I cannot find a 'Record dual-streams if available' option in my BI version 184.108.40.206. Is that option only available in an older or newer version of BI? Is this in any way related to the first quote above from @fenderman? That is, does my version simply always record the substream as well? Or is that comment related to the CAM number (actual different camera within the same enclosure) and not the substreams?
In 5.3.0 when the sub stream recording was added, the option didn't exist yet so it just always recorded the sub stream if your chosen format was BVR. I'm not sure when the option was added. You'll find the option near the bottom of the camera properties > Record tab.
I would strongly recommend recording the dual streams. If you have the substream set to a low bitrate (mine or 256), you will be glad you did if you ever try to do a playback of a cascade tile of watching all your cameras playback at the same time. If you have all your cameras showing playback at the same time with the mainstream, many systems will struggle playing back unless you have few cameras or a screamer system. If you then select playback on one camera, it will show the mainstream then.
But the advantage to running substream helps drop CPU during live recording, but the recording substream helps on playback and watching many cameras at once. Most do not realize that is the benefit. But many also have no need to watch multiple cameras playback at the same time so then it doesn't matter.
The recorded sub stream can also make seeking much more responsive in UI3 (especially since UI3-140 released last night). Loading random video frames from a 4K 30fps H.265 video can be really slow, but from a sub stream it is fast. The downside is sometimes the main and sub streams sent by the camera are out of sync by a few hundred milliseconds.
Thanks for your replies. I guess the option to enable/disable substream recording is indeed simply not in my current version of BI as I suspected. Whenever I upgrade and find that option exists I'll be sure to enable it.
I currently have the Substream set to 352x288(CIF), the difference it makes to CPU usage for recording is pretty extreme, I went from 80% to 10%. Awesome stuff. I find that CIF looks good enough with multiple cameras showing onscreen.