chapter12secondary-storagestructure.ppt

C-LOOK (Cont.) Selecting a Disk-Scheduling Algorithm Which one has better performance? SSTF vs. FCFS? SCAN/C-SCAN vs. SSTF? What does performance depend on? What if the disk queue/load is always 1? Does the disk scheduling algorithm matter? What also matters? File-allocation method. The disk-scheduling algorithm should be written as a separate module of the operating system, allowing it to be replaced with a different algorithm if necessary. Either SSTF or LOOK is a reasonable choice for the default algorithm. Disk Management Low-level formatting, or physical formatting — Dividing a disk into sectors that the disk controller can read and write. Done by factory To use a disk to hold files, the operating system still needs to record its own data structures on the disk. Partition the disk into one or more groups of cylinders. Logical formatting or “making a file system”. Boot block initializes system. The bootstrap is stored in ROM. Master boot record (MBR) on the first sector of disk Boot partition Booting from a Disk in Windows 2000 Bad Sectors Mechanical parts and prone to failures Bad sectors at factory How to detect bad sectors? Error correcting code (ECC) How to handle bad sectors on disks? Reserve some spared sectors … Sector sparing or forwarding Sector slipping Swap-Space Management What is swap space? Virtual memory uses disk space as an extension of main memory. Should swap space be placed in normal file system? Why or why not? Performance vs. space efficiency tradeoff? Cost of External Storage Devices Magnetic Disks NTD 3.5 per Gigabyte (1 T ~ NTD 3560) Access time: 10 ms Tapes: NTD 10 per Gigabyte (old) Can only write/read pages in sequence Flash memory: NTD 50 per Gigabyte (16 G ~ NTD 800) Access time? DRAM: NTD 300 per Gigabyte (2 G ~ NTD 600) Access time: 10s ns Other types of persistent storage devices: Optical storage (CD-R, CD-RW, DVD-R, DVD-RW) * Price per Megabyte of DRAM, From 1981 to 2004 Why DRAM manufacturers are in trouble? 2008 (USD10/GB