RAID Levels Explained

RAID, now known as the Redundant Array of Independent Disks, was previously known as the Redundant Array of Inexpensive Disks. However, hard disk manufacturers probably thought of getting the conception of inexpensive out of the mind of consumers, hence thought to change the full form of RAID to Redundant Array of Independent Disks. Now, let us try to reason as to why was RAID implemented and hence, try to understand the importance of getting RAID levels explained. The term RAID was first coined and defined by David A. Patterson, Garth A. Gibson and Randy Katz at the University of California in 1987. The reason was simple. To combine multiple hard drives to provide high performance as well as high availability.

However, the implementation of RAID was not limited to just that. Different RAID levels were used to deal with different situations. First of all, when hard drives are connected in parallel, then data can be accessed at a faster rate. Other situations that were dealt with by the implementation of RAID were like hard drive data recovery and taking into notice that, if one of the hard disks failed, still the remaining should work. Hence, the combination of hard drives into a single logical unit in a computer server, increases the network availability considerably, hence, increasing the overall system's performance. Now, without wasting much time, let us try to get the RAID levels defined. The explanation of RAID levels given in this article, will not only introduce you to the different RAID levels, but also try to help you in the RAID levels comparison. The RAID levels performance is another aspect, which will be dealt with in this article.

Different RAID Levels Explained

RAID 0 and RAID 1 are the building blocks on which the different RAID levels were further developed. Here, we will try to understand these two RAID levels along with the other RAID levels, RAID 2, RAID 3, RAID 4, RAID 5, RAID 6, RAID 10 and RAID 01. These are the most common and of course, the standard RAID levels.

RAID 0
In RAID 0, the data is split across different hard drives. Hence, as expected, at the server level, the performance of this RAID level is very high. A good number of terminals can access different bits of information at the same time. But the basic reason as to why this RAID level cannot be termed as a proper RAID level is that redundant information is not stored. Hence, a single disk failure can result in the loss of a good amount of data.

Advantages of RAID 0

• Since redundant data is not stored in RAID, hence the capacity of this RAID storage system is excellent, complete 100%.
• This RAID level is very good for large data transfers.
• Splitting up of data across various hard drives provides very high input/output rates.
• There is no parity generation.
• Since, copies of data are not created, hence it is very cost effective. No extra space is used in storing duplicate data.
• It is very easy to implement RAID level 0.

Disadvantages of RAID 0

• The single drive MTBF causes the data availability feature to be very low.
• It is not a proper RAID level, since it cannot provide data redundancy.
• A single disk failure can result in a considerable amount of data loss.
• RAID 0 is not the right RAID level for critical systems, where data holds the prime importance.

RAID 1
This level is known for its mirroring capability. Two hard disks are used, out of which one stores duplicate data. In other words, same data is stored in both the hard disks. Thus, data redundancy is provided very well in this RAID level. However, the cost of implementing this RAID level becomes very high, since one of the hard drives is just used for keeping the duplicate content of the data in the other hard drive. For more information on RAID 0 and RAID 1 comparison, please refer to the article, RAID 0 vs RAID 1.

Advantages of RAID 1

• The capacity of data storage in RAID 1 is not that bad. It is 50%.
• For large data transfers, this RAID level is also very good.
• In RAID 1, reading data is quite fast.
• Most importantly, failure of any one of the disks, cannot cause data loss, as a backup is always there in the other hard disk.
• This is another easy to implement RAID level.

Disadvantages of RAID 1

• It is not very much cost effective, because one of the drives is just storing the duplicate data of the other.
• The writing speed is decreased, since data has to be written twice.
• The disk overhead is also very high.

RAID 2
In RAID 2, data is not stripped at blocks, but at the level of bits. Hamming code is used for error correction. Hamming code is a linear error correcting code. This is very efficient in recovering accurate data from the single bit corruption in data. Thus, this RAID level provides a very high data transfer rate.

Advantages of RAID 2

• High data transfer rates.
• Single bit corruption of data can be accurately recovered.
• Multiple bit corruption can also be detected with much ease.

Disadvantages of RAID 2

• Multiple bit corruption is possible.
• Multiple bit corruption can be detected but not corrected.
• The error bit correction logic is very complex. RAID 2 has become almost an obsolete method of data storage.

RAID 3
In RAID 3, data is split at byte level. In this method, one additional hard disk is used for holding the parity bits. Since data is stored and stripped at the byte level, hence, accessing a single block of data requires access to more than one hard disks. This is another RAID level, whose use is very much limited to certain applications.

Advantages of RAID 3

• For large file transfers, it provides very high read and write speeds.
• It is quite cost effective.
• The capacity of the hard disks used in this system is also very good, since, only one extra hard disk is used for storing the parity bits.

Disadvantages of RAID 3

• RAID 3 is not very good for small data transfers.
• Accessing a block of data means, dealing with more than one hard drive in the hard drive array.
• Application is limited to certain specific fields.

RAID 4
RAID 4 is quite similar to that of RAID 3. It also uses a dedicated parity disk, but the difference is that, it strips the data at block level. This is another RAID level, which became obsolete very soon.

Advantages of RAID 4

• It can provide multiple reads if the controller allows it to do so.
• It is also quite cost effective.
• Unlike RAID 3, it does not require synchronized spindles.

Disadvantages of RAID 4

• Since, only one block of data can be accessed at a time, hence the system does not have a very good performance.
• Writing data to disks is also very slow, as in addition to writing blocks of data, the parity data also needs to entered.
• Implementation of RAID 4 requires solving the problems, which already exist in it, hence making its use almost obsolete.

RAID 5
This is perhaps the most popular RAID level. It also uses block level stripping, but a single dedicated hard drive is not used for holding the parity data. It also provides high storage capacity too. Read more on RAID 5 vs RAID 1 and RAID 5 vs RAID 10.

Advantages of RAID 5

• High read/write speeds are possible. As against RAID 3 and RAID 4, which were quickly replaced by RAID 5, the RAID level 5 allowed multiple writes.
• It is very cost effective. With a minimum of just 3 hard drives, this RAID level can be explained.
• The capacity of this RAID level is also very good.

Disadvantages of RAID 5

• It is not very efficient with large data transfers.
• Though the performance is very good, a disk failure can have a good impact on the system's performance.

RAID 6
RAID 6 was an extension of the RAID 5. Two parity blocks are written in the RAID 6 level, to help in the data recovery process. This RAID level was used for preventing data loss, in case of concurrent disk failures.

Advantages of RAID 6

• The RAID 6 performance is high for read operations.
• Its capacity though not as good as RAID 5 or other RAID levels, but its moderate and hence not very costly to implement.
• It is good for large data transfers.

Disadvantages of RAID 6

• The performance is not very good for small data transfers.
• Writing data requires larger time, as two parity blocks are created.

RAID 10
RAID 10 is often referred to as RAID 1+0. The reason is that this RAID level uses the combined features of RAID 1 and RAID 0. Here, a mirror of each block of data is created and data is also stripped. This is a very good system for handling multiple drive failures.

Advantages of RAID 10

• The disk capacity is moderate, 50%. Here again, copies of the same block of data is stored.
• It is very much fault tolerant, as it can handle multiple hard drive failures.
• It is very good for large data transfers.

Disadvantages of RAID 10

• It is not very cost effective. Duplicate copies of data demands double the number of hard disks, then what is required.
• Just as in the case of RAID 3, the drive spindles need to be synchronized.

RAID 01
RAID level 01 can also be referred to as RAID 0 + 1. It also does not cause any parity generation. Many people get confused between RAID 10 and RAID 01. However, here first data is stripped in an array of hard drives and another array holds the mirror image of the first array of data.

Advantages of RAID 01

• It requires a minimum of 4 drives and is very easy to implement.
• Since mirror images of all the blocks of data is created, hence the capacity is moderate at 50%.
• For large data transfers, the data transfer rate is quite high.

Disadvantages of RAID 01

• Since duplicate copies of the same data is created, hence it is quite costly.
• The write operation takes comparatively larger time.

This was all about the basic RAID levels explanation. Besides these RAID levels, there are other non standard RAID levels like RAID 7, RAID 5E, RAID 1.5, etc. which were created using the logic of the previous RAID levels. I hope the RAID levels explained in this article will prove to be helpful to you in not only understanding these non standard RAID levels, but other RAID levels like, RAID 30, RAID 50, etc.