Periodic frame losses when recording to btrfs volume with OBS

[Sebastian Ochmann <ochmann@cs.uni-bonn.de>]

Hello,

I would like to describe a real-world use case where btrfs does not 
perform well for me. I'm recording 60 fps, larger-than-1080p video using 
OBS Studio [1] where it is important that the video stream is encoded 
and written out to disk in real-time for a prolonged period of time (2-5 
hours). The result is a H264 video encoded on the GPU with a data rate 
ranging from approximately 10-50 MB/s.

The hardware used is powerful enough to handle this task. When I use a 
XFS volume for recording, no matter whether it's a SSD or HDD, the 
recording is smooth and no frame drops are reported (OBS has a nice 
Stats window where it shows the number of frames dropped due to encoding 
lag which seemingly also includes writing the data out to disk).

However, when using a btrfs volume I quickly observe severe, periodic 
frame drops. It's not single frames but larger chunks of frames that a 
dropped at a time. I tried mounting the volume with nobarrier but to no 
avail.

Of course, the simple fix is to use a FS that works for me(TM). However 
I thought since this is a common real-world use case I'd describe the 
symptoms here in case anyone is interested in analyzing this behavior. 
It's not immediately obvious that the FS makes such a difference. Also, 
if anyone has an idea what I could try to mitigate this issue (mount or 
mkfs options?) I can try that.

I saw this behavior on two different machines with kernels 4.14.13 and 
4.14.5, both Arch Linux. btrfs-progs 4.14, OBS 20.1.3-241-gf5c3af1b 
built from git.

Best regards
Sebastian

[1] https://github.com/jp9000/obs-studio