# Irreversible process Entropie-Change/cylinder

Hi,

I have read that for an irreversible process the equation for the entropy is: $$ds>\frac{dQ}{T}$$
But if I regard a cylinder connected to a heatreservoir and want to callculate the entropychange of this cylinder, why can I use the equation: $$ds=\frac{dQ}{T}$$???

THX