改善USB大量儲存裝置於視窗作業系統上的資料傳輸速率

Abstract

理論上，USB2.0高速傳輸埠頻寬達480Mbps（Megabit per second），若USB 匯流排上只傳輸資料，最高速度接近60MB/s（Megabyte per second），但 是在視窗系統下實際應用在讀取USB外接硬碟時，卻只能到達26~32MB/s。先不考慮硬碟啟動磁頭去讀寫的物理性延遲，造成效率不如預期的其中一個原因，可能是整體傳輸花費了太多溝通的時間在主機與裝置間的交握（Handshake）過程。常⾒的USB 外接儲存裝置所遵循的協議是Universal Serial Bus Mass Storage Bulk-Only transport 協議[1]，透過該協議可讓多種類型的儲存媒體透過USB介面作大量的資料傳輸，它提供了便利而可靠的資料交握，但同時也拖累了傳輸速度。為了加快資料傳輸的速度，本文根據Prefetch 和 Cache 原理，提出了Prefetch Read 與Packing Write 等方法。該方法是由掛載在USB 外接儲存裝置的過濾型驅動程式來實現。藉由過濾型驅動程式改變原本的USB封包傳輸流程，在視窗作業系統內獲得最佳的傳輸速度。

Theoretically, the data transfer speed of USB2.0 is 480MB/s. If the data is processed mainly on the USB bus, the highest speed would reach close to 60MB/s. However, on Windows operation system, the reading speed for USB 2.0 hard disk reach merely 26 to 32 MB/s. If we ignore the physical delay caused by the writing and reading of the magnetic head of the hard disk, one the possible reasons for the inefficiency in transfer speed is that the communicating process takes too much time on the handshake between the host and client device. Most USB external mass storage devices follow the Storage Bulk-Only Transport Protocol [4]. With the protocol, many kinds of storage devices can transfer mass data through the USB interface. It offers a convenient and reliable data handshake. But, in the meantime, the protocol also drags down the maximum speed of data transmission. This paper, which aims to speed up the data transmission, offers two methods: Prefetch-Read and Packing-Write. These two methods are implemented by a filter driver loaded on the USB external storage device. This filter driver will change the original process of USB packet transfer to maximize the transfer speed in the Windows O.S.