Clarify mov faststart

[m-sundman]

I have a lot of issues to go through, so in order to make it easier for me to help you, I ask that you please try these things first

• Try with the newest version from GitHub
• Read the documentation and FAQ, Known issues, Troubleshooting
• Search for your problem under Issues or Discussions
• If you have a problem only with a particular file (or file type), use this form instead
• Windows 7, 8 and 8.1 are no longer supported. This is not a question about Windows 8.1 or older.

Operating System

Linux

Steps to reproduce

1. Click the (?) in Export options -> Enable MOV Faststart, showing "Enable this to allow faster playback of the resulting file. This may cause processing to take a little longer."
2. Enable MOV Faststart.
3. Export a large video, perhaps to a (slow) network drive.

Expected behavior

The "processing to take a little longer".

Actual behavior

The processing taking several hours longer. In fact, it takes 3 times as long as without it, because after the initial export it will re-read, and re-write, the whole exported file again, thus tripling the I/O of the exported file.

My suggestion is to change the help text from "Enable this to allow faster playback of the resulting file. This may cause processing to take a little longer." to "Enable this to allow playback without having to read the end of the file. This makes processing use 3 times as much export I/O, which is negligible for small files but might slow down exporting of large files."

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No response

[mifi]

TBh I thought that the difference was negligible, when reading threads like this one: https://superuser.com/questions/856025/any-downsides-to-always-using-the-movflags-faststart-parameter

Someone tried Sintel.2010.1080p.mp4 which is a 523 mb (?) file. How large is the file where you're seeing it taking 3 times as long as without it? that's really something else than "a few extra seconds". This is an interesting datapoint, maybe you could provide your findings in the stack overflow post too?

[m-sundman]

523 MB is tiny. That'll fit into your filesystem cache in RAM. My file was a few hundred gigs.

[m-sundman]

Also, I don't have enough reputation points on stack to reply, but the 2nd highest answer (by thomasrutter) explains what happens in his first bullet point: "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."

And just to clarify, while it doubles the total I/O it triples the export I/O, so if the export filesystem is slow, like when writing to a network drive, that means the whole operation will take 3 times as long.

[mifi]

Ok gotcha. I will add a note about this, thanks