  Gravity/fluid correspondence becomes an important tool to investigate the
strongly correlated fluids. We carefully investigate the holographic fluids at
the finite cutoff surface by considering different boundary conditions in the
scenario of gravity/fluid correspondence. We find that the sonic velocity of
the boundary fluids at the finite cutoff surface is critical to clarify the
superficial similarity between bulk viscosity and perturbation of the pressure
for the holographic fluid, where we set a special boundary condition at the
finite cutoff surface to explicitly express this superficial similarity.
Moreover, we further take the sonic velocity into account to investigate a case
with more general boundary condition. In this more general case, two parameters
in the first order stress tensor of holographic fluid cannot be fixed, one can
still extract the information of transport coefficients by considering the
sonic velocity seriously.
