CompTIA Exam Objective Review: The RAID Solutions
By Aaron Axline
“It’s time to implement a RAID solution on our key servers!” the boss says during your weekly status meeting. You are a little surprised by this, as it means he actually paid attention to the recommendation document you placed on his desk last week. It is a day for miracles, it seems.
“An excellent idea,” you say diplomatically. “I’m glad you thought of it.”
“I want you to implement RAID on our most mission critical servers,” the boss tells you. “It should be fault tolerant, so if a single disk fails we can still recover the data. But, we can only afford to use two hard drives in each server. Make sure you choose the correct option.”
“Ah yes,” you say uncertainly. “That option.”
The answer to this question and a couple more are at the very end of this article but first let us look at what exams this affects as well as more detailed information regarding the characteristics of the different RAID solutions available.
RAID questions can be found in among others the following CompTIA exams: A+ Exam 220-802; Security+ Exam SY0-401 and Server+ Exam SK0-004 (detailed objective descriptions can be found at the very end of this article).
All About RAID
RAID is short for Redundant Array of Independent Disks. It is a data storage solution that uses multiple physical hard drives to create a single data volume. The key benefits associated with using RAID are:
- Performance – faster disk read/write operations
- Capacity – enables large single-volume drive storage
- Fault Tolerance – data can be recovered if a disk fails
RAID solutions are defined by different version numbers. Some older versions of RAID (like RAID 2) are no longer used in the industry. Each version of RAID has its advantages and disadvantages, which we will look at shortly.
When discussing RAID in the context of CompTIA’s A+ 220-801 exam, Security+ SY0-401 exam, or the Server+ SK0-003 exam, candidates need to know about these versions of RAID:
- RAID 0 (zero)
- RAID 1
- RAID 5
- RAID 6
- RAID 10
RAID 0 is known as disk striping. With RAID 0, data is written in segments across all of the disks in the array (a minimum of two disks are required).
This creates a large read/write performance advantage, as data can be read and written from multiple disks simultaneously. The capacity of a RAID 0 array is the sum of all of the disks in the set; for example, three 1TB disks will create a 3TB RAID 0 array.
RAID 0 has one huge drawback: it does not offer any fault tolerance. If one disk in the array fails, the entire RAID volume is compromised. For this reason, RAID 0 should never be used on any system storing critical data.
RAID 0 is primarily known for:
- Excellent read/write performance
- Minimum of two disks required
- No fault tolerance
RAID 1 is known as disk mirroring. In its most common implementation, two identical hard drives are used to create a mirrored set. Data is written identically to both disks, so if one disk fails, the other disk can be used to access and retrieve the data.
RAID 1 offers a modest increase in read performance, as either disk can respond to a read request. RAID 1 is generally considered to be the best compromise between cost and performance.
RAID 1 is primarily known for:
- Good read performance
- Two disks; limited total capacity
- Fault tolerance for a single disk failure
- Relatively inexpensive
RAID 5 is disk striping with parity. Data is striped across all of the disks in the array, along with parity information that can be used to recover the data volume if a disk fails.
RAID 5 requires a minimum of three disks, and the parity information uses the capacity of one of the disks (given that all of the disks are of equal capacity). So, a RAID 5 set made up of five 1TB drives will have a data capacity of 4TB, with 1 TB being used to store the parity data.
RAID 5 offers fault tolerance if one disk in the array fails. However, if a second disk fails before the first failed disk is replaced and the RAID volume is rebuilt, the RAID volume is compromised. This makes RAID 5 a poor choice for very large storage capacities, as the bigger the disk that fails, the longer it takes to rebuild the volume.
RAID 5 is primarily known for:
- Excellent read performance; slower write performance
- Fault tolerance for a single disk failure
- Minimum of three disks
RAID 6 is very much like RAID 5, except it stores dual parity information. This means that two disks can fail in a RAID 6 set, and the data volume can still be restored. This is a preferred solution for very large storage arrays, where it takes a long time to rebuild the RAID volume in the event of a disk failure. With RAID 6, the data is safe if a second disk fails before the RAID set has been fully restored.
RAID 6 requires a minimum of four disks, and uses twice the capacity for parity information as RAID 5. So, a RAID 6 array made up of five 1TB disks would have a data capacity of 3TB; the parity information would use the other 2TB.
RAID 6 is primarily known for:
- Excellent read performance
- Fault tolerance for two disk failures
- Minimum of four disks
Finally, there is RAID 10—also sometimes referred to as RAID 1+0 because it combines the characteristics of RAID 0 and RAID 1. Essentially, RAID 10 is a striped set of mirrored volumes. Data is striped across a disk, which has been mirrored to another disk. This offers excellent read/write speed, with the same fault tolerance as RAID 1 (supports a single disk failure).
RAID 10 requires a minimum of four disks, with each data disk requiring an identical disk for mirroring. For example, the data capacity of a RAID 10 array made up of four 2TB disks would be 4TB. RAID 10 is generally considered to be the best (and most expensive) version of RAID.
RAID 10 is primarily known for:
- Excellent read/write performance
- Same fault tolerance as RAID 1 (single disk failure)
- Most expensive RAID option
RAID as an exam topic
CompTIA exams typically test candidates on RAID knowledge based on these key properties:
- Fault tolerance
The opening of this article offered a version of a typical exam question. Which RAID level offers fault tolerance protection in the event of a single disk failure, and only requires two hard drives to implement? (Answer: RAID 1, disk mirroring.)
Here’s another example: your company has a mission critical server that data can be recovered from in the event of two hard disk failures. What RAID level should you implement on this server? (Answer: RAID 6.)
Here are some quick exam study summaries for the different levels of RAID:
RAID 0: Striped with no parity. Two or more disks. No fault tolerance. Used when fast performance is more important than fault tolerance. Capacity is the sum of all disks used.
RAID 1: Disk mirroring. Fault tolerance. Two hard drives. Capacity is size of one disk (the smaller of the two disks if they are different sizes).
RAID 5: Striping with parity. Fault tolerance. Minimum of three hard drives. Fast disk reads. Ideal for any system that does more reads than writes (example: database server). Capacity is the sum of the disks minus one.
RAID 6: Striped with dual parity. Fault tolerance. Minimum of four hard drives. Can survive two hard disk failures. Same performance as RAID 5. Capacity is the sum of all disks minus two.
RAID 10: Striped set of mirror volumes. Fault tolerance. Excellent performance. Typically the most expensive implementation of RAID.
The more significant CompTIA exam objectives addressing RAID are:
A+ Exam 220-901 Main Domain 1.0 Hardware – Sub-objective 1.5 Install and configure storage devices and use appropriate media.
A+ Exam 220-901 Main Domain 1.0 Hardware – Sub-objective 4.2 Given a scenario, troubleshoot hard drives and RAID arrays with appropriate tools.
Security+ Exam SY0-401 Main Domain 2.0 Compliance and Operational Security – Sub-objective 2.8 Summarize risk management best practices.
Server+ Exam SK0-004 Main Domain 2.0 Server Administration – Sub-objective 2.1 Install and configure server operating systems.
Aaron Axline is a technology writer and knowledge management specialist based in Edmonton, Canada. His work has appeared in titles from Que Publishing and on popular tech blogs and sites.
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