How to calculate the hard disk size for the archive and what HDD types can I use?
Every camera has a preset video value of a stream "density", which is called bitrate. It determines the video stream quality (don’t mix it up with megapixels), and it is measured in megabits per second (Mbit/s). The higher the bitrate, the higher is the load on the server and the network. The total bitrate of all cameras is a good starting point for calculating infrastructure expenses, including disks.
From our experience, a 1 Mbit/s stream is the optimal bitrate for client’s service that transmits video through the Internet. It meets reasonable project expenses for building the infrastructure and provides a good quality image. If your cameras have 4 Mbit/s or more set by default, we recommend that you change the stream to 1 Mbit/s, unless your project requires extra-high quality images. You can do that via the web interface or a camera config.
1 Mbit/s camera recordings take about 10 GB of storage space per day. A 2 Mbit/s stream will sum up into 20 GB per day, etc.
For example, if you want to store a 7-day archive, recorded with a 1 Mbit/s camera, it is necessary to allocate 70 GB at least. If your system includes 500 cameras of this type, you will need 35 TB of disk space.
Disk Requirements: 7200 rpm, SSD (for cache) + HDD for recording.
You can get the number of disks required if you divide a total video stream bitrate (reading and writing) by the drive speed. It’s important to consider that the drive speed can accidentally fall during the constant recording and reading which is a periodical and short-time change.
A hard drive read/write head movement speed is a bottleneck. For example, your disk works at a 100 MB/s speed, but it may produce 3-4 times lower speed for this task because video streams from multiple cameras are recorded into different disk locations, and the disk head jumps all the time. It generates a significant gap which leads to the real speed of 20-30 MB/s. If we convert megabyte per second to megabits, it’ll respectively be 240 Mbit/s for an expected bitrate, and 160 Mbit/s for a real bitrate. For example, one of these drives will let you write an archive from 150-200 cameras at 1 Mbit/s effectively, and provide access to view the video archive to 40 users simultaneously.
Let’s suppose that you have a server with 500 cameras (a total of 500 Mbit/s or ~ 65 MB/s), and your drive has the read/write speed of ~25 MB / sec. In this case, you’ll need three drives at least to meet the disk speed requirements.
You can select the drive size and speed if you know the camera bitrate and the number of users (which is more important during peak hours).
A system for this example includes 500 1 Mbit/s cameras and a 7-day archive, which requires 35 TB total storage, divided into 3-4 disks. Large hard drives are expensive, so we’d better pick nine 4 TB drives, combined into RAID5. It’ll provide sufficient disk speed and storage capacity.
Recommendation: Hard drive prices might go crazy after 4 TB. It’s unreasonably expensive at this time, and you can save a lot if you select disks up to 4 TB.
Here you can see that 100 MB/s, for this project it gives about 3-5 MB/s.