1. A method for caching updates to one target storage device in a first and second memories, wherein the target storage device is one of a plurality of storage devices, comprising: determining an allocation of available space in the second memory to the storage devices that applies throughout an entire session of operation during which updates are received; receiving an update to one target storage device; determining, throughout the entire session, whether adding the update to the second memory will exceed the allocation of available space for the target storage device in the second memory; and writing one copy of the update to the second memory if adding the update to the second memory will not exceed the allocation of available space for the target storage device.