CSE Opera+ng System Principles

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1 CSE Opera+ng System rinciples Mass Storage Mass-Storage Systems Overview of Mass Storage Structure Disk Structure Disk AGachment Disk Scheduling Disk Management Swap-Space Management RAID Structure Stable-Storage Implementa+on CSE Opera+ng System rinciples 2 1

2 Objec+ves To describe the physical structure of secondary storage devices and its effects on the uses of the devices To explain the performance characteris;cs of massstorage devices To evaluate disk scheduling algorithms To discuss opera;ng-system services provided for mass storage, including RAID CSE Opera+ng System rinciples 3 Moving-Head Disk Mechanism track t spindle sector s arm assembly Latency Rota+on +me cylinder c read-write head Seek Time Time to move the head platter arm rotation CSE Opera+ng System rinciples 4 2

3 Disk Structure CSE Opera+ng System rinciples 5 Disk Structure CSE Opera+ng System rinciples 6 3

4 Overview of Mass Storage Structure Magne;c disks provide bulk of secondary storage of modern computers Drives rotate at 60 to 250 +mes per second Transfer rate is rate at which data flow between drive and computer osi;oning ;me (random-access ;me) is +me to move disk arm to desired cylinder (seek ;me) and +me for desired sector to rotate under the disk head (rota;onal latency) Head crash results from disk head making contact with the disk surface Disks can be removable Drive agached to computer via I/O bus Busses vary, including EIDE, ATA, SATA, USB, Fibre Channel, SCSI, SAS, Firewire Host controller in computer uses bus to talk to disk controller built into drive or storage array CSE Opera+ng System rinciples 7 Magne+c Disks lagers range from.85 to 14 (historically) Commonly 3.5, 2.5, and 1.8 Range from 30GB to 3TB per drive erformance Seek ;me from 3ms to 12ms 9ms common for desktop drives (From Wikipedia) Rota;onal latency based on spindle speed Transfer Rate theore+cal 6 Gb/sec (effec+ve Transfer Rate real 1Gb/sec) CSE Opera+ng System rinciples 8 4

5 Magne+c Disk erformance Access Latency = Average access ;me = average seek +me + average latency For fastest disks: 3ms + 2ms = 5ms For slow disks: 9ms ms = 14.56ms Average I/O ;me = average access +me + (amount to transfer / transfer rate) + controller overhead For example to transfer a 4KB block on a 7200 RM disk with a 5ms average seek +me, 1Gb/sec transfer rate with a.1ms controller overhead = 5ms ms + 4KB / 1Gb/sec + 0.1ms = 9.27ms + 4 / sec = 9.27ms +.12ms = 9.39ms CSE Opera+ng System rinciples 9 The First Commercial Disk Drive 1956 IBM RAMAC computer included the IBM Model 350 disk storage system 5M (7-bit) characters 50 x 24 plagers Access +me = < 1 second CSE Opera+ng System rinciples 10 5

6 Modern Disk Drives CSE Opera+ng System rinciples 11 Solid-State Disks (SSD) Nonvola+le memory used like a hard drive Many technology varia+ons Can be more reliable than HDDs More expensive per MB Maybe have shorter life span Less capacity But much faster Quiet Busses can be too slow -> connect directly to CI for example No moving parts, so no seek +me or rota+onal latency CSE Opera+ng System rinciples 12 6

7 Magne+c Tape Was early secondary-storage medium Evolved from open spools to cartridges Rela+vely permanent and holds large quan++es of data Access +me slow Random access ~1000 +mes slower than disk Mainly used for backup, storage of infrequently-used data, transfer medium between systems Kept in spool and wound or rewound past read-write head Once data under head, transfer rates comparable to disk 140MB/sec and greater 200GB to 1.5TB typical storage Common technologies are LTO-{3,4,5} and T10000 CSE Opera+ng System rinciples 13 Magne+c Tapes CSE Opera+ng System rinciples 14 7

8 Disk Structure Disk drives are addressed as large 1-dimensional arrays of logical blocks, where the logical block is the smallest unit of transfer Low-level formamng creates logical blocks on physical media The 1-dimensional array of logical blocks is mapped into the sectors of the disk sequen+ally Sector 0 is the first sector of the first track on the outermost cylinder 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 Logical to physical address should be easy Except for bad sectors Non-constant # of sectors per track via constant angular velocity CSE Opera+ng System rinciples 15 Zone Bit Recording CSE Opera+ng System rinciples 16 8

9 LBA vs. Non-LBA Logical Block Addressing CHS Scheme: Old school must know disk geometry CSE Opera+ng System rinciples 17 N lug and lay CSE Opera+ng System rinciples 18 9

10 Disk AGachment Host-aQached storage accessed through I/O ports talking to I/O busses SCSI itself is a bus, up to 16 devices on one cable, SCSI ini;ator requests opera+on and SCSI targets perform tasks Each target can have up to 8 logical units (disks agached to device controller) FC (fibre channel) is high-speed serial architecture Can be switched fabric with 24-bit address space the basis of storage area networks (SANs) in which many hosts agach to many storage units CSE Opera+ng System rinciples 19 Storage Array Can just agach disks or arrays of disks Storage Array has controller(s), provides features to agached host(s) orts to connect hosts to array Memory, controlling soqware (some+mes NVRAM, etc.) A few to thousands of disks RAID, hot spares, hot swap (discussed later) Shared storage -> more efficiency CSE Opera+ng System rinciples 20 10

11 Storage Area Network Common in large storage environments Mul+ple hosts agached to mul+ple storage arrays - flexible storage array server server LAN/WAN client client client storage array SAN data-processing center tape library web content CSE Opera+ng System provider rinciples 21 Storage Area Network (Cont.) SAN is one or more storage arrays Connected to one or more Fibre Channel switches Hosts also agach to the switches Easy to add or remove storage, add new host and allocate storage to it, etc. Why have separate storage networks and communica+ons networks? CSE Opera+ng System rinciples 22 11

12 Network-AGached Storage Network-aGached storage (NAS) is storage made available over a network rather than over a local connec+on (such as a bus) Remotely agaching to file systems NFS and CIFS are common protocols Implemented via remote procedure calls (RCs) between host and storage over typically TC or UD on I network iscsi protocol is version of SCSI based on I to link data storage facili+es CSE Opera+ng System rinciples 23 Network-AGached Storage CSE Opera+ng System rinciples 24 12

13 Disk Scheduling The opera+ng system is responsible for using hardware efficiently for the disk drives, this means having a fast access ;me and large disk bandwidth Minimize seek +me Seek +me seek distance Disk bandwidth is the total number of bytes transferred, divided by the total +me between the first request for service and the comple+on of the last transfer CSE Opera+ng System rinciples 25 Disk Scheduling (Cont.) Several algorithms exist to schedule the servicing of disk I/O requests The analysis is true for one or many plagers We illustrate scheduling algorithms with a request queue (0-199) 98, 183, 37, 122, 14, 124, 65, 67 Head pointer 53 CSE Opera+ng System rinciples 26 13

14 Disk Scheduling (Cont.) Idle disk can immediately work on I/O request Busy disk means work must queue Op+miza+on algorithms only make sense when a queue exists Note that drive controllers have small buffers and can manage a queue of I/O requests CSE Opera+ng System rinciples 27 FCFS Illustration shows total head movement of 640 cylinders CSE Opera+ng System rinciples 28 14

15 SSTF Shortest Seek Time First selects the request with the minimum seek +me from the current head posi+on SSTF scheduling is a form of SJF scheduling; may cause starva+on of some requests Illustra+on shows total head movement of 236 cylinders CSE Opera+ng System rinciples 29 SSTF (Cont.) CSE Opera+ng System rinciples 30 15

16 SCAN The disk arm starts at one end of the disk, and moves toward the other end, servicing requests un+l it gets to the other end of the disk, where the head movement is reversed and servicing con+nues SCAN algorithm some+mes called the elevator algorithm Illustra+on shows total head movement of 208 cylinders CSE Opera+ng System rinciples 31 SCAN (Cont.) CSE Opera+ng System rinciples 32 16

17 C-SCAN rovides a more uniform wait +me 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 CSE Opera+ng System rinciples 33 C-SCAN (Cont.) CSE Opera+ng System rinciples 34 17

18 LOOK & C-LOOK LOOK a version of SCAN, C-LOOK a version of C-SCAN Arm only goes as far as the last request in each direc+on, then reverses direc+on immediately, without first going all the way to the end of the disk CSE Opera+ng System rinciples 35 C-LOOK (Cont.) CSE Opera+ng System rinciples 36 18

19 Selec+ng a Disk-Scheduling Algorithm SSTF is common and has a natural appeal for low load disks (quickly go to next request) LOOK, C-LOOK, SCAN and C-SCAN perform beger for systems that place a heavy load on the disk (no starva+on, more predictable delays) erformance depends on the number and types of requests Requests for disk service can be influenced by the file-alloca+on method CSE Opera+ng System rinciples 37 Key oints The disk does not understand files It understands blocks It is up to the OS to put things intelligently It does not understand security either CSE Opera+ng System rinciples 38 19

20 Disk Management Low-level formavng, or physical formavng Dividing a disk into sectors that the disk controller can read and write Each sector can hold header informa+on, plus data, plus error correc+on code (ECC) Usually 512 bytes of data but can be selectable To use a disk to hold files, the opera+ng system s+ll needs to record its own data structures on the disk ar;;on the disk into one or more groups of cylinders, each treated as a logical disk Logical formavng or making a file system To increase efficiency most file systems group blocks into clusters Disk I/O done in blocks File I/O done in clusters of blocks CSE Opera+ng System rinciples 39 Disk Management Raw disk access for apps that want to do their own block management, keep OS out of the way (databases for example) Boot block ini+alizes system The bootstrap is stored in ROM Bootstrap loader program stored in boot blocks of boot par++on Methods such as sector sparing used to handle bad blocks CSE Opera+ng System rinciples 40 20

21 Boo+ng from a Disk in Windows CSE Opera+ng System rinciples 41 Swap-Space Management Swap space Virtual memory uses disk space as an extension of main memory Becoming less common due to memory capacity increases Swap space can be carved out of the normal file system, or, more commonly, it can be in a separate disk par++on (raw) CSE Opera+ng System rinciples 42 21

22 Swap-Space Management 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 dirty page is forced out of physical memory, not when the virtual memory page is first created File data wrigen to swap space un+l write to file system requested Other dirty pages go to swap space due to no other home Text segment pages thrown out and reread from the file system as needed What if a system runs out of swap space? Some systems allow mul+ple swap spaces Swap space is a form of graceful degrada;on CSE Opera+ng System rinciples 43 Swap Map (Linux) page slot swap area swap partition or swap file swap map CSE Opera+ng System rinciples 44 22

23 RAID Structure RAID redundant array of inexpensive/ independent disks Mul+ple disk drives provides reliability via redundancy Increases the mean ;me to failure Mean ;me to repair exposure +me when another failure could cause data loss Mean ;me to data loss based on above factors CSE Opera+ng System rinciples 45 Fault Tolerance OK We want to avoid this Fault Error Failure Suppose we have a system with 2 components that all must work Complexity is the bane of fault tolerance success 2 90% components à 81% CSE Opera+ng System rinciples 46 23

24 RAID Structure If mirrored disks fail independently, consider disk with 100,000 mean +me to failure and 10 hour mean +me to repair Mean +me to data loss is 100,000 2 / (2 10) = hours, or 57,000 years! Frequently combined with NVRAM to improve write performance RAID is arranged into six different levels CSE Opera+ng System rinciples 47 RAID (Cont.) Several improvements in disk-use techniques involve the use of mul+ple disks working coopera+vely 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 (RAID 1) keeps duplicate of each disk Striped mirrors (RAID 1+0) or mirrored stripes (RAID 0+1) provides high performance and high reliability Block interleaved parity (RAID 4, 5, 6) uses much less redundancy CSE Opera+ng System rinciples 48 24

25 RAID (Cont.) RAID within a storage array can s+ll fail if the array fails, so automa+c replica;on of the data between arrays is common Frequently, a small number of hot-spare disks are leq unallocated, automa+cally replacing a failed disk and having data rebuilt onto them CSE Opera+ng System rinciples 49 RAID Levels (a) RAID 0: non-redundant striping. (b) RAID 1: mirrored disks. C C C C (c) RAID 2: memory-style error-correcting codes. (d) RAID 3: bit-interleaved parity. (e) RAID 4: block-interleaved parity. (f) RAID 5: block-interleaved distributed parity. (g) RAID 6: Q redundancy. CSE Opera+ng System rinciples 50 25

26 RAID (0 + 1) and (1 + 0) stripe x mirror stripe a) RAID 0 1 with a single disk failure. x stripe mirror mirror mirror mirror b) RAID 1 0 with a single disk failure. CSE Opera+ng System rinciples 51 Ter+ary Storage Low cost Removable media Floppy disks thin flexible disk coated with magne+c material, enclosed in protec+ve plas+c case Magneto-op+c disks rigid plager coated with magne+c material laser heat used to amplify a large/weak magne+c field to record a bit; also used to read data (Kerr effect) very resistant to head crashes CSE Opera+ng System rinciples 52 26

27 Ter+ary Storage Op+cal disks special materials that can be altered by a laser phase-change disks: material freezes into either crystalline or amorphous state WORM disks Write Once, Read Many aluminum film sandwiched between glass or plas+c plagers laser burns small hole through aluminum durable and reliable CSE Opera+ng System rinciples 53 Ter+ary Storage SSD: solid-state disk look like hard-drives, but no moving parts NAND-based flash memory fast, expensive, low-energy MEMS: microelectronic mechanical systems thousands of +ny disk heads CSE Opera+ng System rinciples 54 27

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