Modern high-end disk arrays often have several gigabytes of cache RAM. Unfortunately, most array caches use management policies which duplicate the same data blocks at both the client and array levels of the cache hierarchy: they are inclusive. Thus, the aggregate cache behaves as if it was only as big as the larger of the client and array caches, instead of as large as the sum of the two. Inclusiveness is wasteful: cache RAM is expensive. We explore the benefits of a simple scheme to achieve exclusive caching, in which a data block is cached at either a client or the disk array, but not both. Exclusiveness helps to create the effect of a single, large unified cache. We introduce a DEMOTE operation to transfer data ejected from the client to the array, and explore its effectiveness with simulation studies. We quantify the benefits and overheads of demotions across both synthetic and real-life workloads. The results show that we can obtain useful -- sometimes substantial -- speedups. During our investigations, we also developed some new cache-insertion algorithms that show promise for multi-client systems, and report on some of their properties.

This paper investigates the performance of a multi-disk storage system equipped with a segmented disk cache processing a workload of multiple relational scans. Prefetching is a popular method of improving the performance of scans. Many modern disks have a multisegment cache which can be used for prefetching. We observe that, exploiting declustering as a data placement method, prefetching in a segmented cache causes a load imbalance among several disks. A single disk becomes a bottleneck, degrading performance of the entire system. A variation in disk queue length is a primary factor of the imbalance. Using a precise simulation model, we investigate several approaches to achieving better balancing. Our metrics are a scan response time for the closed-end system and an ability to sustain a workload without saturating for the open-end system. We arrive at two main conclusions: (1) Prefetching in main memory is inexpensive and effective for balancing and can supplement or substitute prefetc...