What is RAID storage and how does it work?

28 March 2022 by Ontrack Team


Redundant Array of Independent Disks (RAID) is a term used to describe computer storage systems that spread or replicate data across multiple drives.

There are two main reasons for RAID storage to work in this way: it increases data reliability and improves I/O (input/output) performance. That said, RAID storage isn't a perfect technology. Data loss can still occur when using it.

In this article, we’ll dive into the workings of RAID and explore its benefits and pitfalls so you can make the right storage choice for your requirements.

How does RAID storage work?

A RAID combines physical disks into a single logical unit by using either special hardware or software. Hardware RAID solutions can come in a variety styles, from built onto the motherboard or add in cards, up to large enterprise NAS or SAN servers. With these setups the operating system (OS) is unaware of the technical workings or the RAID. Software solutions are typically implemented within the OS.

RAID is traditionally used on servers, but can be also used on workstations. The latter is especially true in storage-intensive computers such as those used for video and audio editing, where high storage capacities and data transfer speeds are required.

The RAID storage glossary: commonly used terms

RAID storage is a complex area with plenty of technical terms to get your head around. We’ll explain the most common ones now as we’ll be using some of them later in this article.

  • Parity: distributed information that allows the recreation of data stored within a RAID array, even if a disk fails.
  • Mirroring: when data from one or more hard drives is copied onto another physical disk or disks.
  • Striping: a method that involves writing data across multiple disks. In the example below, data is written across the drives in sequential order until it reaches the last drive. Then it jumps back to the first drive and starts a second stripe before repeating.
  • Block: the logical space on each disk where the data is written. The amount of space is set by the RAID controller.
  • Left/right symmetry: symmetry in a RAID array determines the way in which the data and parity are distributed across the drives. There are four main styles of symmetry that can be used (depending on the RAID vendor) and some companies make proprietary styles to meet their own business needs.
  • Hot spare: a spare disk that can be used in place of a failed disk within a RAID array.
  • Degraded mode: this occurs when a drive in the RAID becomes unreadable and is withdrawn from the array. The new data and parity are then written to the remaining drives within the RAID. If any data is requested from the failed drive, it is worked out with the parity on the others. The decrease in drives degrades the performance of the RAID, hence degraded mode.

RAID storage levels explained

Data recovery engineer checking raid drive

Standard RAID configurations are known as levels. While there were originally five of these, there are now more as variations have evolved to include several nested and many non-standard (usually proprietary) levels.

As previously mentioned, mirroring involves the copying of data to more than one disk, striping occurs when data is split across more than one disk, and error correction occurs when redundant data is stored as a means of allowing problems to be detected and possibly fixed (known as fault tolerance). Depending on the system requirements, one or more of these techniques can be used within different RAID setups.

Different levels have their own types of redundancy, so a trade-off usually has to be made between fault tolerance and performance, depending on the application.

The basic RAID levels include:

  • RAID 0 – Often called striping, this is considered the most basic RAID level. It offers no redundancy but excellent performance. Data is striped across at least two disks and with every disk added, the read/write performance and storage capacity are increased over a single drive. More on what is RAID 0? here.
  • RAID 1 – This level is also called mirroring as the same data is mirrored across two disks, providing the lowest level of RAID redundancy. It offers up to double the read performance of a single drive but no increase in write speed. Stored data is always accessible as long as one disk is still working. More on what is RAID 1? here.
  • RAID 5 – This is a common configuration that provides a good compromise between security and performance. It requires at least three disks and offers increased read speeds but no improvements in write performance. RAID 5 introduces parity to the array, which takes up the space of one disk in total. This level can also tolerate one disk failure. More on what is RAID 5? here.
  • RAID 6 – This takes the concept of RAID 5 a step further as a minimum of four disks are required and dual-parity is introduced, meaning data can be recreated even if two disks fail within the array.
  • RAID 10 – This is a partnership between RAID 1 and RAID 0. Also known as RAID 1 + 0, this configuration consists of assembling two or more RAID 1 devices into a RAID 0 array.  More on what is RAID 10? here.

Modern RAID arrays: benefits and challenges

There are many ways to get more out of your RAID system using non-standard configurations (up to RAID 51 and beyond), mainly as these can be used in tandem with other complex systems for significant efficiency and cost benefits.

As you may have gathered by now, RAID arrays are highly complex. This is often intensified within enterprise IT infrastructures, as RAID systems are used mostly for business critical applications; where availability and efficiency are crucial factors. What’s more, add-on technologies like virtualisation or database applications can spell disaster for a business if the system was to fail.

Modern RAID arrays can also use multiple file systems, like BTRFS or ZFS at the hardware level, with NTFS or HFS layered over the top for application support via virtualisation..

From a data recovery perspective, it would usually be necessary to not only reconstruct the RAID file system and bypass any physical failures, but to also assess any virtualised architecture that may exist. This can often make a recovery attempt extremely complex and time-consuming; however in many cases recoveries can be very successful.

RAID data recovery: the potential challenges

RAID arrays are highly complex, and the challenges they present are intensified when they’re used for business-critical functions within enterprise IT infrastructures, as availability and efficiency are essential.

While useful in some scenarios, add-on technologies like virtualisation and database applications can cause further costly complications in a failing system.

From a data recovery perspective, it would usually be necessary to reconstruct the RAID file system, bypass any physical failures and assess any virtualised architecture. This can make for a complex and time-consuming process, but recovery is possible with the right expertise.

Prepare for drive failure

Sometimes a drive in the RAID can become unreadable; the drive is then considered bad and is withdrawn from the RAID. The new data and parity are then written to the remaining drives within the RAID, if any data is requested from the failed drive it is worked out with the parity on the others. This degrades the performance of the RAID.

Unfortunately, all drives can and will fail at some point.

If a failure occurs involving individual drives (assuming it is within a RAID 1 configuration or greater), the faulty drive can just be replaced with a new one and the data storage map can be rebuilt with zero data loss.

However, if a drive failure exceeds the redundancy capacity of the RAID, you should contact a professional RAID data recovery specialist to minimise the chances of complete data loss. It is imperative to ensure that your chosen provider has the tools and expertise to recover from any configuration or data loss situation. You should also assess whether they have direct partnerships with storage vendors and development capabilities for accommodating new or custom configurations.

We’re here to help with data recovery insight and expertise

Want to know more about how RAID works? Check out this Ontrack podcast from 2015 where we interviewed expert data recovery engineer Robin England on the dos and don’ts of using RAID systems. While the technology has evolved since then, there are still some key points worth considering in 2022.

If you’re already using RAID storage and you’ve recently experienced RAID data loss, we can help with that, too. Get in touch with our experts to start your recovery today