Abstract
RAID storage arrays often possess gigabytes of RAM forcaching disk blocks. Currently, most RAID systems use LRUor LRU-like policies to manage these caches. Since these arraycaches do not recognize the presence of file system buffer caches,they redundantly retain many of the same blocks as those cachedby the file system, thereby wasting precious cache space. In thispaper, we introduce X-RAY, an exclusive RAID array cachingmechanism. X-RAY achieves a high degree of (but not perfect) exclusivitythrough gray-box methods: by observing which files havebeen accessed through updates to file system meta-data, X-RAYconstructs an approximate image of the contents of the file systemcache and uses that information to determine the exclusive set ofblocks that should be cached by the array. We use microbenchmarksto demonstrate that X-RAY's prediction of the file systembuffer cache contents is highly accurate, and trace-based simulationto show that X-RAY considerably outperforms LRU andperforms as well as other more invasive approaches. The mainstrength of the X-RAY approach is that it is easy to deploy - allperformance gains are achieved without changes to the SCSI protocolor the file system above.
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