Maybe it’s because information in digital format has the potential for immortality that some people think hard disks last forever. After all, they are obviously much more durable than videotape. But in fact, hard disks have a limited life and they will eventually fail; but if they fail at a crucial moment it can be quite dramatic. So a core element of the system designer’s task lies in minimizing or even eliminating the effects of disk failure in order to achieve an acceptable level of security and reliability for the end-user. This is accomplished by choosing the most appropriate disk for the job and then incorporating sufficient back up or redundancy to mitigate against malfunction.
Everyone agrees that the ideal disk would work 24 hours a day, have a high capacity, be inexpensive and never ever fail. But since such a disk does not yet exist, DVR manufacturers currently choose from four types with a variety of characteristics
CHOOSING YOUR DISKThe IDE hard disk (also known as the parallel-ATA or P-ATA) is the oldest type of hard disk with integral electronics and currently offers the largest capacities (up to 500 GB per drive) at the lowest price. Its disadvantages are its slow speed and relatively poor durability. Despite a data rate of up to 133 MBytes, it still only permits one kind of data exchange at a time: either reading or writing, whereas a security video system needs picture data to be written and read at the same time. Although most manufacturers give a 3 year guarantee on the IDE drive it may be significant that they no longer specify the Mean Time Between Failures (MTBF) or Power on Hours (POH) on their datasheets. One has to wonder if this is to avoid giving disgruntled customers ammunition for seeking recourse after disk failures.
Serial ATA drives (S-ATA) will soon take over from IDE systems. A clear indication of this is that ATX standard motherboards now have only one IDE connection for two drives, but four or more S-ATA connections. Serial-ATA is a high-speed serial bus with 150 MBytes in the S-ATA I standard. S-ATA II systems with double the transfer rate are already making headway in the market. They have many similarities with IDE drives: the mechanical structure, capacities, price/performance and manufacturer’s guarantee. Datasheets reveal, however, that these hard disks can be expected to have a shorter working life than the SCSI or Fiber Channel disks described below.
SCSI drives were developed for the world of servers and designed for robustness and speed. The parallel bus system gives control rights to up to 15 people at once. SCSI drives can read and write data in parallel, and have an “integrated verify” facility which simultaneously checks the written information and enables the writing process to be repeated if a fault occurs, thereby increasing the security of the data. They are fast because their internal magnetic disks rotate at up to 15,000 revolutions per minute and their reading and writing heads support very fast operation - currently 320 Mbytes/s. Their disadvantages are high cost (around four times as much as an IDE or S-ATA drive) and lower capacities: currently 300 GB max per hard disk.
Fiber Channel (FC) drives are very similar to the SCSI drives in terms of their mechanics. The serial bus achieves data rates of 4 GBit/s, i.e. around 500 MBytes/s. With 300GB, FC drive capacities are equivalent to SCSI ones but they can be more expensive. However, in contrast to other systems, FC drives allow the use of longer cables and other types of cabling including glass fiber, which can reduce or solve cabling problems in large security video systems. FC drives also offer the possibility of using FC switche
SO WHY DO DISKS FAIL?Temperature is the single most important factor in determining the working life of a disk drive. The failure rate of hard disks increases by about 2 percent for each degree Celsius above the standard temperature specified by the manufacturer. But the relationship is not quite linear, as 5oC over the standard operating temperature results in a 15 percent higher failure rate.
A premium choice of disk and high-quality operating conditions can reduce the expected failure rate of disks, but failure can never be completely ruled out; so it is important that appropriate contingency should be made for that possibility.
How much should disk failure worry chief security officers? The simple answer is that this depends on how much you stand to lose. If the potential loss is great then it makes sense to incorporate a significant level of redundancy into the system. This of course has a cost, and every user needs to resolve the security/costs/benefits question before settling on a specific system.
REASONABLE PRECAUTIONSIf you want to prevent candy bar thefts at a corner store, or fuel theft at the local gas station, then the potential financial impact of a hard disk failure is unlikely to justify investment in a security video system with an expensive data backup system. Reasonable precautions might involve using highly visible cameras to deter theft, and using a low cost software backup agent to export data from a DVR to another medium on a daily basis. Or, if additional data security is desired, then mirroring the internal hard disk (RAID-Level 1) would be adequate.
In a warehouse handling situation, with high value goods, the loss of a single pallet might represent $100,000. And, since losses can go unreported for several days or sometimes weeks, the warehouse management needs backup data covering an extended period of time. Instant search and access to all security video recordings by barcode or management data reference is clearly desirable if high tracking and investigation costs are to be avoided, and this would be supported by a high capacity RAID-5 solution. Further financial justification can be provided by the system’s ability to produce security video evidence relating to losses and claims, thus saving in insurance, labor and other costs. A system like this becomes a powerful management tool, which can be used to improve operations and procedures, eventually paying for itself within months.
In a casino, where the stakes are high, a security video system will typically include high-resolution cameras and real-time recording with instant replay for monitoring and scrutinization. In this environment it is easy to justify redundancy in the form of RAID-5 systems to provide short-term storage with instant access and protection against disk failure. However, for the long-term archive demanded by supervising authorities, the tape library is a more practical solution. Access is not so instant, but it can hold vast amounts of data.
Digital video systems which use the latest operating systems also offer other, more affordable, solutions developed in the IT world and which have already proved themselves in data processing centers. For instance using iSCSI (Internet Protocol and the Small Computer System Interface) technology and standard IT skills can build network attached storage and storage area network systems, which include switches for automatically rerouting iSCSI RAID system traffic to alternative paths to avoid any problems that may occur in the network.
Even though a DVR’s hard disk cannot be expected to last forever, there is no reason to lose sleep over its possible demise because there is a range of different ways of incorporating redundancy and protecting against significant data loss. These extend from a low-cost software package, via IDE/S-ATA based and iSCSI RAID systems and on up to fiber channel and IT systems with high levels of redundancy. But for every project the choice always depends on an evaluation of what the impact of disk failure might be and what investment is justified to mitigate against it.
SIDEBARCrime scene video is exploding; so are the concerns of law enforcement of its quality.
Video Forensics Processing From Days to Hours
A case in point: the Crime Scene Investigation (CSI) team in Weber County, Utah. Crime scene video is becoming a major source of evidence for law enforcement, and most Weber County’s CSI team video is from banks and convenience stores, making multiplexed video (a single video with several camera views within it) common. The difference between video and video that can be used as evidence is great. Images from video taken during a criminal act can be damaged, noisy, poorly exposed, multiplexed, low-resolution and shaky. Even worse, sometimes this video comes in a proprietary digital file format. Not every video forensics system addresses these issues, or to the level required for Weber County CSI prosecution.
Mitch Pillkington is a member of the CSI team with the Weber County Sheriff’s Office. Because of the complexity of the county’s existing video forensics system, his job mainly consisted of forensic video analysis. It would take Pillkington hours to demultiplex each image. This process converts the video containing several intermixed camera signals or overlapping images back into its original separate state. Complicated and time consuming, this can require up to four hours to process a single image, while the processed images did not always provide the quality needed to verify information. Thus, valuable time was lost – time that could have been spent identifying and apprehending criminals.
“For three years we had this system that we thought was the standard, and that’s how all systems worked,” said Pillkington. However his frustration led to further research that pointed toward several alternative video forensics tools. Pilkington then solicited demonstrations, so he could compare and choose the best solution overall.
“I used to have stacks of videos on my desk at the beginning of every week waiting for me and the machine to get to them,” said Pillkington. “I no longer have a backlog of videos.” The tool improves the ability to capture and export digital video, work with proprietary video data formats from security systems and print images directly. It can import an extensive range of digital video recorder file types including: .MPG, .WMF and .ASF; and can export as .MOV, .AVI and .WMF, for viewing through QuickTime and Windows Media Player.