What is RAID

Over the past few weeks we have provided reviews on Network Attached Servers (“NAS”) units from Synology and Netgear as well as the multi-disk Direct Storage Array (“DAS”) from Drobo.  While each of these have features and functions that make them unique, they all have one very important thing in common.  They all provide some version of RAID, and that has led some of our readers to as, “What is RAID?”

What is RAID – A Brief History

It is believed that the first use of the term RAID was in a paper presented by three University of California Berkley researcher who published a paper titled; A Case for Redundant Arrays of Inexpensive Disks (RAID).

In their paper, David Patterson, Garth A. Gibson, and Randy Katz proposed that as computers became faster and more efficient, the current storage technology of using a Single Large Expensive Disk (SLED) would not be able to keep up with the processing power and would become a bottleneck in the computer processing system.  They proposed the use of a Redundant Array of Inexpensive Disks as an alternative, and used the acronym RAID.

If you review their paper, you will see that it goes into a great deal of detail on what happens on the disks and is much greater detail than we could ever hope to discuss (and probably understand) here.  In its simplest terms, what they described were methods of repeating data across multiple hard drives using processes that today we call Mirroring, Striping and Parity.  With the data now on multiple disks, there is not only an increase in the efficiency of the storage process, but there is a greater level of protection when a drive fails.  Since the data lives on multiple drives, the RAID system can rebuild and recover the data from that lost drive.

In the early days, it would be rare to see a RAID system outside of a data center and supporting systems other than large mainframes.  We suspect Patterson, Gibson and Katz would have a lot to say about RAID systems now being common in people’s homes.

RAID Types

There are two basic types of RAID system, Software RAID and Hardware RAID.

As the name implies, a Software RAID is usually part of a computer’s operating system and/or run on the local hardware.  While this is cost efficient, it is resource intensive and often time’s cause’s performance problems for other applications running on the computer at the same time.

Until the release of OS X El Capitan, the Mac operating system included a software RAID option right in Disk Utility.  With the redesign of Disk Utility in OS X 10.11, that GUI option no longer exists.  But you still have the ability to create a software RAID using terminal.  For details on how this is done, take a look at a great article by Jim Tanous on TekRevue.com.

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A Hardware RAID system is a separate unit with its own RAID controller built in.  This moves the data processing off of the main computer, freeing up those resources for other purposes.  Many modern consumer RAID systems are part of Network Attached Storage (NAS) appliances like the Synology DS216 and the Netgear ReadyNAS RN202, which we previously reviewed. Of course, these modern NAS devices are much more than just disk storage and have grown into full-fledged servers. And RAID systems are not limited to network use as evidenced by the Drobo 5D which is a 5 disk RAID array designed primarily as a DAS device for a single computer.

There have been many flavors of RAID over the years and some have been abandoned over time as being duplicate functionality or just from lack of popularity.  Here is a brief look at some of the more modern RAID versions in use today.

RAID 0 – is also known as “disk striping.” RAID 0 requires at least 2 hard drives and the data is written across multiple disks which improves overall performance of the computer since read/writes operations are being handled by multiple drives instead of just one.  The drawback to RAID 0 is that the data is not duplicated anywhere so there is no fault tolerance in the system.   In the event of a drive failure, there is a high likelihood of data loss.

RAID1 – is known as “disk mirroring.” RAID 1 also requires at least 2 hard drives, but this time, data is copied simultaneously, from one disk to another, creating a replica, or mirror. Since the exact same data lives on two or more disks, the loss of a drive does not mean the loss of the data on that drive. 

RAID 5 and RAID6 are the most common RAID configuration for business servers and enterprise NAS devices. These version save data and parity (which is additional data used for recovery) across three or more disks. If a disk starts to fail, data is automatically rewritten while the system is still in use.  Another benefit is that it allows many NAS and server drives to be “hot-swappable” meaning in case a drive fails, it can be swapped with a new drive without shutting down the server.

RAID 10 is a combination of RAID 1 and 0 and is often denoted as RAID 1+0. It combines the mirroring of RAID 1 with the striping of RAID 0.

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This is just a very high level description on the most common RAID types.  There is a wealth of information available on the web if you want more detail.

In the early days of RAIDs you needed to have a deep understanding of how they worked and how they were configured because, while the data was recoverable, it required a lot of technical expertise to reconfigure and rebuild the disk array in order to recover the data.  A lot has changed and today’s NAS servers and DAS devices have added functionality that makes drive replacement for failed drives and/or adding new drives to increase storage capacity a plug and play event.  Data is automatically moved and managed by these systems to make data protection effortless.

RAID vs. Backup

Many people are under the impression that if they have a RAID system they do not need to worry about backups. Let’s be clear, RAID IS NOT A BACKUP!

While many people use a NAS or DAS to store their Time Machine backups, most NAS and DAS devices are storing data that is not stored anywhere else.  These could be large audio and video libraries, large photo libraries or other types of data files that need to be retained but not kept on you internal computer hard drive.

While it’s true that the data on the NAS is duplicated and protected from a drive failure in the unit, how would you recover your data in the event of a complete hardware failure or destruction of the device due to fire, flood or other natural disaster?  Data stored in a RAID environment is just as important as the data on your on your Mac and should be backed up on a regular basis and, if possible, stored off site from the primary data.  Fortunately, most NAS and DAS manufacturers provide an easy way to back up your data to an external hard drive and/or many of the popular cloud services available.

The Bottom Line

RAID systems have moved out of the world of enterprise data centers and into the homes of the average user.  It provides a cost effective way to store and protect large volumes of data that won’t fit or doesn’t need to be on your primary internal drive.  With the additional benefits added to many of today’s consumer RAID devices, you get fault tolerant security with ease of use and at a reasonable price.

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