Air RAID

Gaining performance of a video surveillance system

  • By Tri Nguyen
  • Nov 01, 2013

As IP camera usage increases in video surveillance systems, there is a need for larger data storage capacities. The resolution advantage that is inherent with megapixel IP cameras comes with greater data rates and file sizes. Because a 1080p IP camera contains six times the resolution of a VGA analog camera, the storage capacity required for the same amount of recorded time is six times as much. Ensuring that large amounts of data are safeguarded against disk failure should be a requirement for any surveillance system.

However, another challenge posed by larger data rates and file sizes is the video system’s ability to write this data on a disk with sufficient speed and reliability. A RAID 5 storage array increases both the dependability and performance of a video surveillance system, especially a megapixel IP video system.

What is RAID 5?

Redundant Array of Independent Disks (RAID) is a method for configuring multiple disks or hard drives in a single array. While there are many levels of RAID, focusing on RAID 5 for video surveillance is key.

Because RAID 5 requires a minimum of three disks and uses a combination of striping and parity to write data across all the disks rather than sequentially, the data is spread across multiple disks and faster speeds are achieved. Parity is an error-checking technique that is used to add redundancy to a RAID 5 configuration. If a drive in a RAID 5 array fails, the parity information is used with the remaining data on the other drives to recover the missing data. Like the data, the parity information is striped across all the disks.

The effective storage space of a RAID 5 array can be calculated by the formula 1 – 1/n, where n is the number of disks in the array. For example, if you have 8 drives of 1TB each, the effective space is 8TB total x (1 – 1/8) = 8TB x 0.875 = 7TB. In other words, one drive will be used for parity. Overall, RAID 5 increases performance and offers redundancy with efficient space, making it an effective configuration for megapixel IP video systems.

Increased Performance

Hard disk drives have read and write speed limitations. Due to their mechanical nature and physical limitations, by configuring drives in RAID 5, the number of locations that video information can be written increases.

In a megapixel video application, a large amount of random disk operations occur. Video information is being stored along with the indexing of this video. If motion detection and audio is used, the motion detection and audio index must be stored as well. In a non-RAID 5 configuration, these non-linear disk operations are typically handled by the CPU, which can cause the entire system to slow or crash.

In a RAID 5 system, a dedicated hardware RAID 5 controller manages the large number of disk operations. Depending on the quality of the RAID 5 controller, the overall system throughput can be doubled or tripled. For example, a 16 camera, 1080p, megapixel IP video system at 15 fps requires approximately 118 MBps throughput. However, a Windows-based NVR typically has throughput capability of 90 MBps, due to disk write limitations. When configured as RAID 5 and using a robust RAID 5 controller, this throughput increases to 250 MBps, resulting in the ability to connect 33 1080p cameras at 15 fps, compared to only 12 cameras per server.

The Requirements

The main requirements for using RAID 5 are applications that need data redundancy with throughput requirements exceeding the non-RAID 5 capabilities. Data redundancy is on video data. An office environment, for example, may not be as critical as a banking or public safety environment. Often the need for redundancy will be dictated by the specifications of a particular project.

Calculations may be needed to determine the required throughput and the throughput capability of the surveillance system. The camera bit rate should be determined and multiplied by the total number of cameras to figure the minimum throughput requirement. Overhead requirements, such as remote client connections and backups, need to be considered. This throughput value should be compared to the manufacturers’ system throughput to determine if RAID 5 is needed.

Advantages and Disadvantages

RAID 5 gives an end user the security of data redundancy and the ability to recover data in the event of a disk failure. It increases the throughput performance of a system because of its ability to write on multiple drives at once. Coupled together, these two factors make RAID 5 a good option for high camera count and high megapixel applications.

Because of the added hardware, and in many cases, a higher grade of hard disk drives, RAID 5 will be more costly than a non-RAID 5 system. The effective storage of a RAID 5 system will be decreased as well, due to the extra parity information. Other disadvantages include longer build time of a RAID 5 system, the complexity of adding hard drives in the field, and the knowledge level needed to recover lost data in the event of a failed drive.

Today’s IP megapixel applications require more data storage and performance capabilities of a video surveillance system. This requires the system to be robust enough to write large amount of data onto disk without compromising the performance and reliability of the system.

RAID 5 adds data redundancy as well as performance increases to handle megapixel IP video applications. Critical video should be recorded with a fault tolerant system and high IP applications need greater performance. RAID 5 is an effective solution to both of those needs.

This article originally appeared in the November 2013 issue of Security Today.

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