G scsi 31 may 2012 exports data as a logical

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Unformatted text preview: are queued wai<ng for the disk   FCFS (do nothing) reasonable when load is low   long waiKng Kme for long request queues   SSTF (shortest seek Kme first)   minimize arm movement (seek Kme), maximize request rate   unfairly favors middle blocks   SCAN (elevator algorithm)   service requests in one direcKon unKl done, then reverse   skews wait Kmes non ­uniformly (why?)   C ­SCAN   like scan, but only go in one direcKon (typewriter)   uniform wait Kmes   31 May 2012 Disks and File Systems 15 University of Washington Interac<ng with disks   In the old days…   OS would have to specify cylinder #, sector #, surface #, transfer size     i.e., OS needs to know all of the disk parameters Modern disks are even more complicated   not all sectors are the same size, sectors are remapped, …   disk provides a higher ­level interface, e.g., SCSI           31 May 2012 exports data as a logical array of blocks [0 … N] maps logical blocks to cylinder/surface/sector OS only needs to name logical block #, disk maps this to cylinder/ surface/sector on ­board cache as a result, physical parameters are hidden from OS –  both good and bad Disks and File Systems 16 University of Washington Seagate Barracuda 3...
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