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This paper presents a novel hierarchy cache architecture for the purpose of optimizing IO performance. The main idea of the hierarchy cache is to use a few megabytes of RAM and a pagefile to form a two-level cache architecture. The pagefile is equivalent to the cache disk in DCD(Disk Caching Disk). The pagefile outperforms data disks, because data are accessed in different units and different ways. Small writes are collected in the RAM cache first, and data will be transferred to the pagefile in large writes later. When the system is idle, it will destage data from the pagefile to data disks. The performance test results show that the hierarchy cache can improve IO performance dramatically for small writes, and the mail server using the hierarchy cache driver can handle transactions about 2.2 times faster than the normal mail server. The hierarchy cache is implemented as a filter driver, so its transparent to the current Windows 2000 / Windows XP operating system.