CCTV Recorder Upgrade at a Garden Centre
A garden centre has 16 IP cameras. Each streams H.264 video at an average 4 Mbps, 24 hours a day. The manager wants 30 days of continuous recording. The existing recorder has a 12 TB hard disk, a dual-core CPU, 4 GB RAM and one 1 Gbps network port; it is also used as the staff file server. Recordings currently overwrite after about 17 days, and playback freezes when three staff copy files across the network. A supplier proposes replacing the 12 TB disk with a single 24 TB disk and adding a second 1 Gbps network port. The centre's insurance policy requires that recorded footage is protected against a single disk failure. The manager asks whether the proposal is sufficient and whether SSDs would be better. Electricity costs £0.28 per kWh; the recorder runs continuously.
1. Calculate the storage required for 30 days of continuous recording and compare it with the existing and proposed disks. State whether the supplier’s single 24 TB disk meets the requirement.
Read the worked answer
Using decimal units, 4 Mbps = 4,000,000 bits per second ÷ 8 = 500,000 bytes per second = 0.5 MB per second. Per camera per day: 0.5 MB/s × 86,400 s = 43,200 MB = 43.2 GB. For 16 cameras: 16 × 43.2 = 691.2 GB per day. For 30 days: 691.2 × 30 = 20,736 GB = 20.736 TB.
The existing 12 TB disk would hold 12,000 ÷ 691.2 ≈ 17.4 days, which matches the observed overwrite after about 17 days. A single 24 TB disk would hold 24,000 ÷ 691.2 ≈ 34.7 days, so it has enough capacity for 30 days.
However, the insurance policy requires protection against a single disk failure. One 24 TB disk gives no redundancy; if it fails, all footage is lost. The proposal is therefore insufficient even though the capacity calculation passes. A suitable design would provide at least 20.736 TB of usable space after one failure, for example two 24 TB disks in RAID 1 (24 TB usable), or three 12 TB disks in RAID 5 (24 TB usable), or four 12 TB disks in RAID 6 (24 TB usable and two-failure tolerance). RAID is not a backup: a separate offline or offsite copy is still needed for fire, theft or ransomware.
2. Explain why adding a second 1 Gbps network port may not stop playback freezing, and identify the more likely hardware bottleneck.
Read the worked answer
The camera load is 16 × 4 Mbps = 64 Mbps inbound, which is only about 6.4% of one 1 Gbps port. Even with protocol overhead and bursts, the network is not the likely limit. Adding a second 1 Gbps port does not help if the same disk and CPU are serving both the recordings and the staff file copies.
The recorder is performing continuous sequential writes from 16 cameras while staff copy files and someone plays back video. Playback needs reads from the same disk, so the disk head has to move between write and read areas. That causes queueing and visible freezing. The dual-core CPU and 4 GB RAM may also be too small for indexing, file sharing and video serving at the same time. The symptom appears specifically when three staff copy files, which points to disk I/O contention rather than network saturation.
Before buying hardware, measure disk queue length, IOPS, CPU utilisation and RAM pressure. A better fix is to separate the surveillance recorder from the general file server, use surveillance-grade disks, put recordings on a dedicated volume or RAID array, and add RAM for caching. A second network port would help only if measurements showed the network link itself was saturated, which the 64 Mbps camera total does not suggest.
3. The manager asks whether SSDs would be better than hard disks for this recorder. Give a reasoned recommendation covering capacity, cost, endurance and reliability.
Read the worked answer
SSDs would improve random read performance and make playback and file copying more responsive. They use less power and have no moving parts, so they are quieter and less sensitive to vibration. But the decision is not one-sided.
The system needs over 20 TB of usable storage after redundancy. Large SSDs are much more expensive per terabyte than surveillance hard disks, so an all-SSD array would be costly. Endurance is also a concern. The recorder writes about 691.2 GB per day, or 252 TB per year. A consumer SSD rated at 600 TBW would be exhausted in roughly 2.4 years, before the footage-retention requirement has been met comfortably. High-endurance enterprise SSDs exist, but they increase cost further.
The power saving is small relative to purchase price. A 5 W saving for 24 hours a day for a year is 43.8 kWh, about £12.30 at £0.28 per kWh; even a 10 W saving is about £24.50 per year. That will not repay a large SSD premium quickly.
A sensible recommendation is to use SSDs only for the operating system, indexing or a read cache, and use surveillance-grade hard disks in RAID for bulk video. That gives fast playback responsiveness where it matters while retaining capacity and single-disk-failure protection at reasonable cost.