X-RAY: a non-invasive exclusive caching mechanism for RAIDs

RAID storage arrays often possess gigabytes of RAM for caching disk blocks. Currently, most RAID systems use LRU or LRU-like policies to manage these caches. Since these array caches do not recognize the presence of file system buffer caches, they redundantly retain many of the same blocks as those cached by the file system, thereby wasting precious cache space. In this paper, we introduce X-RAY, an exclusive RAID array caching mechanism. X-RAY achieves a high degree of (but not perfect) exclusivity through gray-box methods: by observing which files have been accessed through updates to file system meta-data, X-RAY constructs an approximate image of the contents of the file system cache and uses that information to determine the exclusive set of blocks that should be cached by the array. We use microbenchmarks to demonstrate that X-RAY's prediction of the file system buffer cache contents is highly accurate, and trace-based simulation to show that X-RAY considerably outperforms LRU and performs as well as other more invasive approaches. The main strength of the X-RAY approach is that it is easy to deploy - all performance gains are achieved without changes to the SCSI protocol or the file system above.