Why does recording a game with Fraps decrease performance?

Question

I've run into some performance problems whilst recording some gameplay videos for Youtube.

As I understand, Fraps saves the current frame pixel for pixel in a video file (in a way similar to a RAW file). What I don't understand is why the game's performance decreases with it (as an example, my FPS drop from ~40 to unplayable).

I can understand that loading new content etc. would slow down while recording, because the HDD is busy writing data, but the game I tried loads the complete map/character/objects etc. into the RAM and the HDD is idle from then (which I could prove myself by some testing).

My second thought was that I could simply use an external device which I would plug into the HDMI output and set it to duplicate. As the screen resolution is the same I would say there is no performance loss.

Why does the game's performance decrease when recording it and would my second thought be viable?

Answer 1

I can understand that loading new content etc. would slow down while recording, because the HDD is busy writing data

This is exactly what is happening with screen recording software.

You are capturing often up to 30 files of (likely) uncompressed image data every second. 30 large files depending on resolution and color space settings (1366 X 768 = 1,049,088 pixels, 1920 x 1080 = 2,073,600 pixels -- plus the extra 3-4 bits per pixel for color data). This kind of data writing slows everything down, which is why one of the recommendations when screen recording is to capture to a second HDD (not another partition, a second physical disk which doesn't house the OS/program/game).

As far as a second device is concerned, if you could pipe a picture to it and have that second device record the action, then using Fraps as a utility is likely a moot point.

A decent post on Fraps optimization is here.