Bài giảng Hệ điều hành nâng cao - Chapter 12: Mass - Storage Systems

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Bài giảng Hệ điều hành nâng cao - Chapter 12: Mass - Storage Systems

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  1. Chapter 12: Mass-Storage Systems Operating System Concepts – 8th 8th Edition Operating System Concepts – Edition 12.1 Silberschatz, Galvin and Gagne ©2009
  2. Chapter 12: Mass-Storage Systems s Overview of Mass Storage Structure s Disk Structure s Disk Attachment s Disk Scheduling s Disk Management s Swap-Space Management s RAID Structure s Stable-Storage Implementation s Tertiary Storage Devices Operating System Concepts – 8th Edition 12.2 Silberschatz, Galvin and Gagne ©2009
  3. Objectives s Describe the physical structure of secondary and tertiary storage devices and the resulting effects on the uses of the devices s Explain the performance characteristics of mass-storage devices s Discuss operating-system services provided for mass storage, including RAID and HSM Operating System Concepts – 8th Edition 12.3 Silberschatz, Galvin and Gagne ©2009
  4. Overview of Mass Storage Structure s Magnetic disks provide bulk of secondary storage of modern computers q Drives rotate at 60 to 250 times per second q Transfer rate is rate at which data flow between drive and computer q 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) q Head crash results from disk head making contact with the disk surface 4 That’s bad s Disks can be removable s Drive attached to computer via I/O bus q Busses vary, including EIDE, ATA, SATA, USB, Fibre Channel, SCSI, SAS, Firewire q Host controller in computer uses bus to talk to disk controller built into drive or storage array Operating System Concepts – 8th Edition 12.4 Silberschatz, Galvin and Gagne ©2009
  5. Magnetic Disks s Platters range from .85” to 14” (historically) q Commonly 3.5”, 2.5”, and 1.8” s Range from 30GB to 3TB per drive s Performance q Transfer Rate – theoretical – 6 Gb/sec q Effective Transfer Rate – real – 1Gb/sec q Seek time from 3ms to 12ms – 9ms common for desktop drives q Average seek time measured or calculated based on 1/3 of tracks q Latency based on spindle speed 4 1/(RPM * 60) q Average latency = ½ latency (From Wikipedia) Operating System Concepts – 8th Edition 12.5 Silberschatz, Galvin and Gagne ©2009
  6. Magnetic Disk Performance s Access Latency = Average access time = average seek time + average latency q For fastest disk 3ms + 2ms = 5ms q For slow disk 9ms + 5.56ms = 14.56ms s Average I/O time = average access time + (amount to transfer / transfer rate) + controller overhead s For example to transfer a 4KB block on a 7200 RPM disk with a 5ms average seek time, 1Gb/sec transfer rate with a .1ms controller overhead = q 5ms + 4.17ms + 4KB / 1Gb/sec + 0.1ms = q 9.27ms + 4 / 131072 sec = q 9.27ms + .12ms = 9.39ms Operating System Concepts – 8th Edition 12.6 Silberschatz, Galvin and Gagne ©2009
  7. Moving-head Disk Mechanism Operating System Concepts – 8th Edition 12.7 Silberschatz, Galvin and Gagne ©2009
  8. The First Commercial Disk Drive 1956 IBM RAMDAC computer included the IBM Model 350 disk storage system 5M (7 bit) characters 50 x 24” platters Access time = < 1 second Operating System Concepts – 8th Edition 12.8 Silberschatz, Galvin and Gagne ©2009
  9. Magnetic Tape s Was early secondary-storage medium q Evolved from open spools to cartridges s Relatively permanent and holds large quantities of data s Access time slow s Random access ~1000 times slower than disk s Mainly used for backup, storage of infrequently-used data, transfer medium between systems s Kept in spool and wound or rewound past read-write head s Once data under head, transfer rates comparable to disk q 140MB/sec and greater s 200GB to 1.5TB typical storage s Common technologies are LTO-{3,4,5} and T10000 Operating System Concepts – 8th Edition 12.9 Silberschatz, Galvin and Gagne ©2009
  10. Disk Structure s Disk drives are addressed as large 1-dimensional arrays of logical blocks, where the logical block is the smallest unit of transfer s The 1-dimensional array of logical blocks is mapped into the sectors of the disk sequentially q Sector 0 is the first sector of the first track on the outermost cylinder q 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 q Logical to physical address should be easy 4 Except for bad sectors 4 Non-constant # of sectors per track via constant angular velocity Operating System Concepts – 8th Edition 12.10 Silberschatz, Galvin and Gagne ©2009
  11. Disk Attachment s Host-attached storage accessed through I/O ports talking to I/O busses s SCSI itself is a bus, up to 16 devices on one cable, SCSI initiator requests operation and SCSI targets perform tasks q Each target can have up to 8 logical units (disks attached to device controller) s FC is high-speed serial architecture q Can be switched fabric with 24-bit address space – the basis of storage area networks (SANs) in which many hosts attach to many storage units s I/O directed to bus ID, device ID, logical unit (LUN) Operating System Concepts – 8th Edition 12.11 Silberschatz, Galvin and Gagne ©2009
  12. Storage Array s Can just attach disks, or arrays of disks s Storage Array has controller(s), provides features to attached host(s) q Ports to connect hosts to array q Memory, controlling software (sometimes NVRAM, etc) q A few to thousands of disks q RAID, hot spares, hot swap (discussed later) q Shared storage -> more efficiency q Features found in some file systems 4 Snaphots, clones, thin provisioning, replication, deduplication, etc Operating System Concepts – 8th Edition 12.12 Silberschatz, Galvin and Gagne ©2009
  13. Storage Area Network s Common in large storage environments s Multiple hosts attached to multiple storage arrays - flexible Operating System Concepts – 8th Edition 12.13 Silberschatz, Galvin and Gagne ©2009
  14. Storage Area Network (Cont.) s SAN is one or more storage arrays q Connected to one or more Fibre Channel switches s Hosts also attach to the switches s Storage made available via LUN Masking from specific arrays to specific servers s Easy to add or remove storage, add new host and allocate it storage q Over low-latency Fibre Channel fabric s Why have separate storage networks and communications networks? q Consider iSCSI, FCOE Operating System Concepts – 8th Edition 12.14 Silberschatz, Galvin and Gagne ©2009
  15. Network-Attached Storage s Network-attached storage (NAS) is storage made available over a network rather than over a local connection (such as a bus) q Remotely attaching to file systems s NFS and CIFS are common protocols s Implemented via remote procedure calls (RPCs) between host and storage over typically TCP or UDP on IP network s iSCSI protocol uses IP network to carry the SCSI protocol q Remotely attaching to devices (blocks) Operating System Concepts – 8th Edition 12.15 Silberschatz, Galvin and Gagne ©2009
  16. Disk Scheduling s The operating system is responsible for using hardware efficiently — for the disk drives, this means having a fast access time and disk bandwidth s Minimize seek time s Seek time seek distance s 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 Operating System Concepts – 8th Edition 12.16 Silberschatz, Galvin and Gagne ©2009
  17. Disk Scheduling (Cont.) s There are many sources of disk I/O request q OS q System processes q Users processes s I/O request includes input or output mode, disk address, memory address, number of sectors to transfer s OS maintains queue of requests, per disk or device s Idle disk can immediately work on I/O request, busy disk means work must queue q Optimization algorithms only make sense when a queue exists s Note that drive controllers have small buffers and can manage a queue of I/O requests (of varying “depth”) s Several algorithms exist to schedule the servicing of disk I/O requests s The analysis is true for one or many platters s We illustrate scheduling algorithms with a request queue (0-199) 98, 183, 37, 122, 14, 124, 65, 67 Head pointer 53 Operating System Concepts – 8th Edition 12.17 Silberschatz, Galvin and Gagne ©2009
  18. FCFS Illustration shows total head movement of 640 cylinders Operating System Concepts – 8th Edition 12.18 Silberschatz, Galvin and Gagne ©2009
  19. SSTF s Shortest Seek Time First selects the request with the minimum seek time from the current head position s SSTF scheduling is a form of SJF scheduling; may cause starvation of some requests s Illustration shows total head movement of 236 cylinders Operating System Concepts – 8th Edition 12.19 Silberschatz, Galvin and Gagne ©2009
  20. SSTF (Cont.) Operating System Concepts – 8th Edition 12.20 Silberschatz, Galvin and Gagne ©2009

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