Abstract
A log-structured merge-tree-based key value store (LSMKV) is an append-only database for storing and retrieving unstructured data, especially in a write-intensive environment. This database uses hierarchical components to store and manage data. Upper-level components have a shorter data lifespan and a higher access locality than lower-level components. Hence, the data access latency of the upper-level components significantly affects the performance of the entire database. Hybrid solid-state drives (SSD) composed of media with different access speeds can improve the performance of an LSMKV by storing the upper-level components using a fast storage space. However, many hybrid SSDs use fast storage spaces to store data that are frequently allocated to the same logical address; they are not suitable for storing append-only component data, which are allocated to adjacent logical addresses. This article proposes a hybrid SSD-management method to reduce the data access latency of append-only LSMKVs and increase the durability of hybrid SSDs. The proposed method allocates the data of upper-level components to a fast storage space using the level information of the data as a hint. This study utilizes dynamic data separation to determine the components to be placed in the fast storage space, NAND block management to store the data with similar lifespans in the same fast NAND block, and a data-relocation method to migrate long-lived data from the fast NAND region to another NAND region. Experimental results indicate that the proposed method reduces the average I/O latency by an average of 12% and increases the device durability by an average of 22%.
Highlights
Various systems, such as machine learning, blockchain, and content delivery networks, use key-value stores for unstructured data processing
DATA PLACEMENT TECHNIQUE OF HYBRID solid-state drives (SSD) FOR LOG-STRUCTURED MERGE-TREE BASED KEY-VALUE STORE This study aims to effectively estimate the lifespan of sorted string table (SSTable) in storage and allocate short-lifespan SSTables to the single-level cell (SLC) NAND region
Because the amount of data which are requested as small random-writes are not large in the log-structured mergetree-based key-value (LSMKV) operating environment, the amount of host-requested data which were allocated to the SLC region space, in the case of ComboFTL, was from 0.001 to 0.033 multiplied that of the proposed scheme
Summary
Various systems, such as machine learning, blockchain, and content delivery networks, use key-value stores for unstructured data processing. LSMKVs have low write latency as key-value data are written to the storage device in an append-only manner [16]. In the hybrid SSD, the higher the proportion of the SLC block in the entire NAND block, the higher the lifespan and response speed of the flash storage, but the lower the total capacity (Table 1). According to Chang [4], a hybrid SSD can reduce the storage access latency by placing small and frequently updated data on SLCs. In addition, cold data recorded in SLCs can be relocated to other NAND regions to secure SLC space, and hot data recorded in MLC can be moved to SLCs to reduce data access latency. To increase the durability of the device, the hybrid SSD should prevent the NAND region from wearing out significantly
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