Device Management PDF

Device Management Angelika S. Balagot Instructor Storage Media Storage media are divided into two groups: - Sequential Access Storage Media - store records sequentially, one after the other - Direct Access Storage Devices (DASD) - store either sequ...

Device Management Angelika S. Balagot Instructor Storage Media Storage media are divided into two groups: - Sequential Access Storage Media - store records sequentially, one after the other - Direct Access Storage Devices (DASD) - store either sequential or direct z access files There are vast differences in their speed and shareability. Direct Access Storage Devices (DASD) Devices that can directly read or write to a specific place. Grouped into 3 categories: - Magnetic disks (Fixed-Head, Movable-Head) - Optical discs - Flash memory z Although the variance in DASD access times isn’t as wide as with magnetic tape, the location of the specific record still has a direct effect on the amount of time required to access it. Direct Access Storage Devices Fixed-Head Magnetic Disk Storage Looks like CD or DVD covered with magnetic film that has been formatted. Each circle is called track. Data is recorded serially on each z track by the fixed read/write head position over it. Direct Access Storage Devices Fixed-Head Magnetic Disk Storage Faster than movable-head disks Used when speed is of the utmost importance High costs z Reduced storage space compare to movable disk Direct Access Storage Devices Movable-Head Magnetic Disk Storage Like a computer hard drives. Have one read/write head that floats over each surface of each disk. Disk can be a single platter or partz of a disk pack/stack of magnetic platters. The number of tracks typically a thousand or more Direct Access Storage Devices Movable-Head Magnetic Disk Storage Each platter has two surfaces for recording, and each surface is formatted with a specific number of concentric z tracks where the data is recorded. Each track on each surface is numbered: Track 0 identifies the outermost concentric circle on each surface; the highest- numbered track is in the center Direct Access Storage Devices Movable-Head Magnetic Disk Storage The arm moves two read/write heads between each pair of surfaces: 1 surface above and 1 surface below. The arm moves all of the z heads in unison, so if one head is on Track 36, then all of the heads are on Track 36 - in other words, they’re all positioned on the same track but on their respective surfaces creating a virtual cylinder. Direct Access Storage Devices Movable-Head Magnetic Disk Storage z Direct Access Storage Devices Optical Disc Storage Was made possible by developments in laser technology. Single spiraling track of same-sized sectors. Running from the center to the rim of the disc. Direct Access Storage Devices Optical Disc Storage Optical disc consists of a single spiraling track of same-sized sectors running from the center to the rim of the disc. Direct Access Storage Devices Optical Disc Storage To put data on an optical disc, a high-intensity laser beam burns indentations on the disc that are called pits. These pits, which represent 0s, contrast with the unburned flat areas, called lands, which represent 1s. The first sectors are located in the center z of the disc the laser moves outward reading each sector in turn. Direct Access Storage Devices Optical Disc vs Magnetic Disk Among the many differences between an optical disc and a magnetic disk is the design of the disc track and sectors. A magnetic disk, which consists of concentric tracks of sectors, spins at a constant speed—this is called constant angular velocity (CAV). Because the sectors at the outside of the disk spin faster past the read/write head than the inner sectors, outside sectors are much larger than sectors located near the center of the disk. Direct Access Storage Devices Optical Disc vs Magnetic Disc On the other hand, an optical disc consists of a single spiraling track of same-sized sectors running from the center to the rim of the disc. This single track also has sectors, but all sectors are the same size regardless of their locations on the disc. The disc drive adjusts the speed of the disc’s spin to compensate for the sector’s location on the disc—this is called constant linear velocity (CLV). Direct Access Storage Devices Flash Memory Storage A type of electrically erasable programmable read-only memory (EEPROM). A nonvolatile removable medium that emulates random access memory, but, unlike RAM, stores data securely even when it’s removed from its power source. Historically, flash memory was primarily z used to store startup (boot up) information for computers, but is now used to store data for cell phones, mobile devices, music players, cameras, and more. Direct Access Storage Devices Flash Memory Storage The bits can be erased only by applying the flash to a large block of memory with each flash erasure, the block becomes less stable. In time (after 10,000 to 1,000,000 uses), a flash memory device will no longer z reliably store data. Examples: - USB flash drives - SSDs - Memory cards, - Smartphones, tablets, etc. Summary Device Management Types of Devices (Dedicated, Shared, Virtual Devices) Storage Media 2 Groups of Storage Media (Sequential Access Storage Media,zDirect Access Storage Devices (DASD)) 3 categories of DASD (Magnetic Disk, Optical Disc, Flash Memory) End of Discussion Angelika S. Balagot Instructor