity.
* RAID helps protect against disk failures, but it is **not a substitute for regular backups**.
