Joint management of performance-predictable virtualized storage devices with hard disk and flash memory

Recently, the significance of storage virtualization has been high-lighted due to the growing performance demands of next-generation applications. However, with its unpredictable long seek time and rotational delays, hard disk fails to provide sufficient performance guarantees to fulfill the service-level objectives of the applications, especially on random accesses. To resolve the random-access problem of hard disk, it is a common approach to adopt flash memory in a hard-disk-based storage system. Nevertheless, flash memory requires garbage collection to reclaim the space with invalid data, where the garbage collection process might introduce long and unpredictable time overheads and considerably reduce the performance benefits of flash memory. This work proposes a joint management strategy for virtualized storage systems with hard disk and flash memory. The major idea of this work is, to exploit flash memory to enhance the random access performance of hard disk, while exploiting hard disk to aid flash memory in avoiding the worst-case delays incurred by garbage collection. Analytical and experimental studies revealed that the resulted virtualized storage device shows surprising behaviors of the worst-case performance higher than both hard disk and flash memory.