Introduction to RAID

Introduction

One of the most common techniques to improve either data reliability or data performance (or both) is called RAID (Redundant Array of Inexpensive Disks). The concept was developed in 1977 by David Patterson, Garth Gibson, and Randy Katz as a way to use several inexpensive disks to create a single disk from the perspective of the OS while also achieving enhanced reliability or performance or both.

Before anyone erupts and says that RAID does not stand for “Redundant Array of Inexpensive Disks”, let me start by stating that was the original definition. Over time, the definition has become more commonly known as “Redundant Array of Independent Disks” perhaps so the word “inexpensive” isn’t associated with RAID controllers or disks. Personally I use the original definition but regardless, either definition means that the disks are independent of one another. Feel free to use either definition since it won’t change the content of this article. Now, back to our discussion of RAID.

When the original paper was issued, five different RAID levels or configurations were defined. Since that time other RAID configurations have been developed including what are referred to as “hybrid” RAID configurations.

The RAID Advisory Board (RAB) was created to help advise the IT community on the defined RAID configurations and to help the creation of new RAID configuration definitions. While it is not an organization that creates legally binding standards and labeling, it does help in clarifying what the RAID levels mean and what is commonly accepted in the community. There was a time where companies were creating very strange RAID configurations and using strange labels, causing great confusion. The RAB has helped to reduce the proliferation of “weird” RAID configurations and labeling and standardize the meaning of various RAID levels.

In this article I want to review the seven most common standard RAID configurations. But I will also very briefly touch on some of the hybrid RAID configurations. For each RAID level, I will describe how it works as well as the configuration’s particular pros and cons. However, before starting I want to clarify one thing: RAID is not meant as a replacement for backups. RAID can help improve data reliability which really means data availability (improving uptime for data) and/or data performance (I/O performance). It is not intended as a replacement for backups or keeping multiple independent copies of your data.

RAID Configurations

As mentioned above, there were five original RAID levels or configurations that were defined but others have been developed since that original article. In RAID terminology each distinct RAID configuration is given a number which can also be called a RAID “level”. The core RAID configurations are listed as: RAID-0, RAID-1, RAID-2, RAID-3, RAID-4, RAID-5, and RAID-6.

RAID-0
This RAID configuration is really focused on performance since the blocks are basically striped across multiple disks. Figure 1 from wikipedia (image by Cburnett) illustrates how the data is written to two disks.

Figure 1: RAID-0 layout (from Cburnett at wikipedia under the GFDL license)

In this illustration, the first block of data, A0, is written to the fist disk, the second block of data, A1, is written to the second disk, the third block of data, A3, is written to the first disk, and so on. If the I/O is happening fast enough data blocks can be written almost simultaneously (i.e. A0 and A1 are written at just about the same time). Since the data is broken up into block sized units between the disks, it is commonly said that the data is striped across the disks. As you can see, striping data across the disks means that the overall write performance of the disk set is very fast, usually much faster than a single disk.

Reading from a RAID-0 group is also very fast. A read request comes in and the RAID controller, which controls the placement of data, knows that it can read A0 and A1 at the same time since they are on separate disks, basically doubling the potential read performance relative to a single disk.

You can have as many disks as you want in a RAID-0 array (a group of disks in a RAID-0 configuration). However, one of the downsides to RAID-0 is that there is is no additional data redundancy provided by RAID-0 (it is all focused on performance). No data parity is computed and stored meaning that if you lose a disk in a RAID-0 array, you will lose access to all of the data in the array. If you can bring the lost disk back into the array without losing any data on it, then you can recover the RAID-0 array, but this is a fairly rare occurrence.

Consequently, we can see that RAID-0 is focused solely on performance with no additional data redundancy beyond the redundancy in a single disk. This affects how RAID-0 is used. For example, it can be used in situations where performance is paramount and you have a copy of your data elsewhere or the data is not important. A classic usage case is for scratch space where data is written while an application is running but is not needed once the application is done and the final output is copied to a more resilient storage device. If a scratch space disk is lost while the application is running, you can rebuild the RAID-0 array with one fewer drives, and rerun the application.

The capacity and failure rate of a RAID-0 array is the fairly simple to compute. The capacity is computed as,

Capacity = n * min(disk sizes)

where n is the number of disks in the array and min(disk sizes) is the minimum common capacity across the drives (this indicates that you can use drives of different sizes). This equation also means that RAID-0 is very capacity effective since it doesn’t waste any space for parity or any other error correction. It uses all of the space for data focusing on performance.

The failure rate is a little more involved but can also be estimated.

MTTFgroup = MTTFdisk / n

where MTTF is the Mean Time To Failure and “group” refers to the RAID-0 array and “disk” refers to a single disk. So as you add disks, you greatly reduce the MTTF for the RAID-0 array. Having two disks decreases the MTTF by half. Three disks reduces the MTTF by a factor of 3, and so on. So you can tell why people are reluctant to use RAID-0 for file systems where data availability and reliability is important. But, RAID-0 is the fastest RAID configuration and has the best capacity utilization of any RAID configuration discussed in this article.

Table 1 below is a quick summary of RAID-0 with a few highlights.

Table 1 – RAID-0 Highlights

RAID-1
RAID-1 is almost the exact opposite of RAID-0 because it uses multiple drives that are mirrors of one another. Typically two drives are used in RAID-0 but three drive RAID-1 configurations are becoming more common. RAID-1 takes an incoming block of data to one drive and creates a mirror image (copy) of it on a second drive. So RAID-1 doesn’t compute any parity of the block – it just copies the entire block to a second drive. Figure 2 from wikipedia (image by Cburnett) illustrates how the data is written to two disks in RAID-1.

Figure 2: RAID-1 layout (from Cburnett at wikipedia under the GFDL license)