[FFmpeg-user] Why is concat so slow on a powerful machine?

[Christian Johannesen]

On Thu, Oct 19, 2017 at 11:51 AM, Kevin Duffey <
kevinmduffey-at-yahoo.com at ffmpeg.org> wrote:

>  So I would assume that if I can copy the two files in a matter of seconds
> from one dir to another on the SSD, that copying them together would be
> similar speed. However I did try that and it seemed to hang.. or appear to
> do nothing. I unfortunately had to stop it but if it was working, it was
> taking a long while as well.
> Which now has me wondering..why? If it is just copying two files into
> one.. why is that way way slower than copying the two files to another
> location individually? A copy is a copy. It is not rendering.. it is merely
> moving the bits to another location (er.. copying them) no different than
> copying the two files individually.
> Is there any other setting for ffmpeg, like I should specify the output
> codec, or something.. to ensure it is copying and not rendering?
>
>
I tested this using FFmpeg running on a windows server with fiber channel
attached storage. I am able to duplicate the input file at a rate of
460MB/sec. I tested joining two 6 minute 48 second DNxHR SQ 8bit sources at
3840x2160 23.98 fps with this command: ffmpeg.exe -f concat -safe 0 -i
C:\FFMPEG\concat.txt -c:v copy -c:a copy
C:\Volumes\PTLabSAN\ChrisJ\DNxHR\out.mov. With this process, I achieved 126
fps. I would suspect the storage as the concat process only used 5% of my
CPU.

  Duration: N/A, start: 0.000000, bitrate: 463496 kb/s
    Stream #0:0(eng): Video: dnxhd (DNXHR SQ) (AVdh / 0x68645641),
yuv422p(tv, bt709, progressive), 3840x2160, 461960 kb/s, SAR 1:1 DAR 16:9,
23.98 fps, 23.98 tbr, 24k tbn, 24k tbc
    Metadata:
      creation_time   : 2017-10-19T17:06:59.000000Z
      handler_name    : Apple Alias Data Handler
      encoder         : DNxHR/DNxHD
      timecode        : 00:00:00:00
    Stream #0:1(eng): Audio: pcm_s16le (sowt / 0x74776F73), 48000 Hz,
stereo, s16, 1536 kb/s
    Metadata:
      creation_time   : 2017-10-19T17:06:59.000000Z
      handler_name    : Apple Alias Data Handler
      timecode        : 00:00:00:00
    Stream #0:2: Unknown: none
Output #0, mov, to 'C:\Volumes\PTLabSAN\ChrisJ\DNxHR\out.mov':
  Metadata:
    encoder         : Lavf57.71.100
    Stream #0:0(eng): Video: dnxhd (DNXHR SQ) (AVdh / 0x68645641),
yuv422p(tv, bt709, progressive), 3840x2160 [SAR 1:1 DAR 16:9], q=2-31,
461960 kb/s, 23.98 fps, 23.98 tbr, 24k tbn, 24k tbc
    Metadata:
      creation_time   : 2017-10-19T17:06:59.000000Z
      handler_name    : Apple Alias Data Handler
      encoder         : DNxHR/DNxHD
      timecode        : 00:00:00:00
    Stream #0:1(eng): Audio: pcm_s16le (sowt / 0x74776F73), 48000 Hz,
stereo, s16, 1536 kb/s
    Metadata:
      creation_time   : 2017-10-19T17:06:59.000000Z
      handler_name    : Apple Alias Data Handler
      timecode        : 00:00:00:00
Stream mapping:
  Stream #0:0 -> #0:0 (copy)
  Stream #0:1 -> #0:1 (copy)
Press [q] to stop, [?] for help
frame=19576 fps=126 q=-1.0 Lsize=46196179kB time=00:13:36.48
bitrate=463500.7kbits/s speed=5.26x
video:46042752kB audio:153090kB subtitle:0kB other streams:0kB global
headers:0kB muxing overhead: 0.000728%