  We explore several aspects of the relation between gravity and entanglement
in the context of AdS/CFT, in the simple setting of 3 bulk dimensions.
Specifically, we consider small perturbations of the AdS metric and the CFT
vacuum state and study what can be learnt about the metric perturbation from
the Ryu-Takayanagi (RT) formula alone. It is well-known that, if the RT formula
holds for all boundary spacelike segments, then the metric perturbation
satisfies the linearized Einstein equations throughout the bulk. We generalize
this result by showing that, if the RT formula holds for all spacelike segments
contained in a certain boundary region, then the metric perturbation satisfies
the linearized Einstein equations in a corresponding bulk region (in fact, it
is completely determined in that region). We also argue that the same is true
for small perturbations of the planar BTZ black hole and the CFT thermal state.
We discuss the relation between our results and the ideas of
subregion-subregion duality, and we point out that our argument also serves as
a holographic proof of the linearized RT formula for boundary segments.
