112.1Silberschatz, Galvin and Gagne ©2005Operating System ConceptsCSMC 412CSMC 412Operating SystemsProf. Ashok K Agrawala© 2005 Ashok AgrawalaSet 1212.2Silberschatz, Galvin and Gagne ©2005Operating System ConceptsMassMass--Storage SystemsStorage Systems Overview of Mass Storage Structure Disk Structure Disk Attachment Disk Scheduling Disk Management Swap-Space Management RAID Structure Disk Attachment Stable-Storage Implementation Tertiary Storage Devices Operating System Issues Performance Issues212.3Silberschatz, Galvin and Gagne ©2005Operating System ConceptsObjectivesObjectives Describe the physical structure of secondary and tertiary storage devices and the resulting effects on the uses of the devices Explain the performance characteristics of mass-storage devices Discuss operating-system services provided for mass storage, including RAID and HSM12.4Silberschatz, Galvin and Gagne ©2005Operating System ConceptsOverview of Mass Storage StructureOverview of Mass Storage Structure Magnetic disks provide bulk of secondary storage of modern computersz Drives rotate at 60 to 200 times per secondz Transfer rate is rate at which data flow between drive and computerz Positioning time (random-access time) is time to move disk arm to desired cylinder (seek time) and time for desired sector to rotate under the disk head (rotational latency)z Head crash results from disk head making contact with the disk surface That’s bad Disks can be removable Drive attached to computer via I/O busz Busses vary, including EIDE, ATA, SATA, USB, Fibre Channel, SCSIz Host controller in computer uses bus to talk to disk controller built into drive or storage array312.5Silberschatz, Galvin and Gagne ©2005Operating System ConceptsMovingMoving--head Disk head Disk MachanismMachanism12.6Silberschatz, Galvin and Gagne ©2005Operating System ConceptsOverview of Mass Storage Structure (Cont.)Overview of Mass Storage Structure (Cont.) Magnetic tapez Was early secondary-storage mediumz Relatively permanent and holds large quantities of dataz Access time slowz Random access ~1000 times slower than diskz Mainly used for backup, storage of infrequently-used data, transfer medium between systemsz Kept in spool and wound or rewound past read-write headz Once data under head, transfer rates comparable to diskz 20-200GB typical storagez Common technologies are 4mm, 8mm, 19mm, LTO-2 and SDLT412.7Silberschatz, Galvin and Gagne ©2005Operating System ConceptsDisk StructureDisk 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.z Sector 0 is the first sector of the first track on the outermostcylinder.z Mapping proceeds in order through that track, then the rest of the tracks in that cylinder, and then through the rest of the cylinders from outermost to innermost.12.8Silberschatz, Galvin and Gagne ©2005Operating System ConceptsDisk AttachmentDisk Attachment Host-attached storage accessed through I/O ports talking to I/O busses SCSI itself is a bus, up to 16 devices on one cable, SCSI initiatorrequests operation and SCSI targets perform tasks z Each target can have up to 8 logical units (disks attached to device controller FC is high-speed serial architecturez Can be switched fabric with 24-bit address space – the basis of storage area networks (SANs) in which many hosts attach to many storage unitsz Can be arbitrated loop (FC-AL) of 126 devices512.9Silberschatz, Galvin and Gagne ©2005Operating System ConceptsNetworkNetwork--Attached StorageAttached Storage Network-attached storage (NAS) is storage made available over a network rather than over a local connection (such as a bus) NFS and CIFS are common protocols Implemented via remote procedure calls (RPCs) between host and storage New iSCSI protocol uses IP network to carry the SCSI protocol12.10Silberschatz, Galvin and Gagne ©2005Operating System ConceptsStorage Area NetworkStorage Area Network Common in large storage environments (and becoming more common) Multiple hosts attached to multiple storage arrays - flexible612.11Silberschatz, Galvin and Gagne ©2005Operating System ConceptsDisk SchedulingDisk 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 componentsz Seek time is the time for the disk are to move the heads to the cylinder containing the desired sector.z 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.12.12Silberschatz, Galvin and Gagne ©2005Operating System ConceptsDisk Scheduling (Cont.)Disk 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 53712.13Silberschatz, Galvin and Gagne ©2005Operating System ConceptsFCFSFCFSIllustration shows total head movement of 640 cylinders.12.14Silberschatz, Galvin and Gagne ©2005Operating System ConceptsSSTFSSTF 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.812.15Silberschatz, Galvin and Gagne ©2005Operating System ConceptsSSTF (Cont.)SSTF (Cont.)12.16Silberschatz, Galvin and Gagne ©2005Operating System ConceptsSCANSCAN The disk arm starts at one end of the disk, and moves toward theother 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.912.17Silberschatz, Galvin and Gagne ©2005Operating System ConceptsSCAN (Cont.)SCAN (Cont.)12.18Silberschatz, Galvin and Gagne ©2005Operating System ConceptsCC--SCANSCAN Provides a more uniform wait time than SCAN. The head moves from one end of the disk to the other. servicing
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