NCQ vs. I/O scheduler

Abstract

Native Command Queueing (NCQ) is an optimization technology to maximize throughput by reordering requests inside a disk drive. It has been so successful that NCQ has become the standard in SATA 2 protocol specification, and the great majority of disk vendors have adopted it for their recent disks. However, there is a possibility that the technology may lead to an information gap between the OS and a disk drive. A NCQ-enabled disk tries to optimize throughput without realizing the intention of an OS, whereas the OS does its best under the assumption that the disk will do as it is told without specific knowledge regarding the details of the disk mechanism. Let us call this expectation discord , which may cause serious problems such as request starvations or performance anomaly. In this article, we (1) confirm that expectation discord actually occurs in real systems; (2) propose software-level approaches to solve them; and (3) evaluate our mechanism. Experimental results show that our solution is simple, cheap (no special hardware required), portable, and effective.

An efficient scheduling algorithm for NCQ within SSDs

“…To alleviate this problem, we supplemented our model with a simple aging scheme. In other words, whenever a command in NCQ is overtaken by other newly-arrived commands, its I/O service time can be revised as in (4).…”

“…system software techniques designed for hard disk drives are being redesigned by considering the characteristics of SSDs [2,3]. In this paper, we focus on NCQ (Native Command Queueing), which is one of the techniques that improve the I/O performance of hard disk drives [4].…”