Understanding RAID Systems for Surveillance Storage

In recent years, the physical security industry has seen more changes than just about any other time in modern memory. Digital or IP (Internet Protocol) cameras, based on CCD technology, have replaced more familiar analog surveillance cameras in most new installations. And to support them, DVRs (Digital Video Recorders) have virtually eliminated the use of tape and VCRs.

Combined, these two IP technologies have opened the way for the physical security industry to dramatically improve video surveillance and other common applications while greatly reducing costs and increasing service levels.


Learn More:
IP-based video surveillance storage was the topic of a recent webinar from SecurityInfoWatch.com. You can register to view this archived webinar for free.

Yet moving to IP also has its own challenges, especially as security users come to rely more and more on the improved resolution, higher frame rates, and megapixel quality that the technology offers. In fact, many security practitioners and integrators admit that over half of their support and customer satisfaction problems relate to DVR drive failures, which can result in the loss of all recorded video and data.

So is it a mistake to move from tape to IP? The answer is a resounding "no", as the benefits of scalable, open IP far outweigh the problems. In fact, most IP storage problems in DVRs can be either fully eliminated or the risk mitigated with the application of basic techniques and strategies that are common in the Information Technology (IT) industry.

One way to avoid data losses is to use RAID technology, either built into the DVR system or added on as an external IP storage array.

What Is RAID, and Why Do We Need It?

RAID is the acronym for Redundant Array of Independent (or Inexpensive) Disks, in wide IT use since the late 1980s and as defined in the SIGMOD paper, "A Case for Redundant Arrays of Inexpensive Disks". The Storage Networking Industry Association (SNIA) defines RAID in part as "...a family of techniques for managing multiple disks to deliver desirable cost, data availability, and performance characteristics to host environments."

And like many families, not all members are suitable - or desirable - for every need or situation. Nowhere is that more true than in physical security for video surveillance. In fact, some RAID levels would be extremely bad choices for our industry.

To determine which RAID makes the most sense for video surveillance, let's take a quick look at the basic techniques, strengths, and weaknesses of each.

An Overview of RAID Levels

RAID 0
RAID 0 provides no redundancy or data protection from the failure of any disk. So why use it? For our purposes in physical security, you shouldn't. But in IT there are some specific situations where the absolute best speed is more important than protecting data. RAID 0 delivers just that, by splitting or "striping" the recorded data across all of the drives in the array so as to improve read/write times and effectively spreading the load. RAID 0 was not one of the original levels adopted by the IT industry, but was added later specifically to deliver this improved performance capability.

Since losing any drive in the array to a failure would result in ALL recorded data being lost, this is not a suitable technique for use in physical security applications in general and video surveillance storage in particular.

RAID 1
RAID 1 is also referred to as a 'storage mirror' or a 'mirroring solution'. All data is recorded on not just one but two identical drives, so that two copies exist at all times. If one drive fails, the exact copy on the other continues to function and no data is ever lost. While many DVRs have no RAID protection at all, some do feature RAID 1 support.

So is RAID 1 the answer for physical security systems? In many cases, the answer is no. That's because by doubling the disks used to record a mirror, you effectively also double the cost per megabyte of storage. And that can add up very quickly with video workloads requiring longer retention and higher frame rates and better resolution, to the point that DVRs quickly run out of recording capacity.

While RAID 1 is undoubtedly a useful technology for some applications, it is clearly even better financially for those selling disk drives, who will sell double the capacity for every storage application using it compared with those that don't. That is why we would say that RAID 1 should not typically be your first choice.

RAID 2
We don't have to spend a lot of time on RAID 2, as it is not used in any commercial storage implementation today and you can skip ahead. But if you just have to know why, read on.

When the RAID definitions were originally laid out, RAID 2 made sense because it striped data to a bunch of drives, with an ECC (Error Checking and Correction) code symbol stored on a specific drive for the entire array for data recovery purposes. As ECC codes are now embedded in disk drives, however, this is no longer required and RAID 2 is no longer commercially used.

RAID 3
RAID 3 copies or 'stripes' data across three or more drives at the byte level. One of the drives becomes the parity drive and performs this task. The parity drive is not used to store video or other digital data. By using the information on the parity drive, it is possible to rebuild and recover all of the information on the remaining drives in the event of a disk failure.

Only in the extreme case of two drives failing at once will RAID 3 falter and stored data become lost. So while RAID 3 has some risk (if a second drive were to fail before the first drive was rebuilt), it greatly reduces the cost in comparison to RAID 1 and improves the availability of the data over having no RAID or only performance-oriented RAID 0 in use.

RAID 3 has been used in some IT systems, but due to the inherent complexity of the storage controller design needed and some configuration challenges when rebuilding lost data, it has been largely superseded by RAID 5. It is not commonly used today.

RAID 4
RAID 4 functions like RAID 3, but uses larger stripes so as to allow read speed to be increased. While an improvement in some environments over RAID 3, it offers no real advantage over RAID 5.

Thus like RAID 3, RAID 4 has been largely replaced by RAID 5 and is not found in most storage systems today.

RAID 5
We've already talked about several RAID levels giving way to RAID 5, so briefly here's what is and why it is so popular.

Simply put, RAID 5 delivers the best overall balance of data protection and performance and makes the most efficient use of drive capacity of all RAID techniques. As a result, RAID 5 is the most widely used technique in IT and is also found in the best DVR and IP storage systems.

RAID 5 requires three or more identical drives to be in a RAID group. Data is striped across drives, and parity is distributed amongst them all.

While RAID 5 is not as fast as RAID 1 since its parity data is spread across multiple drives, it is clearly the best overall choice for many physical security and IT applications.

Data loss risks are greatly lessened with RAID 5. Only in the unlikely event of a second drive failure, just as with the other RAID levels we have reviewed, will data be at risk.

RAID 6
RAID 6 is an extended version of RAID 5 that it is able to protect against not just one, but two simultaneous disk drive failures without loss of recorded data.

RAID 6 stripes data on a block level, just as RAID 5 does. But it also adds a second set of parity that is calculated and then copied to all of the drives in the array. This extra step is what allows RAID 6 to survive a double disk drive failure, but it also requires more disk space and thus is less efficient than RAID 5.

In particular, RAID 6 is considered to be unacceptably inefficient when a small number of low capacity disk drives are deployed, such as in a typical DVR. However, the ability of RAID 6 to support two simultaneous failures is growing in importance as the number of hard drives in individual IT storage systems dramatically increases to support larger numbers of megapixel digital cameras at higher resolution and frame rates.

RAID 6 is also important to consider as denser, higher capacity disk drives are used, such as 1 Terabyte (TB) devices. Because these latest drives have so much capacity, the time to rebuild the data after a failure is greatly increased over less dense, lower capacity drives. The risk then becomes that there is a higher likelihood that while one drive is rebuilding after a failure, a second could conceivably fail during the long rebuild process, and all data be lost. RAID 6 eliminates this risk.

RAID 6 will thus grow in importance in the future, but is secondary to RAID 5 for physical security applications today.

RAID 10
RAID 10 is a newer standard, which like both RAID 0 and 6 was not one of the original RAID techniques. As in RAID 1, mirrors are created, and then additionally a RAID 0 stripe is applied to all of the drives, linking them together.

RAID 10 is very fast, and offers good drive failure tolerance, but requires double the storage to create the mirror as without. Thus, it is also twice as expensive per TB as RAID 5. While found in some large capacity, high performance IT systems, and useful for very large video surveillance storage requirements, RAID 10 is not common or required in most smaller implementations today.

Like RAID 6, RAID 10 will grow in importance in the future as larger storage deployments become more common for physical security implementations.


RAID Chart:
This simple Powepoint file/chart demonstrates how the different RAID categories write the data [download the RAID Chart]

There Are More?

While these are the basic RAID techniques, there are other RAID levels that individual IT manufacturers sometimes promote. As well, there are some newer, double-digit levels that are being adopted by the largest IT systems for speed and fault tolerance or to address scalability issues. Since they are often vendor-specific and largely not appropriate for most of today's physical security applications, these additional RAID levels (like vendor-specific RAID 7) will not be discussed here.

In Conclusion: What Physical Security Pros Should Know About RAID Storage

So now you know that RAID isn't all that complicated a concept to understand, and a couple of its techniques offer real advantages to the physical security industry for video surveillance and other data storage needs.

Today, many DVRs do not offer RAID protection, and as a result non-redundant systems may leave your recorded data open to complete loss. Fortunately this can be easily avoided. A subset of available DVRs offer RAID 1 and the best also feature RAID 5. For most physical security installations today, RAID 1 offers great protection and performance, but is often too expensive to deploy effectively.

For most video surveillance deployments, RAID 5 is usually the best choice, with its strong mix of affordable protection and good performance.

If your DVR vendor doesn't offer RAID 5, or you need additional capacity but don't want to replace those expensive DVRs you just paid for, you may want to consider adding an inexpensive external IP storage solution that will increase both reliability and performance of all of your video surveillance systems. Today, there are external IP solutions which can be connected in just a few minutes to popular brands of DVRs without noticeable disruption, and will continue to support any mix of analog and digital cameras as before.

Finally, ask your vendor about the type of drives that they use in their DVR systems. To lessen the frequency of disk drive failures, the best storage solutions use a higher grade disk drive than is typically found in PCs and some DVRs. These "server grade" drives often cost slightly more, but are required to pass more stringent manufacturer tests than the drives that are found in most PCs and many DVRs.

IP offers many advantages over tape and the proprietary, closed systems of our industry's past. IP can be less costly, yet much more capable. By using well designed IP systems with RAID, and ensuring that you are using server-grade drives within redundant storage systems, you can enjoy all of the benefits of IP, worry free.

About the author: Jeff Whitney is vice president of marketing at scalable IP storage vendor Intransa. He offers extensive physical and network security knowledge, combined with IT storage expertise developed over twenty years as an industry practitioner, with a successful leadership career spanning Antares Technologies, Amdahl Corporation, HAL Computer, Fujitsu Technology Solutions, Spinnaker Networks, Network Appliance, and MaXXan Systems. He is a volunteer to the ASIS International Physical Security Council, and is the primary Intransa representative to the Security Industry Association, while also representing the company to the GreenGrid.org and the Storage Networking Industry's Green Initiative. He can be reached via email for discussion of this article.

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