  Quantum error correction methods use processing power to combat noise. The
noise level which can be tolerated in a fault-tolerant method is therefore a
function of the computational resources available, especially the size of
computer and degree of parallelism. I present an analysis of error correction
with block codes, made fault-tolerant through the use of prepared ancilla
blocks. The preparation and verification of the ancillas is described in
detail. It is shown that the ancillas need only be verified against a small set
of errors. This, combined with previously known advantages, makes this `ancilla
factory' the best method to apply error correction, whether in concatenated or
block coding. I then consider the resources required to achieve $2 \times
10^{10}$ computational steps reliably in a computer of 2150 logical qubits,
finding that the simplest $[[n,1,d]]$ block codes can tolerate more noise with
smaller overheads than the $7^L$-bit concatenated code. The scaling is such
that block codes remain the better choice for all computations one is likely to
contemplate.
