Popularity Based Cache Management Scheme for RAID Which Uses an SSD as a Cache

Abstract

In this paper, we propose a static cache management policy for RAID architecture which uses an SSD as a disk cache. Because SSD has several drawbacks, dynamic cache management scheme such as LRU can cause serious problem i.e., too much write operations can be occurred in order to contain most recently used data. Frequent write operations on SSD not only slow down the performance but also shorten the lifetime. For this reason, we propose a static cache management policy which takes popularity and size into account. Popular data of some period is copied into SSD and too large files are excluded since too large files can lower the hit ratio. We used real trace to evaluate our scheme and the results showed that write counts are reduced to 1/37 of LRU policy as we desired. Unfortunately, the average hit ratio was lower than LRU policy but when popular data is similar between two periods, our scheme showed better hit ratio than LRU. This implies that our scheme can be used on web server which follows zipf’s distribution and has smooth changing of popular data.