| The rapid development of information technology and the rapid increase of digital information have brought serious challenges to the storage system. The traditional disk storage devices have been unable to meet the needs of current data intensive applications due to their inherent mechanical properties. The appearance of the solid state drives, which is based on flash memory, improves this situation, and it is widely used as a cache to reduce the performance gap between memory and disk.Because the data server of distributed file system Cappella has low performance, we introduce SSD device and use it and DRAM to compose a two-level hybrid cache. Charm, a configurable and history information aware replacement policy for a multi-level cache, allows users to configure multiple content such as access patterns, read-write mode and so on for applications, and can also determine access pattern by watching historical access information. It is possible to adapt to changes in access patterns. In addition, Charm filters out the data which is only visited once, and caches the hot data which is visited once or more into the SSD. Thereby it reduces the number of writes to the SSD, and extends the life time of SSD. On Cappella data server, we designed and implemented a multi-level cache based on DRAM and SSD. The multi-level cache efficiently manages the cache space, adopts Charm algorithm as a cache replacement policy, and implements data read ahead and persists the dirty data to disk after aggregation.The experiments with the real life workload on MCsim show that Charm increases the overall cache hit rate by 13%-46% and reduces the number of writes to the SSD by 14%-30%, compared to existing multi-level cache replacement algorithms. The tests for Cappella system with multi-level cache show large file sequential read/write performance is increased by 9%-37%, and small file random access latency is reduced by 68%-89%. It effectively enhances the performance of the Cappella data server. |
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