20091105 - Disk Organization and Scheduling One of the functions of a disk driver is to schedule requests taking into account physical drive

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Unformatted text preview: Disk Organization and Scheduling One of the functions of a disk driver is to schedule requests, taking into account physical drive characteristics in order to achieve efficient operation. Hardware Components of Disk Subsystem Disk Drive: May be • fixed head or movable head • fixed media or removable media Controller: Controls one or more disk drives. • Contains internal buffer memory. • May be operated in programmed I/O or DMA mode. Disk Subsystem Hardware Drum Physical Characteristics of Modern Magnetic Disk Drives • Consists of a collection of platters each with two surfaces . • There is one read/write head for each surface. • Head assembly can be moved radially to one of a number of cylinders . • Intersection of a cylinder and a surface is a track . • Each track is logically divided into sectors . Disk Physical Characteristics Sources of Delay in Accessing a Disk Drive Seek Time: Time taken to move heads to the proper cylinder. • Usually the dominant delay: on order of 10ms. Rotational Latency: Time taken for the desired sector to rotate into position under the head. • Usually somewhat smaller than seek time: average 4ms at 7200 rpm. Transfer Time: Time taken to transfer data in to main memory, once sector has been located. • Lower limit: imposed by DMA transfer rate ( e.g. 133MB/sec for PCI bus). • Upper limit: imposed by recording density and disk rotational speed. Typical Parameters • Rotational speed : 7200 rpm (= 120 rps)....
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This note was uploaded on 12/19/2009 for the course CSE 306 taught by Professor Kifer,m during the Spring '08 term at SUNY Stony Brook.

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20091105 - Disk Organization and Scheduling One of the functions of a disk driver is to schedule requests taking into account physical drive

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