Music Player Daemon

I have tried to set mpd's buffer to 100% and buffer size to 200mb.
It works normally.
I want to know that, Is this setting means that the playback will be done after whole file stored to memory?
My theory with this setting is, the audio data would transferred along less path to cpu than normal setting.
Can get more precise data to cpu. I assume that this can improve the quality.
Is it possible?

Don't mess with those settings.
It will not improve quality - I mean, how could it possibly? MPD plays back digital audio, and changing buffer settings does not change the bits which are sent to the sound hardware.
It will not be any more efficient - quite the contrary! It is less efficient, because sometimes, the buffer has to be discarded (when switching songs and seeking), and that means more CPU time for decoding was wasted.
Don't mess with those settings, because that will not give any advantage, but it has the chance that you introduce problems.
There's a lot of bad advice on the internet about MPD and buffer settings, but it's all wrong. After dealing with support cases about that, I decided years ago to deprecate those settings and remove its documentation.
Since MPD 0.21 (two years ago), the "buffer_before_play" setting (which you set to 100%) is gone completely. Whatever you configure, MPD will ignore it. (see https://github.com/MusicPlayerDaemon/MP ... 1/NEWS#L17)