Mass Storage Systems - Operating Systems - Lecture Slides, Slides of Computer Science

Download Mass Storage Systems - Operating Systems - Lecture Slides and more Slides Computer Science in PDF only on Docsity! Mass-Storage Systems Chapter 12 Docsity.com Objectives • 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 HSM Docsity.com Overview of Mass Storage Structure (Cont.) • Magnetic tape – Was 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, 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 – 20-200GB typical storage – Common technologies are 4mm, 8mm, 19mm, LTO-2 and SDLT Docsity.com Disk 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 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. Docsity.com 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 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 Docsity.com Disk 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. Docsity.com 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, 67 Head pointer 53 Docsity.com FCFS Illustration shows total head movement of 640 cylinders. Docsity.com SCAN • 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. Docsity.com SCAN (Cont.) queue = 98, 183, 37, 122, 14, 124, 65, 67 head starts at 53 37 536567 98 122124 | Lu | Ll Docsity.com C-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. Docsity.com 0 | 14 ! C-LOOK (Cont.) 98, 183, 37, 122, 14, 124, 65, 67 queue head starts at 53 37 536567 | 98 122124 183199 Docsity.com Selecting 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. Docsity.com Disk 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. Docsity.com Data Structures for Swapping on Linux Systems /#—————_ Swap area page — slot — Swap partition or swap file swap map ® Docsity.com RAID Structure • RAID – multiple disk drives provides reliability via redundancy. • RAID is arranged into six different levels. Docsity.com RAID (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. Docsity.com Stable-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. Docsity.com Tertiary Storage Devices • Low cost is the defining characteristic of tertiary storage. • Generally, tertiary storage is built using removable media • Common examples of removable media are floppy disks and CD-ROMs; other types are available. Docsity.com Removable Disks • Floppy disk — thin flexible disk coated with magnetic material, enclosed in a protective plastic case. – Most floppies hold about 1 MB; similar technology is used 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. Docsity.com Tapes • Compared to a disk, a tape 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 between 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. Docsity.com Operating System Issues • Major OS jobs are to manage physical devices and to present a virtual machine abstraction to applications • For hard 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. Docsity.com Application Interface • Most OSs handle removable disks almost exactly like fixed disks — a new cartridge is formatted and an empty file 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 system 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. Docsity.com 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 jukebox of tapes or removable disks. • Usually incorporate tertiary storage by extending the file system. – Small and frequently used files remain on disk. – Large, old, inactive files are archived to the jukebox. • HSM is usually found in supercomputing centers and other large installations that have enormous volumes of data. Docsity.com 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 over the entire I/O time, including seek or locate, and cartridge switching. Drive’s overall data rate. Docsity.com 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 requires 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 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. Docsity.com Price per Megabyte of DRAM, From 1981 to 2004 Docsity.com Price per Megabyte of Magnetic Hard Disk, From 1981 to 2004 Docsity.com Price per Megabyte of a Tape Drive, From 1984-2000 Docsity.com