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Anchor10466341046634 Using Volume Cache Anchor10466351046635File accesses often incur repeated reading of the same volume sectors. On a FAT volume, these may be sectors in the root directory, the area of the file allocation table (FAT) from which clusters are being allocated or data from important (often-read) files. A cache wedged between the system driver and volume layers (as shown in Figure 5 Figure 7-8) will eliminate many unnecessary device accesses. Sector data is stored upon first read or write. Further reads return the cached data; further writes update the cache entry and, possibly, the data on the volume (depending on the cache mode). Anchor10500851050085 Image Removed

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Figure 7-8 Volume cache architecture

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A cache is defined by three parameters: size, sector type allocation and mode. The size of the cache is the number of sectors that will fit into it at any time. Every sector is classified according to its type, either management, directory or file; the sector type allocation determines the percentage of the cache that will be devoted to each type. The mode determines when cache entries are created (i.e., when sectors are cached) and what happens upon write. Anchor10483541048354  

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Table 7-3 Cache types