  Caches exploit temporal and spatial locality to allow a small memory to
provide fast access to data stored in large, slow memory. The temporal aspect
of locality is extremely well studied and understood, but the spatial aspect
much less so. We seek to gain an increased understanding of spatial locality by
defining and studying the Granularity-Change Caching Problem. This problem
modifies the traditional caching setup by grouping data items into blocks, such
that a cache can choose any subset of a block to load for the same cost as
loading any individual item in the block.
  We show that modeling such spatial locality significantly changes the caching
problem. This begins with a proof that Granularity-Change Caching is
NP-Complete in the offline setting, even when all items have unit size and all
blocks have unit load cost. In the online setting, we show a lower bound for
competitive ratios of deterministic policies that is significantly worse than
traditional caching. Moreover, we present a deterministic replacement policy
called Item-Block Layered Partitioning and show that it obtains a competitive
ratio close to that lower bound. Moreover, our bounds reveal a new issue
arising in the Granularity-Change Caching Problem where the choice of offline
cache size affects the competitiveness of different online algorithms relative
to one another. To deal with this issue, we extend a prior (temporal) locality
model to account for spatial locality, and provide a general lower bound in
addition to an upper bound for Item-Block Layered Partitioning.
