Indexing in flash storage devices: a survey on challenges, current approaches, and future trends

Indexes are special purpose data structures, designed to facilitate and speed up the access to the contents of a file. Indexing has been actively and extensively investigated in DBMSes equipped with hard disk drives (HDDs). In the recent years, solid-state drives (SSDs), based on NAND flash technology, started replacing magnetic disks due to their appealing characteristics: high throughput/low latency, shock resistance, absence of mechanical parts, low power consumption. However, treating SSDs as simply another category of block devices ignores their idiosyncrasies, like erase-before-write, wear-out and asymmetric read/write, and may lead to poor performance. These peculiarities of SSDs dictate the refactoring or even the reinvention of the indexing techniques that have been designed primarily for HDDs. In this work, we present a concise overview of the SSD technology and the challenges it poses. We broadly survey 62 flash-aware indexes for various data types, analyze the main techniques they employ, and comment on their main advantages and disadvantages, aiming to provide a systematic and valuable resource for researchers working on algorithm design and index development for SSDs. Additionally, we discuss future trends and new lines of research related to this field.