Storage sizing and retention
From bitrate to terabytes, retention policy, recording modes, RAID and headroom, tiering and cloud.
- Convert bitrate and retention into storage capacity
- Account for recording mode, RAID, headroom and growth
- Explain retention policy drivers and archive tiers
The core formula
= bitrate (Mbps) × 10.8 × duty cycle
Total TB = Σ (GB/day per camera) × retention days ÷ 1,000
So 1 Mbps recorded continuously is about 10.8 GB per day, or roughly 324 GB per 30 days. This single rule of thumb lets you sanity-check any storage quote in your head.
Recording modes
| Mode | Description | Storage impact |
|---|---|---|
| Continuous | Records all the time | Highest; most predictable |
| On motion / on event | Records when motion or another event occurs, with pre- and post-event buffers | Lower; depends heavily on the scene (estimate a duty cycle, e.g. 20–50%) |
| Scheduled | Continuous during set hours, motion outside them | Mixed |
| Dual stream / tiered quality | High resolution for a short period, lower resolution or frame rate kept longer | Reduces long-term storage while keeping recent evidence detailed |
Motion-based recording looks cheap on paper but is risky to size: wind, rain, headlights and IR noise can trigger constant motion. Many enterprise designs record continuously and rely on smart codecs to keep quiet scenes small.
Retention policy
- Retention is a business, legal and regulatory decision: industry regulations, internal policy, insurance, and privacy law (which may set a maximum) all apply. Get it in writing from the customer.
- Common enterprise retention ranges from 30 to 90 days; some regulated environments require more. Do not assume; ask.
- Evidence that must be kept longer should be exported or protected (e.g. bookmarked/protected video) so it is not overwritten by normal retention.
- In Security Center, retention is configured per camera or through Archiver defaults; an Auxiliary Archiver can keep a second copy with different settings (for example a lower-quality stream kept longer, or a copy in another location).
RAID, headroom and growth
- RAID overhead: RAID 5 loses one drive's capacity, RAID 6 two, RAID 10 half. With today's large drives, RAID 6 (or equivalent erasure coding) is common for video because rebuilds take a long time.
- Free-space headroom: do not plan to fill volumes to 100%; keep roughly 10% free (check the VMS vendor's guidance).
- Decimal vs binary: drives are sold in decimal TB (10¹²) while operating systems often show TiB (2⁴⁰), which reads about 9% smaller.
- Growth: add capacity for future cameras and higher resolutions (often 20–30%).
- Write performance matters as well as capacity: many cameras writing simultaneously is a sustained write workload. Use storage validated by the VMS vendor.
Worked example
Office building: 50 cameras, 30 days, continuous
50 cameras, measured average 4 Mbps each, continuous recording, 30-day retention.
- Per camera per day: 4 × 10.8 = 43.2 GB.
- All cameras per day: 43.2 × 50 = 2,160 GB = 2.16 TB/day.
- 30 days: 2.16 × 30 = 64.8 TB of video.
- 10% free-space headroom: 64.8 ÷ 0.9 = 72 TB usable.
- RAID 6 with 12 drives (10 of 12 usable = 83.3%): 72 ÷ 0.833 ≈ 86.4 TB raw → 12 × 8 TB drives = 96 TB raw = 80 TB usable. ✔
- Add growth: if 20% more cameras are expected, step up to 12 × 10 TB (100 TB usable).
Same site with motion recording at a 40% duty cycle: 64.8 × 0.4 = 25.9 TB of video, but only if the 40% estimate holds. Validate with a pilot.
Try it: bandwidth and storage calculator
Decimal units (1 TB = 1,000 GB), matching how drives are sold. Operating systems often show binary TiB, which reads about 9% smaller. Use measured bitrates from a pilot camera whenever possible.
Edge, on-prem and cloud storage
| Option | Description | Use when |
|---|---|---|
| Edge (SD card / camera storage) | Camera records locally; VMS can pull the recording later | Failover during network outages, small remote sites, low-bandwidth links |
| On-prem Archiver storage | Direct-attached, NAS or SAN storage on VMS servers | Most enterprise deployments; predictable cost and performance |
| Cloud / hybrid | Video kept or copied to cloud storage, sometimes via an on-site appliance | Long retention, distributed sites, reduced on-site hardware, off-site resilience |
Genetec Security Center supports edge recording with archive transfer (copying recordings from edge devices or between Archivers on a schedule or on demand). In Security Center SaaS, Genetec Cloudlink appliances provide on-site recording with cloud-managed configuration. Details in the VMS architecture module.
Common mistakes
- Using datasheet maximum bitrate (overkill) or a guessed bitrate (too little) instead of a measured average.
- Forgetting RAID overhead and free-space headroom.
- Confusing TB and TiB in the quote.
- Assuming a motion duty cycle without testing the scene at night and in bad weather.
- Not asking for the customer's written retention requirement, including legal maximums.
- Choosing storage on capacity alone without checking sustained write performance.
Check your understanding
1.Roughly how much storage does 1 Mbps of continuous recording use per day?
2.20 cameras at 2 Mbps, continuous, 30 days. Raw video total?
3.RAID 6 on 12 drives leaves how much capacity usable (ignoring filesystem)?
4.Why is motion-based recording risky to size?
5.Who decides retention, and what can limit it besides cost?