How to determine the right plan in a Carlsbad structure on the queenside for examples similar to this one

Question

Let's say that I'm play from the Black side this position:

[FEN ""] 
1. e4 c6 2. d4 d5 3. exd5 cxd5 4. f4 Nf6 5. Nf3 Bg4 6. h3 Bxf3 7. Qxf3 e6 8. c3 Nc6 9. Bd3 Be7 10. Be3 a6 11. Nd2 b5 12. a3 Rb8 13. O-O 

My question is: How do you know when to play Na5 and when to play a5 continuing with the minority attack in such Carlsbad structures? I'm asking in general, not just for this position. And I would be happy if you have a reference like a book where you read about the strategy of playing this.

Answer 1

The plan defusing the minority attack is know since ages: b4, Nb3 (or d3), Nc5. The attacker only remains with a weak pawn on b5 and White's c3 is totally safe. (Your example has reversed colors to the "usual", I reversed everything too)

So if it should succeed nevertheless, some none-excluding possibilities are...

• Black's d-pawn is still on d6. Only after a5,b4 etc. has been done, it moves to d5.
• All knights of White have been exchanged.
• Pressure on c3. b4 simply hangs the pawn.
• None of the above, but b5 is safe (e.g. White can only pester it with a B but the K protects it), Black just ignores the Nc5 and invades on the a file.
• Specific to your example, Black must first do prophylactic measurements against f5, which is the "mirrored" minority attack for White.

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