114.1Operating System ConceptsCSMC 412Operating SystemsProf. Ashok K Agrawala© 2004 Ashok AgrawalaSet 1414.2Operating System ConceptsMass-Storage Systems Disk Structure Disk Scheduling Disk Management Swap-Space Management RAID Structure Disk Attachment Stable-Storage Implementation Tertiary Storage Devices Operating System Issues Performance Issues214.3Operating System ConceptsDisk Structure Disk drives are addressed as large 1-dimensional arrays of logical blocks, where the logical block is the smallest unit of transfer. The 1-dimensional array of logical blocks is mapped into the sectors of the disk sequentially.) Sector 0 is the first sector of the first track on the outermostcylinder.) Mapping proceeds in order through that track, then the rest of the tracks in that cylinder, and then through the rest of thecylinders from outermost to innermost.14.4Operating System ConceptsDisk Scheduling The operating system is responsible for using hardware efficiently — for the disk drives, this means having a fast access time and disk bandwidth. Access time has two major components) Seek time is the time for the disk are to move the heads to the cylinder containing the desired sector.) Rotational latency is the additional time waiting for the disk to rotate the desired sector to the disk head.Minimize seek time Seek time ≈ seek distance Disk bandwidth is the total number of bytes transferred, divided by the total time between the first request for service and the completion of the last transfer.314.5Operating System ConceptsDisk Scheduling (Cont.) Several algorithms exist to schedule the servicing of disk I/O requests. We illustrate them with a request queue (0-199).98, 183, 37, 122, 14, 124, 65, 67Head pointer 5314.6Operating System ConceptsFCFSIllustration shows total head movement of 640 cylinders.414.7Operating System ConceptsSSTF Selects the request with the minimum seek time from the current head position. SSTF scheduling is a form of SJF scheduling; may cause starvation of some requests. Illustration shows total head movement of 236 cylinders.14.8Operating System ConceptsSSTF (Cont.)514.9Operating System ConceptsSCAN The disk arm starts at one end of the disk, and moves toward the other end, servicing requests until it gets to the other end of the disk, where the head movement is reversed and servicing continues. Sometimes called the elevator algorithm. Illustration shows total head movement of 208 cylinders.14.10Operating System ConceptsSCAN (Cont.)614.11Operating System ConceptsC-SCAN Provides a more uniform wait time than SCAN. The head moves from one end of the disk to the other. servicing requests as it goes. When it reaches the other end, however, it immediately returns to the beginning of the disk, without servicing any requests on the return trip. Treats the cylinders as a circular list that wraps around from the last cylinder to the first one.14.12Operating System ConceptsC-SCAN (Cont.)714.13Operating System ConceptsC-LOOK Version of C-SCAN Arm only goes as far as the last request in each direction, then reverses direction immediately, without first going all the way to the end of the disk. 14.14Operating System ConceptsC-LOOK (Cont.)814.15Operating System ConceptsSelecting a Disk-Scheduling Algorithm SSTF is common and has a natural appeal SCAN and C-SCAN perform better for systems that place a heavy load on the disk. Performance depends on the number and types of requests. Requests for disk service can be influenced by the 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.14.16Operating System ConceptsDisk Management Low-level formatting, or physical formatting — Dividing a disk into sectors that the disk controller can read and write. 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.) Bootstrap loader program.Methods such as sector sparing used to handle bad blocks.914.17Operating System ConceptsMS-DOS Disk Layout14.18Operating System ConceptsSwap-Space Management Swap-space — Virtual memory uses disk space as an extension of main memory. Swap-space can be carved out of the normal file system,or, more commonly, it can be in a separate disk partition. Swap-space management) 4.3BSD allocates swap space when process starts; holds text segment (the program) and data segment.) Kernel uses swap maps to track swap-space use.) Solaris 2 allocates swap space only when a page is forced out of physical memory, not when the virtual memory page is first created.1014.19Operating System Concepts4.3 BSD Text-Segment Swap Map 14.20Operating System Concepts4.3 BSD Data-Segment Swap Map1114.21Operating System ConceptsRAID Structure RAID – multiple disk drives provides reliability via redundancy. RAID is arranged into six different levels.14.22Operating System ConceptsRAID (cont) Several improvements in disk-use techniques involve the use of multiple disks working cooperatively. Disk striping uses a group of disks as one storage unit. RAID schemes improve performance and improve the reliability of the storage system by storing redundant data.) Mirroring or shadowing keeps duplicate of each disk.) Block interleaved parity uses much less redundancy.1214.23Operating System ConceptsRAID Levels14.24Operating System ConceptsRAID (0 + 1) and (1 + 0)1314.25Operating System ConceptsDisk Attachment Disks may be attached one of two ways:1. Host attached via an I/O port2. Network attached via a network connection14.26Operating System ConceptsNetwork-Attached Storage1414.27Operating System ConceptsStorage-Area Network14.28Operating System ConceptsStable-Storage Implementation Write-ahead log scheme requires stable storage. To implement stable storage:) Replicate information on more than one nonvolatile storage media with independent failure modes.) Update information in a controlled manner to ensure that we can recover the stable data after any failure during data transfer or recovery.1514.29Operating
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