It seems many sites like YouTube suggest moov atom at the front of the file (Fast Start).

ffmpeg does not make this a default behavior, but you can specify it with the -movflags faststart option. I'm wondering if there is any downside to always using this parameter?


Only downside is it can take a few more seconds

Depending on the size of your input it can take a few more seconds usually to perform the second pass to move the moov atom to the beginning of the file.

Example of re-muxing a 1 hour video:

Without -movflags +faststart

ffmpeg -i input.mkv -c copy output.mp4

With -movflags +faststart

ffmpeg -i input.mkv -c copy -movflags +faststart output.mp4

Note this option is only for MP4, M4A, M4V, MOV output.


I guess this thread needs an update. On latest ffmpeg (3.4.1) I get:

$ time ffmpeg -y -f lavfi -i nullsrc=s=hd720:d=600 -preset ultrafast out.mp4
real 0m26.578s
$ time ffmpeg -y -f lavfi -i nullsrc=s=hd720:d=600 -movflags +faststart -preset ultrafast out.mp4
real 0m26.849s

Same results. Now try with a real video:

$ time ffmpeg -y -i Sintel.2010.1080p.mp4 -preset:v ultrafast out.mp4
real 3m38.829s
$ time ffmpeg -y -i Sintel.2010.1080p.mp4 -preset:v ultrafast -movflags +faststart out.mp4
real 3m43.674s

About 2% difference, that could be just noise. Also need to note that "Starting second pass: moving the moov atom to the beginning of the file" phase took no more than couple seconds on 600Mb output file.

  • Agreed. I have a 50MB input (mjpeg stream) that is being encoded to mp4 with h264. Without faststart real 0m30.676s. With faststart real 0m30.599s. My ffmpeg version is 4.2. I guess it's safe to say that this doesn't make a big difference anymore, if any at all. – confetti Oct 21 '19 at 0:36
  • On a local hard drive for a short video it shouldn't make any significant difference. It might make a significant difference on certain mediums that are slow to seek or where seeking is impossible (such as when piping output). – thomasrutter Jul 21 '20 at 23:54

As you may already know, the information required to write the moov atom or even to know the size of the moov atom isn't available until after the entire file is processed.

The drawbacks to having the moov atom at the start, and the reason many tools don't do it by default, are therefore all related to this fact.

If none of the following are a problem for you, then you can put the moov at the start all times and have no drawbacks.

  • A second pass is required. This potentially doubles the amount of disk I/O as data must be read from input, written to disk, then read from disk and re-written. This can be impractical where I/O speed is very slow, such as when writing to a network drive.

    Note that some software may optimize this by roughly estimating the required size of the moov atom near the start of encoding and reserving this much space, plus a margin of error, at the start of the output. In many cases this would remove the need to read-back and re-write the rest of the file and instead seek to the start and overwrite just a small part, at the cost of using slightly more space for the moov atom than necessary.

  • Output cannot be piped to another command or sent to stdout on the fly, because those mechanisms have no way of doing a second pass.

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