Eventually, it always happens with a technology discussion; we get to the alphabet soup section. Although the need for storage of your surveillance is evident, it is not always clear which solution is right for your organisation.
There are a variety of storage options available, the most prevalent being direct-attached storage (DAS), network-attached storage (NAS) and storage area networks (SAN). Several key criteria to consider include:
* Capacity – the amount of data (images) that needs to be stored and shared.
* Performance – I/O and throughput requirements.
* Scalability – long-term surveillance system growth.
* Availability and reliability – how important are your recordings?
* Data protection – backup and recovery requirements.
* IT staff and resources available.
* Budget concerns.
While one type of storage media is usually sufficient for smaller companies, large enterprises will often have a mixed storage environment, implementing different mediums for specific departments, workgroups and remote offices.
DAS: ideal for local data sharing requirements
Direct-attached storage (DAS) is the most basic level of storage. The storage devices are part of the host computer, like the drive in your laptop, or directly connected to a single server, as you typically find in a small office, in which the workstation must access the server (typically one of the workstations) in order to connect to the storage device.
This is in contrast to networked storage such as NAS and SAN, which are connected to workstations and servers over a network. That is what you find being used in large companies. For all the local user knows, the storage devices could be in Singapore or San Francisco.
In the DAS model, if the server is down or experiencing problems, users cannot store and access data. Surveillance images cannot be retrieved or stored. If the organisation grows and needs new servers, storage for each server must be administered separately. That can get quite messy. They will want to move to a networked storage solution as quickly as possible.
Thankfully, organisations transitioning to networked storage can protect their investment in their legacy DAS system. The most popular option is to place it on the network via a bridge device. As a result, the current storage resource can be used in a new networked infrastructure without incurring the immediate costs of networked storage. Once the transition is made, DAS can still be used locally to store less critical data, such as keeping images from the cameras watching over the building’s hallways.
NAS: file-level data sharing across the enterprise
For those systems that must store a large amount of images for many days, NAS is a good option. Network-attached storage (NAS) is a special purpose device, comprised of both hard disks and management software. It is 100% dedicated to serving files over a network. As a result, NAS relieves the server of storage and file serving responsibilities and provides a lot more flexibility in data access by virtue of being independent.
NAS and storage area networks (SAN) are sometimes competitive when creating a system but just as likely to become part of a NAS/SAN convergence scheme. High reliability features, such as RAID and hot swappable drives and components (covered below), are standard even in lower end NAS systems, while midrange offerings provide enterprise data protection features such as replication and mirroring for business continuance.
NAS is an attractive investment that provides tremendous value, considering that the main alternatives are adding new servers, which is an expensive proposition, or expanding the capacity of existing servers, a long and arduous process that is usually more trouble than it is worth. NAS systems can provide many terabytes of storage in high-density form factors, making efficient use of data centre space.
SAN: high availability for block-level data transfer
A storage area network (SAN) is a dedicated, high-performance storage network that transfers data between servers and storage devices, separate from the local area network. With their high degree of sophistication, management complexity and cost, SANs are traditionally implemented for mission-critical applications in the enterprise space.
In contrast to DAS or NAS, which are optimised for data sharing at the file level, the strength of a SAN lies in its ability to move large blocks of data or images. This is especially important for bandwidth-intensive applications such as large IP/megapixel camera system recording. The distributed architecture of a SAN also enables it to offer higher levels of performance and availability than any other storage medium. By dynamically balancing loads across the network, SANs provide fast data transfer while reducing latency and server workload. Large numbers of users can simultaneously access images without creating bottlenecks on the local area network and servers.
SANs ensure predictable performance and 24x7 data availability and reliability. They also offer excellent scalability for large enterprises that anticipate significant growth in surveillance storage requirements. And unlike DAS, excess capacity in SANs can be pooled, resulting in a very high utilisation of resources.
There has been much debate in recent times about choosing SAN or NAS in the purchasing decision, but the truth is that the two technologies can prove quite complementary. Today, SANs are increasingly implemented in conjunction with NAS. With SAN/NAS convergence, companies can consolidate block-level and file-level data on common arrays.
What are RAID 1 and RAID 5?
RAID (redundant array of independent disks – originally redundant array of inexpensive disks) is often used as an umbrella term for computer data storage schemes that can divide and replicate data among multiple physical drives. It is a storage technology that provides increased reliability and functions through redundancy. Multiple disk drive components are combined into a single logical unit. Data is distributed across the drives.
Surveillance systems tend to use either RAID 1 or RAID 5. RAID 1 data is written identically to multiple drives, thereby producing a ‘mirrored set’. The array continues to operate as long as at least one drive is functioning. RAID 5 distributes parity along with the data and requires all drives, but one to be present to operate. Thus, the array is not destroyed by a single drive failure.
Assuring continuous recording
Hot swapping is used when one wants to change the configuration or repair a working system without interrupting its operation. It may simply be for convenience, to avoid the delay and nuisance of shutting down and then restarting complex equipment, or because it is essential that the equipment be permanently available.
For instance, hot swapping may be used to add or remove hard drives without interrupting equipment operation. For example, computer RAID disk arrays allow a faulty disk to be hot-swapped for a new one; the new one is configured to become part of the array automatically or by user command.
Options to increase time stored
In the perfect world, you would be able to save all recordings forever; assuring that key evidence would always be available. However, budgets dictate otherwise. Therefore, some simple options that let recordings be stored longer include adding more drives to the system. Many lower their frame-rate over time, with older recordings being kept at a lower frame-rate than newer recordings. For example, video can initially be viewed and stored at 8 fps. Then after a week, every other frame is deleted, leaving 4 fps of storage. A week later, the same thing occurs, leaving 2 fps, and so on.
Storing surveillance recordings is like operating any complex filing system. The size of the container must fit the application. Unfortunately, no practical universal storage medium exists and all forms of storage have some drawbacks. And, we can never forget a key role for the recordings – they must stand up in court. They must be crisp and be able to show that they have experienced no tampering.
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