Curious question: How does mpv run a file even after it has been deleted

[lostb053]

For context, I don't have a background in computer related subjects, but I'm interested in exploring tech. So I might not understand extremely technical answers.

I was playing a video (~15GB), and without closing the player or moving to a new video, I deleted the file. Returned to the player, and got curious if it would still access the file in its entirety, which to my surprise it did.
So basically:

• File isn't visible in explorer
• I was using HDD
• Ram utilization seemed normal, so unlikely that it was cached there

My guesses are:

• Either mpv creates a local copy of the file played
• Or it stores the data sectors (I might be using wrong term to explain what I mean) for the file, and can still access the data as long as the player remains open, even if it's marked as free space on HDD (well the data remains unless overwritten)

I'm sorry if this isn't the place for this, but I found this quite interesting and had to ask.

[kasper93]

This is good observation. This is how kernel manages resources, in this cases files. Kernel will not remove resource that you have open handle to, this would cause wrong application behavior and most likely crashes.

This is not related to mpv and there are better publications/books to read about the resource management and file systems. But I can give you high level overview. Mostly from UNIX point of view.

File isn't visible in explorer

Those things that you see in explorer are just pointers to where the data is. Or in fact where the metadata about the file is (inode). You can have multiple "paths" (hard links) pointing to single resource. inode is index of a "bucket" that keeps information about the file, like mode, size, links count and blocks building the file.

Now if you delete a file, inode will be deleted only if links count is 0 and there are no open file descriptors to this file. And once inode is gone, the blocks will be available to be reused.

In short when mpv is reading file, you can remove the file, but in reality inode will still be alive until you close the mpv.

Either mpv creates a local copy of the file played

No, mpv can cache some of the data in memory, but it's not a full copy and by default it is only small portion of a file, for local files it is 1s only.

Or it stores the data sectors (I might be using wrong term to explain what I mean) for the file, and can still access the data as long as the player remains open, even if it's marked as free space on HDD (well the data remains unless overwritten)

mpv doesn't do that, kernel does. But like said before, it doesn't mark the blocks as free until all references to this file are closed.

I'm bad at explaining things, I googled, and this may be interesting for you on this topic of filesystem basics.
https://compas.cs.stonybrook.edu/~nhonarmand/courses/fa17/cse306/slides/16-fs_basics.pdf

This was very simplified, there are many nuisances how it works, depending on the kernel, filesystem and so on. For example NT kernel (Windows) doesn't allow removing files that are in use at all, while Linux will happily unlink() your file, but internally keep it alive as long as needed.

[lostb053]

Hey, while I won't say I completely understand the explanation (lack of knowledge on my part), but I get the gist of it. Thanks for the reply, and it makes sense 👌.

[lostb053]

If you could clarify one more thing:

Windows creates a lock or something for files in use (and I'm indeed using Windows), i.e. won't allow deletion of file as long as file is in the use. So how is the file that is being accessed by mpv, can be yet deleted? Does mpv access the file in such a way, that the lock is not triggered when deleting the file? I mean I would have otherwise gotten that "File in use" dialog box

[kasper93]

mpv opens file with FILE_SHARE_DELETE flag which allows deletion of in-use file. Without this flag you will get the "File in use" dialog.

[lostb053]

Understood. Also, just noticed

• File, tho not visible in the explorer, while still in use indeed isn't marked as free space.
• When mpv switches to other video file or closes, only then is the free space reflected.

Basically it's indeed as you explained earlier. Thanks for the informative reply 👌