If a single drive fails, a PC can go from “fine” to “down” in seconds. RAID (Redundant Array of Independent Disks) reduces that risk, improves speed, or both, by combining multiple drives into one logical storage unit. For CompTIA A+ 220-1201 Objective 3.4, you don’t need to design enterprise storage, but you do need to recognize common RAID levels and predict what happens when a disk dies.
This guide focuses on RAID 0, 1, 5, 6, and 10. You’ll learn what each level does, the minimum number of drives, how usable capacity is calculated, how failure tolerance works, and where each option fits in real support work. The goal is simple: see a RAID label on a ticket and know what it implies.
RAID basics you must know before you compare RAID 0, 1, 5, 6, and 10
RAID turns several physical drives into one array that the system treats like a single disk. Depending on the level, RAID can improve performance (often reads), improve uptime (survive a drive failure), or provide a mix of both. What RAID does not do is protect against accidental deletion, file corruption, malware, theft, or fire. Those are backup problems.
RAID can be implemented in two main ways. Hardware RAID uses a dedicated controller (on a RAID card or built into a motherboard). The controller manages the array and presents it to the system as one device. Software RAID is managed by the operating system. Both can work well, but hardware RAID often has better performance and management features on servers.
In many systems, you’ll first see RAID in firmware. The BIOS or UEFI setup may show the array as a virtual disk (or logical volume) even though it is made of several drives. That matters during troubleshooting, because the OS might only report one disk, while the RAID tool reports each member drive and its health.
A practical rule for the A+ exam is to assume drives in an array should match. Mixed sizes can work, but the array will usually size itself to the smallest drive, wasting extra space on larger disks.
Striping, mirroring, and parity in plain language
Striping splits data across drives in chunks. If two drives share the work, each can read or write part of a file, which can increase speed.
Mirroring writes the same data to two drives. If one fails, the other still has a complete copy.
Parity stores calculated information that can re-create missing data if a drive fails. Parity is not a full copy of files; it’s more like a repair formula stored across the array. Rebuilds can take hours and they stress the remaining drives.
Capacity, fault tolerance, and performance, the three trade-offs
Every RAID level trades three things: usable capacity, fault tolerance, and performance. You can’t maximize all three at once.
These quick capacity rules cover most exam questions (assume equal-sized drives):
- RAID 0 uses all space.
- RAID 1 uses about half the total space (with a two-drive mirror).
- RAID 5 loses the space of one drive.
- RAID 6 loses the space of two drives.
- RAID 10 loses about half the total space.
Performance also has patterns. Reads often improve with striping and sometimes with mirroring. Writes can slow down on parity arrays (RAID 5 and 6) because parity must be calculated and written along with the data.
RAID 0, 1, 5, 6, and 10 compared for the CompTIA A+ 220-1201 exam
The exam favors predictable facts: minimum drives, capacity math, and failure behavior.