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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?

3 Answers 3

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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
0m11.695s

With -movflags +faststart

ffmpeg -i input.mkv -c copy -movflags +faststart output.mp4
0m12.252s

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

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  • 3
    In the latest version of ffmpeg (3.x+ or 4.x+), the correct syntax is -movflags faststart indeed (without the + sign). See: ffmpeg.org/ffmpeg-formats.html#Options-8 Dec 23, 2018 at 14:59
  • 1
    @SiuChingPong-AsukaKenji- It doesn't really matter if you include the + or not.
    – llogan
    Dec 26, 2018 at 0:17
  • So it doesn't apply to .mkv output? I don't get an error, so does that mean it's simply being ignored? Mar 9, 2021 at 2:07
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    @HashimAziz This option is only for the MOV family such as MP4, M4A, M4V, MOV. It will get ignored for anything else.
    – llogan
    Mar 9, 2021 at 20:30
  • 1
    @HashimAziz As far as I know, there is no global-metadata block in MKV, so no need to move it to the start of the file. In MKV, metadata such as time-index is attached to each chunk of the file, which means skipping to a desired time-index is more or less a game of guessing a byte-offset first, and verifying the nearest time-index. Of course, this is an imprecise and slow process, and it would be quite similar for MP4 without "faststart" optimization (there are subtle differences, though).
    – Robin479
    Feb 13, 2022 at 12:37
18

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.

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  • "About 2% difference, that could be just noise." Could be. One can tell whether it's noise by repeating it and seeing if the difference happens again without having varied the parameters. Here, both times the fast start is slower, so I would be inclined to say that it's still slower (not just noise) even if not noticeable on the scale of 12 seconds as in the accepted answer. What would be interesting is whether it's exponential, since it's 2.2% in the ~3.7 minutes example and only 1% in the 27s example. A 1-hour video being 100% slower would be kind of annoying.
    – Luc
    Jan 8 at 9:42
18

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 or seeking backwards.

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  • You wrote: "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". Is ffmpeg such a software?
    – Totor
    Feb 18, 2021 at 23:00
  • Ffmpeg looks like it has a -moov_size bytes option on the MP4 muxer to reserve a certain amount of bytes for the moov at the start of the file. I don't know exactly how it interacts with -movflags faststart. I would assume if you use just moov_size bytes, it should work like you want. But you'd need to test. Also, estimating the size of the moov atom yourself sounds a bit hit and miss. Feb 19, 2021 at 0:34

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