A file opened in raw mode wouldn’t work with IO.write and IO.binwrite, I think?

iex(1)> {:ok, fd} = File.open("some.file", [:raw, :write])
{:ok,
 {:file_descriptor, :prim_file,
  %{
    handle: #Reference<0.3732712904.755892237.167770>,
    owner: #PID<0.105.0>,
    r_ahead_size: 0,
    r_buffer: #Reference<0.3732712904.755892226.167656>
  }}}
iex(2)> IO.write(fd, "hello")
** (FunctionClauseError) no function clause matching in :io.request/2

    The following arguments were given to :io.request/2:

        # 1
        {:file_descriptor, :prim_file,
         %{
           handle: #Reference<0.3732712904.755892237.167770>,
           owner: #PID<0.105.0>,
           r_ahead_size: 0,
           r_buffer: #Reference<0.3732712904.755892226.167656>
         }}

        # 2
        {:put_chars, :unicode, "hello"}

    (stdlib 3.12.1) io.erl:565: :io.request/2
    (stdlib 3.12.1) io.erl:63: :io.o_request/3

:file.write can be used instead.

iex(2)> :file.write(fd, "hey")
:ok

Also there are some ownership complications similar to sockets where only the process that opened the file can write to it, not sure if it’s a problem for the Task.async_stream approach described in OP (it shouldn’t be a problem if the file is opened and worked with within the same task process).

Since it sounds like you don’t need to persist the files in memory, you could mount a ramdisk so the files never have to touch the disk (assuming you have enough ram)