Cómo entender y manejar el almacenaje de video

Fredrik Nilsson
Recording and saving video in an IP surveillance environment requires the ability to store large amounts of data for sometimes unspecified lengths of time. There are a number of different factors to consider when selecting the appropriate storage system for an installation including scalability, redundancy and performance.
Similar to the way a PC can «save» documents and other files, video can be stored on a server or PC hard disk. Specialized equipment is not needed because a storage solution does not differentiate video data – it is viewed as any other large group of files that is stored, accessed and eventually deleted. However, video storage puts new strains on storage hardware because it may be required to operate on a continual basis, as opposed to during normal business hours with other types of files. In addition, video by nature generates very large amount of data creating high demand on the storage solution.
Calculating the storage needs
In order to appropriately calculate the storage requirements of a network surveillance system, there are a number of elements to factor in, such as the number of cameras required in your installation, the number of hours a day each camera will be recording, how long the data will be stored, and whether the system uses motion detection or continuous recording. Additional parameters like frame rate, compression, image quality and complexity should also be considered.
The type of video compression employed also effects storage calculations. Systems employing JPEG or Motion-JPEG compression vary storage requirements by changing the frame rate, resolution and compression. If MPEG compression is used, then bit rate is the key factor determining the corresponding storage requirements.
Storage is usually measured in Megabytes (MB) per hour or in Gigabytes (GB) per day. One MB equals one million bytes, and one GB is one billion bytes. There are eight bits per byte, and these bits are essentially small «pulses» of information.
Fortunately, there are very specific formulas available for calculating the proper amount of storage to buy. These formulas are different for Motion-JPEG and MPEG compression because Motion-JPEG consists of one individual file for each image, while MPEG is a stream of data, measured in bits per second. The formulas are as follows:
Motion JPEG
1. Image size x frames per second x 3600s = KB per hour / 1000 = MB per hour
2. MB per hour x hours of operation per day / 1000 = GB per day
3. GB per day x requested period of storage = Storage need
Camera Resolution Image size (KB) Frames per second MB/hour Hours of operation GB/day
No.1 CIF 13 5 234 8 1,9
No.2 CIF 13 15 702 8 5,6
No.3 4CIF 40 15 2160 12 26
Total for the 3 cameras and 30 days of storage=1002 GB
1. Bit rate / 8(bits in a byte) x 3600s = KB per hour / 1000 = MB per hour
2. MB per hour x hours of operation per day / 1000 = GB per day
3. GB per day x requested period of storage = Storage need
Camera Resolution Bit Rate (kBit/s) Frames per second MB/hour Hours of operation GB/day
No.1 CIF 170 5 76,5 8 0,6
No.2 CIF 400 15 180 8 1,4
No.3 4CIF 880 15 396 12 5
Total for the 3 cameras and 30 days of storage= 204 GB
Storage Options
As previously mentioned, IP surveillance does not require specialized storage solutions – it simply utilizes standard components commonly found in the IT industry. This provides lower system costs, higher redundancy, and greater performance and scalability than found in DVR counterparts.
Storage solutions depend on a PC’s or server’s ability to store data. As larger hard drives are produced at lower costs, it is becoming less and less expensive to store video. There are two ways to approach hard disk storage. One is to have the storage attached to the actual server running the application. The other is a storage solution where the storage is separate from the server running the application, called network attached storage (NAS) or storage area networks (SANs).
Direct server attached storage is probably the most common solution for hard disk storage in small to medium-sized IP surveillance installations (See image 1, server attached storage). The hard disk is located in the same PC server that runs the video management software. The PC and the number of hard disks it can hold determine the amount of storage space available. Most standard PCs can hold between two and four hard disks. With today’s technology, each disk can store approximately 300 gigabytes of information for a total capacity of approximately 1.2 terabytes (one thousand gigabytes).
When the amount of stored data and management requirements exceed the limitations of direct attached storage, a NAS or SAN and allows for increased storage space, flexibility and recoverability.
NAS provides a single storage device that is directly attached to a Local Area Network (LAN) and offers shared storage to all clients on the network (See image 2, network attached storage). A NAS device is simple to install and easy to administer, providing a low-cost storage solution. However, it provides limited throughput for incoming data because it has only one network connection, which could become problematic in high-performance systems.
SANs are high-speed, special-purpose networks for storage, typically connected to one or more servers via fiber. Users can access any of the storage devices on the SAN through the servers, and the storage is scalable to hundreds of terabytes. Centralized storage reduces administration and provides a high-performance, flexible storage system for use in multi-server environments. In a SAN system, files can be stored block by block on multiple hard disks. Technologies such as Fiber Channel are commonly used, providing data transfers at four gigabits per second (Gbps).
This type of hard disk configuration allows for very large and scalable solutions where large amounts of data can be stored with a high level of redundancy. For example, the Kentucky Department of Juvenile Justice (DJJ) recently updated an analog tape storage system to a SAN system, allowing the department to install a greater number of cameras throughout its locations and centralize the storage of remote video feeds. The DJJ employed EMC Corp.’s Surveillance Analysis and Management Solution (SAMS) to make the video searchable. This system, which handles hundreds of cameras, is easily expanded and managed as each individual facilities’ needs change.
Redundant Storage
SAN systems build redundancy into the storage device. Redundancy in a storage system allows for video, or any other data, to be saved simultaneously in more than one location. This provides a backup for recovering video if a portion of the storage system becomes unreadable. There are a number of options for providing this added storage layer in an IP surveillance system, including a Redundant Array of Independent Disks (RAID), data replication, tape backups, server clustering and multiple video recipients.
RAID — RAID is a method of arranging standard, off-the-shelf hard drives such that the operating system sees them as one large hard disk. A RAID set up spans data over multiple hard disk drives with enough redundancy that data can be recovered if one disk fails. There are different levels of RAID – ranging from practically no redundancy, to a full-mirrored solution in which there is no disruption and no data loss in the event of hard disk failure.
Data replication — This is a common feature in many network operating systems. File servers in the network are configured to replicate data among each other providing a back up if one server fails (See image 3, data replication).
Tape backup — Tape backup is an alternative or complementing method where a tape backup machine is installed on the server and records copies of all materials saved on a periodic basis, i.e. daily or weekly. There is a variety of software and hardware equipment available, and backup policies normally include taking tapes off-site to prevent possible fire damage or theft.
Server clustering — A common server clustering method is to have two servers work with the same storage device, such as a RAID system. When one server fails, the other identically configured server takes over. These servers can even share the same IP address, which makes the so called «fail-over» completely transparent for users.
Multiple video recipients — A common method to ensure disaster recovery and off-site storage in network video is to simultaneously send the video to two different servers in separate locations. These servers can be equipped with RAID, work in clusters, or replicate their data with servers even further away. This is an especially useful approach when surveillance systems are in hazardous or not easily accessible areas, like mass-transit installations or industrial facilities.
The variety of storage options available for IP surveillance systems makes it crucial to consider the different ways the information will be used and stored for the long term. As hard drive technology continues to advance, it is important to utilize open standards to ensure that storage is scalable and future proof. In addition, advances in IP-surveillance – such as intelligent video algorithms – will make it even more critical to select open storage devices that can handle combinations of data from different sources. Storage systems should be able to accommodate new and upcoming applications so that equipment investments are not lost as technology advances.

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