The overall key of this piece in A minor (it is Bach's prelude from Prelude and Fugue in A minor BWV 895). Here is what I understood :
At the end of bar 6, there seems to be a change of key: this might be conceivable because of the very last G natural and the 2nd inversion of the dominant seventh chord without its root (D, F, B) which belongs to C major. That being said, the fact that C is not part of bar 7's second chord creates a kind of ambiguity: it could thus conceal an E minor triad which, followed by a D natural, could lead very well to the following chord: a nice A major triad. However, neither the chord progression C major-A major nor E minor 7-A major sound very baroque to me.
For the next part, I am convinced that this A major triad goes on a D minor modulation (with the leading tone of the scale be that C♯). The next chords belongs entirely to D minor (1st inversion of its triad, then A major [dominant triad] as bar 8's second chord). And so on until the first chord in bar 9, G major triad followed by a F (to form the dominant seventh of C major) which naturally leads to a nice C major triad, whose dominant is immediately substitued with an A. Finally, this takes us back to our first key (A minor), as the G♯ reappears on the last beat of bar 9 – back home!
I have to say that I find myself a bit confused about the whole thing and I would like to read a more precise and accurate analysis of these 4 measures.