Solid state disks are receiving prospects because of their high performance, which exceeds several gigabytes per second throughput with random I/O workloads. However, the capacity-to-cost rate of SSDs remains several times more than that of hard disks, thus it limits the application of SSDs.As a solution to the capacity problem, the rapid random access of SSDs can be exploited to enable efficient searching of redundant data and their deduplication. However, the space earned from deduplication cannot be used as permanent storage because it must be reclaimed when deduplication is cancelled as a result of an update to the deduplicated data.
To overcome this limitation, we propose a novel FTL scheme that enables the gained capacity to be used as permanent storage space for the file system layer. The proposed approach determines the safe amount of gained capacity that can be provided to the upper layer based on the compression rate prediction scheme. It then secures the required space by selectively compressing cold data when capacity overflow occurs from broken deduplication. Also, an efficient deduplication scheme and a selective compression method are introduced to retain the performance of the SSD.
We evaluated the FTL with a simulator and a prototype implementation on OpenSSD Cosmos, and it shows that the file system obtained approximately 79% additional capacity by the proposed scheme.