Performance Enhancement For Flash Memory Based Storage System

As a high performance, low power consumption, non-volatile storage device, flash memory has been widely applied in embedded systems, desktop computer storage systems and even data centers. However, flash memory has some design challenges in general, bad random write performance, inability of in-place updates and limited lifetime. These challenges have been holding back the further development and applications of flash memory. To design effective approaches for flash memory with considering its write performance, inability of in-place updates and limited lifetime have been the key work in the previous work. In this thesis, with the awareness of the following characteristics, we propose a set of approaches for flash memory based storage systems to solve these problems:first, flash memory has write and read performance asymmetric; second, in-place updates are not applicable to flash memory; third, the operating units of erase operation and write operation are different. With awareness of these three access characteristics, this thesis proposes approaches from three topics to cope with the challenges for flash memory based storage systems.For the first topic, we discuss flash memory as hard disk cache. Flash memory as hard disk cache has been studied for years. However, flash memory as hard disk cache cannot satisfy the lifetime and write performance requirements in common. There are many works on flash memory based hard disk cache to enhance lifetime and performance. However, under the intensive write applications, these approaches are not able to solve these problems in essential. In this topic, with the awareness of flash memory as hard disk cache, a hybrid hard disk cache architecture is proposed. The hybrid hard disk cache is composed with an emerging non-volatile memory and flash memory with the objectives of reducing the number of the low efficient write operations on flash memory and reducing the number of erase operations on flash memory to improve write performance and enhance lifetime. In this work, the emerging non-volatile memory is phase change memory (PCM), which is one of the most promising non-volatile memories.For the second topic, we consider the host systems of flash memory based storage system. Currently, there are large amount of buffer cache management approaches applied to virtual memory with exploring the asymmetric of read and write performance. However, these approaches are separated with the design of write buffer cache, which is a common component embedded in flash memory based storage systems. In this topic, we the first time design a virtual memory management approach with the awareness of write buffer cache. The proposed management approach cooperates with the management approaches of write buffer cache and virtual memory closely to achieve performance improvement. In addition, a write buffer cache management approach is also proposed to closely work with the proposed approach in the virtual memory.For the last topic, we still consider the flash memory based secondary storage systems with the embedded write buffer cache. Different from the second topic considering the cooperation between virtual memory and write buffer cache, we find the case that the proposed cooperation is invalid in many systems since write buffer cache is always embedded in storage systems while virtual memory is managed by operating systems. In this topic, we propose a unified write buffer cache management approach. Based on the studies of the current work on the write buffer cache, we find that there is a key problem in the design of write buffer cache:embedded write buffer cache is unaware of the cache line access information. With the analysis of the embedded write buffer cache, we propose a write buffer cache model. Based on the proposed model, a unified write buffer cache management approach is proposed to reduce the write activities and the number of erase operations.