While running Linkbench with S3, sulabh.mahajan@mongodb.com noticed that the chunk cache filled with objects fairly quickly, but almost nothing was evicted. The "chunk cache chunks evicted" stat was zero, but it was hard to debug further than that with the time available.

Sulabh managed to get his testing done anyway, but this issue highlights the need for (1) more detailed eviction stats, and (2) some improvments to the algorithm (e.g. perhaps we remember the least-used chunk rather than waiting for a ref count to drop to zero.)