The issue It’s not related with any blocking in Erlang per se… As I mentioned in the post I linked previously, it’s because the BEAM has a default of 2 seconds to write data to the disk when the file is open with the defaults for delayed_write, as per docs :
delayed_write
The same as {delayed_write, Size, Delay} with reasonable default values for Size and Delay (roughly some 64 KB, 2 seconds).
You can confirm this with a script that writes directly to the file system:
defmodule FileIO do
@moduledoc """
iex> fd = FileIO.open! "test.txt"
#PID<0.292.0>
iex> FileIO.append!(fd, "test it") && FileIO.read!("test.txt")
"test it\n"
iex> FileIO.close fd
:ok
"""
@write_mode [:append, :binary]
# @write_mode [:append, :binary, :delayed_write]
# @write_mode [:append, :binary, {:delayed_write, 1, 1}]
def open!(path, write_mode \\ @write_mode) do
File.open!(path, write_mode)
end
def close(file_descriptor) do
File.close(file_descriptor)
end
def append!(file_descriptor, data) do
{:ok, start_position} = :file.position(file_descriptor, :cur)
:ok = IO.binwrite(file_descriptor, "#{data}\n")
{:ok, end_position} = :file.position(file_descriptor, :cur)
%{
file_descriptor: file_descriptor,
start_position: start_position,
end_position: end_position,
size: byte_size(data)
}
end
def read!(path) do
{:ok, data} = :file.read_file(path)
data
end
end
So, I can confirm that the delay of 2 seconds is indeed coming from the BEAM when the file is open to write with delayed_write. If in the above script I use @write_mode [:append, :binary] or @write_mode [:append, :binary, {:delayed_write, 1, 1}] I can immediately read the content of the file after I write to it and see that my last write is persisted on disk, but if I use instead @write_mode [:append, :binary, :delayed_write] I cannot see the last write in the file, unless I wait 2 seconds to read it.
I totally agree with you here…
In my opinion calling Mnesia ACID, therefore durable is stretching a little the definition of ACID and durable, but it seems the reply is always replication, that then leads to netsplits, that then leads to data loss when recovering from it, thus the ACID definition is again being streched.
