1、Chapter 12: Mass-Storage Systems,Adapted to COP4610 by Robert van Engelen,Overview of Mass Storage Structure: Tape,Magnetic tape Early secondary-storage medium Relatively permanent and holds large quantities of data Access time slow Random access 1000 times slower than disk Mainly used for backup, s
2、torage 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 20-200GB typical storage Common technologies are 4mm, 8mm, 19mm, LTO-2 and SDLT,Overview of Mass Storage Structure: Disk,M
3、agnetic disks provide bulk of secondary storage of modern computers Drives rotate at 60 to 200 times per second 3,600 -12,000 RPM Transfer rate is the rate at which data flows between drive and computer Positioning time (random-access time) is time to move disk arm to desired cylinder (seek time) an
4、d time for desired sector to rotate under the disk head (rotational latency) Head crash results from disk head making contact with the disk surface Thats bad,Overview of Mass Storage Structure: Disk,Disks can be removable, mounted as neededDrive attached to computer via I/O bus Busses vary, includin
5、g EIDE, ATA, SATA, USB, Fiber Channel, SCSI, and FireWire Host controller in computer uses bus to talk to disk controller built into drive or storage array,Disk Structure,Disk drives are addressed as large 1-dimensional arrays of logical blocks The logical block is the smallest unit of transfer, usu
6、ally 512 bytes The 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 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 cylind
7、ers from outermost to innermost,Moving-head disk mechanism,Disk 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 initiator requests operation and SCSI targets perform tasks Each target can have up to 8 logical
8、 units (disks attached to device controller FC 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 attach to many storage units Can be arbitrated loop (FC-AL) of 126 devices,Network-Attached Storage,Network-
9、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 protocol,Storage Area Netw
10、ork,Common in large storage environments (and becoming more common) Multiple hosts attached to multiple storage arrays - flexible,Disk Scheduling,The operating system is responsible for using hardware efficiently for the disk drives, this means having a fast access time and high disk bandwidth Acces
11、s 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 bandwidt
12、h 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,Disk Scheduling (Cont.),Several algorithms exist to schedule the servicing of disk I/O requests FCFS SSTF SCAN and C-SCAN LOOK and C-LOOKWe illustrate t
13、hem with a request queue (0-199):98, 183, 37, 122, 14, 124, 65, 67Head pointer 53,FCFS,Illustration shows total head movement of 640 cylinders using FCFS,SSTF,FCFS is sub-optimal and results in many head movements FCFS only sees the topmost request in the queue The head may move over a later request
14、ed position multiple timesSSTF selects the request from a batch of requests that has minimum seek time from the current head positionSSTF scheduling is a form of SJF scheduling; may cause starvation of some requests,SSTF (Cont.),Illustration shows total head movement of 236 cylinders,SCAN,SSTF may c
15、ause starvation when many requests are closely clustered and the outer track requests never get a chance to be servicedSCAN algorithm keeps moving in a single direction and then reverses The disk arm starts at one end of the disk, and moves toward the other end, servicing requests The head movement
16、is reversed when it gets to the other end of the disk, and servicing continues in the other directionAlso called the elevator algorithm,SCAN (Cont.),Illustration shows total head movement of 208 cylinders,C-SCAN,Provides a more uniform wait time than SCANThe head moves from one end of the disk to th
17、e 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 tripTreats the cylinders as a circular list that wraps around from the last cylinder to the first one,C-SCAN (Cont.),Illust
18、ration shows total head movement of 382 cylinders,C-LOOK,Version of C-SCANArm 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,C-LOOK (Cont.),Illustration shows total head movement of 322 cylinders,Sel
19、ecting 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-schedul
20、ing 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,Disk Management,Low-level formatting, or physical formatting Dividing a disk into sect
21、ors 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” Methods such as sector sparing used to hand
22、le bad blocks Boot block reserved to initialize the system The bootstrap is stored in ROM Bootstrap loader program,Booting from a Disk in Windows 2000,Swap-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
23、, 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
24、 memory, not when the virtual memory page is first created,Data Structures for Swapping on Linux Systems,Linux supports more than one swap area Each swap area consist of 4KB page slots With each swap area a swap map with integer counters indicating how many processes share the page slot,RAID Structu
25、re,RAID Redundant Arrays of Inexpensive DisksMultiple disk drives provides reliability via redundancy 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 le
26、ss redundancyDisk striping uses a group of disks as one storage unit,RAID Levels,RAID is arranged into six different levels,RAID (0 + 1) and (1 + 0),Stable-Storage Implementation,Write-ahead log scheme requires stable storage To implement stable storage: Replicate information on more than one nonvol
27、atile 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,Tertiary Storage Devices,Low cost is the defining characteristic of tertiary storage Generally, tertiary stora
28、ge is built using removable media Common examples of removable media are floppy disks and CD-ROMs; other types are available,Removable Disks,Floppy disk thin flexible disk coated with magnetic material, enclosed in a protective plastic case Most floppies hold about 1.44 MB; similar technology is use
29、d for removable disks that hold more than 1 GB Removable magnetic disks can be nearly as fast as hard disks, but they are at a greater risk of damage from exposure,Removable Disks (Cont.),A magneto-optic disk records data on a rigid platter coated with magnetic material Laser heat is used to amplify
30、 a large, weak magnetic field to record a bit Laser light is also used to read data (Kerr effect) The magneto-optic head flies much farther from the disk surface than a magnetic disk head, and the magnetic material is covered with a protective layer of plastic or glass; resistant to head crashes Opt
31、ical disks do not use magnetism; they employ special materials that are altered by laser light,WORM Disks,The data on read-write disks can be modified over and over, such as CD-RW disks WORM (“Write Once, Read Many Times”) disks can be written only once Thin aluminum film sandwiched between two glas
32、s or plastic platters To write a bit, the drive uses a laser light to burn a small hole through the aluminum; information can be destroyed but not altered Very durable and reliable Read Only disks, such ad CD-ROM and DVD, come from the factory with the data pre-recorded,Tapes,Compared to a disk, a t
33、ape is less expensive and holds more data, but random access is much slower Tape is an economical medium for purposes that do not require fast random access, e.g., backup copies of disk data, holding huge volumes of data Large tape installations typically use robotic tape changers that move tapes be
34、tween tape drives and storage slots in a tape library stacker library that holds a few tapes silo library that holds thousands of tapes A disk-resident file can be archived to tape for low cost storage; the computer can stage it back into disk storage for active use,Tape Drives,The basic operations
35、for a tape drive differ from those of a disk drive locate positions the tape to a specific logical block, not an entire track (corresponds to seek) The read position operation returns the logical block number where the tape head is The space operation enables relative motion Tape drives are “append-
36、only” devices; updating a block in the middle of the tape also effectively erases everything beyond that block An EOT mark is placed after a block that is written,Operating System Issues,Major OS jobs are to manage physical devices and to present a virtual machine abstraction to applications For har
37、d disks, the OS provides two abstraction: Raw device an array of data blocks File system the OS queues and schedules the interleaved requests from several applications,Application Interface,Most OSs handle removable disks almost exactly like fixed disks a new cartridge is formatted and an empty file
38、 system is generated on the disk Tapes are presented as a raw storage medium, i.e., and application does not not open a file on the tape, it opens the whole tape drive as a raw device Usually the tape drive is reserved for the exclusive use of that application Since the OS does not provide file syst
39、em services, the application must decide how to use the array of blocks Since every application makes up its own rules for how to organize a tape, a tape full of data can generally only be used by the program that created it,File Naming,The issue of naming files on removable media is especially diff
40、icult when we want to write data on a removable cartridge on one computer, and then use the cartridge in another computerContemporary OSs generally leave the name space problem unsolved for removable media, and depend on applications and users to figure out how to access and interpret the dataSome k
41、inds of removable media (e.g., CDs) are so well standardized that all computers use them the same way,Hierarchical Storage Management (HSM),A hierarchical storage system extends the storage hierarchy beyond primary memory and secondary storage to incorporate tertiary storage usually implemented as a
42、 jukebox of tapes or removable disksUsually incorporate tertiary storage by extending the file system Small and frequently used files remain on disk Large, old, inactive files are archived to the jukeboxHSM is usually found in supercomputing centers and other large installations that have enormous v
43、olumes of data,Speed,Two aspects of speed in tertiary storage are bandwidth and latency Bandwidth is measured in bytes per second Sustained bandwidth average data rate during a large transfer; # of bytes/transfer time = Data rate when the data stream is actually flowing. Effective bandwidth average
44、over the entire I/O time, including seek or locate, and cartridge switching = Drives overall data rate,Speed (Cont.),Access latency amount of time needed to locate data Access time for a disk move the arm to the selected cylinder and wait for the rotational latency; 35 milliseconds Access on tape re
45、quires winding the tape reels until the selected block reaches the tape head; tens or hundreds of seconds Generally say that random access within a tape cartridge is about a thousand times slower than random access on disk The low cost of tertiary storage is a result of having many cheap cartridges
46、share a few expensive drives A removable library is best devoted to the storage of infrequently used data, because the library can only satisfy a relatively small number of I/O requests per hour,Reliability,A fixed disk drive is likely to be more reliable than a removable disk or tape drive An optic
47、al cartridge is likely to be more reliable than a magnetic disk or tape A head crash in a fixed hard disk generally destroys the data, whereas the failure of a tape drive or optical disk drive often leaves the data cartridge unharmed,Cost,Main memory is much more expensive than disk storage The cost
48、 per megabyte of hard disk storage is competitive with magnetic tape if only one tape is used per drive The cheapest tape drives and the cheapest disk drives have had about the same storage capacity over the years Tertiary storage gives a cost savings only when the number of cartridges is considerably larger than the number of drives,Price per Megabyte of DRAM, From 1981 to 2004,Price per Megabyte of Magnetic Hard Disk, From 1981 to 2004,Price per Megabyte of a Tape Drive, From 1984-2000,End of Chapter 12,
