Flash memory-based solid-state drives (SSDs) offer several attractive features and benefits compared to hard disk drive (HDD), such as shock resistance and better performance especially for random data access. Depending on the number of bits in each cell, Flash memory can be designed as single/multi/triple level cell (SLC/MLC/TLC), which have different performance, density, cost and write endurance characteristics. To bring the best of these together, several researchers have proposed designing SSD using hybrid SLC/MLC/TLC Flash memory. However, these SSDs also present several challenges such as buffer management, placement of hot/cold data in suitable portion, and intelligent garbage collection. Several recent techniques aim to address these challenges.In this paper, we present a survey of techniques for managing SSDs designed with SLC/MLC/TLC Flash memory. We classify the works on several axes to bring out their similarities and differences.We aim to synthesize the state-of-art progress in hybrid SSD management and also spark further research in this area.
KEYWORDSflash translation layer, garbage collection, hybrid solid state disk, NAND flash memory, wear leveling
INTRODUCTIONAs the amount of digital data continues to grow at an exponential rate and key applications become more data intensive, efficient storage architectures and management techniques have become more important than ever before. Conventionally, hard disk drive* has been used as a storage device; however, its limitations such as poor performance (especially for random accesses), higher form factor, and vulnerability to shocks and magnetic fields have encouraged researchers to explore its alternatives.Flash memory is a promising technology for designing storage devices due to its several attractive properties, eg, high performance and density, low power consumption, noise-free operation, and immunity to shocks and magnetic fields. 2-6 Also, its cost has been decreasing in recent years and it is expected to provide even better cost efficiency than HDD in near future. Based on the number of bits stored in each cell, Flash can be characterized as SLC (1 bit), MLC † (2 bit) and TLC (3 bits). As shown in Table 1, these cell types provide a spectrum of properties and trade-offs. Specifically, on going from SLC to MLC to TLC, the performance and write endurance decrease whereas density and cost efficiency improve.To achieve the best of these three cell types, hybrid SSD designs have been proposed, which use Flash memories of multiple cell types to improve performance, energy efficiency, and reliability. 16 However, these hybrid SSD designs also bring challenges, such as selection of relative proportion of SLC/MLC/TLC, efficient mapping/moving of hot/cold data to them, accounting for their disparate write endurance and density values, and
