This work investigates how to protect numerical iterative algorithms from all types of errors that can strike at scale: fail-stop errors (a.k.a. failures) and silent errors, striking both as computation errors and memory bit-flips. We combine various techniques: detectors for computation errors, checksums for memory errors, and checkpoint/restart for failures. The objective is to minimize the expected time per iteration of the algorithm. We design a hierarchical pattern that combines and interleaves all these fault-tolerance mechanisms, and we determine the optimal periodic pattern that achieves this objective. We instantiate these results for the performance analysis of the Preconditioned Conjugate Gradient (PCG) algorithm: we report several scenarios where the optimal pattern dramatically decreases the overhead due to error mitigation.