  In master-field simulations of lattice QCD, the expectation values of
interest are obtained from a single or at most a few representative gauge-field
configurations on very large lattices. If the light quarks are included, the
generation of these fields using standard techniques is however challenging in
view of various algorithmic instabilities and precision issues. Ways to
overcome these problems are described here for the case of the O(a)-improved
Wilson formulation of lattice QCD and the viability of the proposed measures is
then checked in extensive simulations of the theory with 2+1 flavours of
quarks.
