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UE CS 470 - Lecture 29

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Lecture 29OutlineDisk Drives 1Disk Drives 2Disk Drives 3Disk Drives 4Disk Drives 5Disk Drives 6Disk Drives 7Disk Drives 8Disk Scheduling 1Disk Scheduling 2FormattingBoot Block 1Boot Block 2Bad BlocksSwap Space 1Swap Space 2RAID 1RAID 2RAID 5RAID 6RAID 7RAID LevelsRAID 0+1 and RAID 1+0Friday, March 25 CS 470 Operating Systems - Lecture 29 1Lecture 29Reminder: Homework 7 is due on Monday at class time for Exam 2 review; no late work accepted. Memory management project is now due on Friday, April 1.Reminder: Exam 2 is on Wednesday. Exam 2 review sheet is posted.Questions?Friday, March 25 CS 470 Operating Systems - Lecture 29 2OutlineDisk systemsDisk schedulingDisk managementRAIDFriday, March 25 CS 470 Operating Systems - Lecture 29 3Disk DrivesA disk is viewed logically as a linear array of blocks. How is it mapped onto a circular disk drive?A disk drive is one or more platters rotating on a spindle. Each side of a platter has a head that reads the data off that side of the platter. Each platter side has concentric grooves called tracks. The vertical extent of the same track position on each platter is a cylinder. Each track/cylinder is divided into sectors.Friday, March 25 CS 470 Operating Systems - Lecture 29 4Disk DrivesFriday, March 25 CS 470 Operating Systems - Lecture 29 5Disk DrivesGenerally, block numbers are mapped with Block 0 at cylinder/track 0 (outermost groove), head 0, sector 0. The next block is sector 1 until the track is full, then the next block is head 1, sector 0, etc., until the cylinder is full, then the next block is cylinder/track 1, head, 0, sector 0, and so forth.Conceptually, it is possible for OS's to map logical block numbers to <cyl, head, sector> addresses, but this does not happen any more with mapping handled by the disk controller.Friday, March 25 CS 470 Operating Systems - Lecture 29 6Disk DrivesOne reason mapping is done in disk controller is that disks have been getting larger. Density has increased in three dimensions.# sectors/track (higher rotation speed)# tracks/platter (shorter seek separation)# bits/space (vertical writes in groove)Components of disk performance areseek time: disk arm movement to correct cylinderrotational delay (latency): wait for correct sector to rotate under the headFriday, March 25 CS 470 Operating Systems - Lecture 29 7Disk DrivesTaken together, data access time is determined byBandwidth (bytes transferred/unit time): buffer to disk, buffer to hostBuffer sizeDisk drives come in various speeds and sizes optimized for various applicationsFriday, March 25 CS 470 Operating Systems - Lecture 29 8Disk DrivesDisk drive Application Sizes RPM Cache Buffer to hostNotes, Street priceWD Caviar Blue Standard, internal desktop80GB-1TB7200 32MB SATA 6Gb/s1TB ~$65WD Caviar Black Maximum speed, internal desktop80GB-2TB7200 64MB SATA 6Gb/s2TB ~$160WD Caviar Green Maximum capacity, low power, internal desktop320GB-3TBvariable 64MB SATA 3Gb/s3TB ~$215WD VelociRaptor Internal, enterprise server150-600GB10000 32MB SATA 6Gb/s600GB ~$250WD Scorpio Blue Standard, internal laptop80GB-1TB5200 8MB SATA 3Gb/s1TB ~$110WD Scorpio Black Maximum power, internal laptop160GB-750GB7200 16MB SATA 3Gb/s750GB ~$110Using Western Digital as a prototypical lineFriday, March 25 CS 470 Operating Systems - Lecture 29 9Disk DrivesDisk drive Application Sizes RPM Cache Buffer to hostNotes, Street priceWD AV-25 24/7 surveillance 160-500GB5400 32MB SATA 3Gb/sMTBF 1 million hours, 500GB ~$72WD My Book EssentialExternal desktop 80GB-2TBUSB 3.0 5Gb/s2TB ~$120WD My Passport EliteExternal portable 250-640GBUSB 2.0 480Mb/s640GB ~$113WD My Book World Edition IINetworked 2-4TB Ethernet RAID 1/0 (2 drives in box), 4TB ~$340Toshiba makes a 240GB, 4200 RPM, 8MB cache disk drive that sells for ~$230. Why would anyone want to buy this small, slow, expensive drive?Friday, March 25 CS 470 Operating Systems - Lecture 29 10Disk DrivesWhat is the limit on the capacity of a disk drive using conventional magnetic media?Typical drives are ~250Gb/sq.in.The Toshiba drive is ~344Gb/sq.in.Current limit is ~500Gb/sq.in.Theoretical limit is ~1Tb/sq.in., any smaller grains and heat will change the magnetization of the bitsSeagate research into ways of packing more bits. Theoretically up to 50Tb/sq.in.Friday, March 25 CS 470 Operating Systems - Lecture 29 11Disk SchedulingAs with all resources, can extract best perfor-mance if schedule disk accesses. Now mostly done in the disk controller because:Original IDE interface has maximum 16383 cylinders x 16 heads x 63 sectors = 8.4GB to be reported. All disks to this now and the EIDE interface was added to find the actual geometry using LBA (linear block addressing).Most disks map out defective sectors to spare ones.# sectors/track is not constant. About 40% more sectors on outer tracks than on inner tracks.Friday, March 25 CS 470 Operating Systems - Lecture 29 12Disk SchedulingOS generally just makes requests to the controller. The controller has a queue and a scheduling algorithm to choose which request is serviced next.The algorithms are straightforward and have similar properties to other scheduling algorithms that we have studied.OS's are now more concerned with disk management. I.e., how to make a disk usable to users.Friday, March 25 CS 470 Operating Systems - Lecture 29 13FormattingLow-level, physical formatting is done at the factory, but OS can do this, too.File system formattingCreate a partition table that groups cylinders into a virtual disk. Tools like fdisk, sfdisk, PartitionMagicCreate file system. In Unix, makefs allocates inodes (index blocks).Create swap space.Friday, March 25 CS 470 Operating Systems - Lecture 29 14Boot BlockHow does a computer find the OS to boot? Cannot require that it be in a particular location on a particular disk, since we can choose between more than one.Bootstrap loader is a program that loads OS's. It could be stored in ROM, but then would be hard to change. Usually very small loader is stored in ROM that knows where the loader program is in the boot block (aka MBR - master boot record). Example loaders include grub, lilo, the Windows loader,...Friday, March 25 CS 470 Operating Systems - Lecture 29 15Boot BlockBoot loaders know how to initialize the CPU and bring up the file system. They are configured to know where the OS program code resides.


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