One of the most important functions of your video recorder is the storage and archiving of recorded footage for later playback and evidence collection. As a prudent user, you should assess your storage both when you first acquire your unit and on an ongoing basis to determine if the current storage meets your requirements. We will address the following questions with this article.
Where do you find the information required to assess your system and how to interpret it?
What is the recording capacity of your system?
How should you assess the recording capacity on the basis of time?
1) Checking your system’s HDD status
All Swann recorders have a status page where you can examine the details of the HDD(s) connected. Some also have the ability to interface with the HDD’s internal reporting mechanism (called SMART) which can alert the user to possible faults before they compromise recording.
While we will not describe the exact procedure here to access the HDD page (refer to your system documentation) we will show our 2 most common recorders’ HDD pages so you are familiar with the layout.
The information presented is fairly basic, usually containing just a label for each HDD installed, the Total Capacity of each drive and the Free Space left on each drive. For multiple drives, you can usually just add up each Total Space indicator to get the total recording space available. You need this information to calculate available recording time.
You will notice that the total space displayed for each drive does not appear to match up exactly with the HDD capacity that was provided by our information or by the HDD manufacturer (if you installed your own HDD). This is because the HDD manufacturers calculate capacity using decimal notation, whereas a computer file system (and therefore your DVR) calculates file system capacity in binary. Unfortunately, binary and decimal does not match up exactly, so the space reported will be slightly less due to this. Also, file system overheads consume some available HDD space which combined with the binary-decimal differences result in a number that is a bit smaller than the capacity that the manufacturer assigns to the HDD. When we list the HDD capacity on our product information, we use the figure that is provided by the HDD manufacturer.
1a) SMART information
S.M.A.R.T., or Self-Monitoring, Analysis and Reporting Technology, is a feature of modern HDDs to alert the user to potential future hardware failures. It is possible to determine from performance metrics when a HDD is more likely to fail, although this is not always possible with all failures. A thorough explanation on S.M.A.R.T. would be too long for this article. There are many on-line resources that explain what S.M.A.R.T. does and how to read the output to determine if there may be a problem.
2) Recording settings; which ones affect HDD usage?
The primary recording setting that affects your HDD space consumed is the recording bit rate set on the camera channel. Each camera will usually have the option of setting individual bit rates, but it is recommended that you use a consistent bit rate across all channels for ease of use.
It is a common error to consider FPS (frames/second) as affecting the space consumed by recording. FPS does not directly translate into HDD consumption, although by setting a lower FPS you can potentially achieve a higher image quality on any particular bit rate. This would enable you to set a lower bitrate and achieve similar image quality.
3) What is the recording capacity of your system?
While technically Mb/s (Megabits per second or bit rate) can also be used as a measure of storage used over time, it’s not very useful to us when making calculations. While the marketing material will provide for X days of recording, a general ‘days’ figure may not address your recorder’s usage pattern. To be precise, you will need to make your own calculations.
Firstly, we need to convert the bit rate into a more usable unit. For this, we select MB/h (Megabytes per hour). This is best suited for determining how many hours of recording our storage solution will hold and can then be used for calculation of days of recording taking into account various other factors.
Let’s look at the formula for this calculation:
MB/h = Mb/s x 60 x 60 ÷ 8
Firstly, we have to multiply by 60 twice to convert from seconds to hours, then divide by 8 to convert from bits to bytes. We can consolidate that formula, and if we do so:
MB/h = Mb/s x 450
Therefore, we can see that by strict calculation, each 1Mb/s will equal 450MB/h. In practice, with compression and variable bit rate encoding, the figure is closer to 400MB/h but 450Mb/h is a perfectly usable and easy figure to remember.
Now, we take that figure and multiply our camera bit rate to get the usage of one camera for an hour. We’ll use a popular example, the default bit rate for the NHD-815 3-megapixel camera:
Camera storage utilisation (MB/h) = 5(Mb/s) x 450(MB/h)
This works out at 2250MB/h or just under 2.2GB/h. HD cameras use a lot of space!
NB: To convert from MB/h to GB/h, you divide by 1024, not 1000.
Once we have a figure for the amount of space used by a camera per hour, we can then divide our total storage by that figure to arrive at a figure for Camera Hours. Quite simply, the amount of hours of recording for one camera that will fit on the installed storage. Once you have that then you can adapt the figure to any number of cameras and time requirements.
Example: A standard 1TB HDD, after formatting and file system overhead, has approximately 920GB space available. Dividing 2.2GB into that yields a figure of 418 camera hours.
4) Estimating total storage time and determining storage requirements based on recording time required
Now that we have a solid understanding of how to calculate the storage we have available, we can proceed to estimate how many days of recording we can achieve. We can also use this information to determine a total storage requirement based on a number of days of recording requirement.
Using the camera hours figure, we can determine an estimate of days of recording using any number of cameras. Simply divide by 24 to convert hours to days, then divide that by the number of cameras attached to get the final figure.
Example: Using the previous example previous of 418 camera hours for the 1TB HDD with NHD-815 cameras we will calculate the days of recording for 8 cameras:
418 (camera hours) ÷ 24 ÷ 8 cameras = 2.2 days of recording
The important element to note here is that we are calculating based on the cameras recording 24 hrs/day, i.e. that the cameras are recording non-stop. However, this discounts one of the most important and useful features of a recorder, motion detection. Using motion detection allows us to capture footage only when activity is occurring in front of the cameras. This means that you aren’t recording a lot of footage that has no use.
Example: Let’s do the same calculation again, but change the number of hours/day that the cameras record to 4 hours:
418 ÷ 4 ÷ 8 = 13.1 days of recording
That’s much better right? You may be thinking that 4 hours/day doesn’t seem like a substantial amount of recording. However, don’t forget that under motion detection recording not all cameras will be recording at the same time nor will they record for a full hour for each event. Some recorders have very short event time triggers so you may have events that last only a minute or 2. 4 hours/day estimates that each of your cameras will be recording about 16% of the day. This is a commonly accepted figure to calculate from when dealing with CCTV. Any figure is possible to calculate on and depends on how much activity is expected to be recorded. You can also change the bit rate to reduce the storage used per hour for the cameras, although that needs to be balanced against the desired quality of the recording.
Now that we have worked out how many days are estimated to be captured, it’s an easy task to determine if you need any additional storage and how much. Using the previous days of recording example, if we wanted 30 days of storage with all the settings already made, we would need to use a 3TB HDD.
30 days ÷ 13.1 days/TB = 2.29TB
As you can see, each calculation builds on the last to generate a profile of your system and determine your storage needs according to the requirements you set on the system.
5) Which HDD should you use to expand your recorder?
There are a number of HDD manufacturers that produce a wide variety of devices for different purposes. Of these variants, Swann uses and recommends HDDs that are specifically suited for surveillance recorders.
We use HDDs from two of the major manufacturers of HDDs, Western Digital and Seagate. For the Western Digital line of drives, the surveillance variant is currently labelled the 'WD Purple' family of drives. For Seagate, the surveillance variant is the 'Video 3.5' family of drives. Both families are perfectly suited to a surveillance purpose, being low-heat, low-power, high-reliability drives. Please ensure that your Swann recorder is capable of utilising the size of drive you plan to upgrade or add before you acquire a new HDD.
This concludes our article on HDD capacity. Hopefully, you find it useful in determining the best system for your needs.