Mass Storage Systems and Disk Management

Virtual Memory Review

  • Logical address space can be much larger than physical address space.
  • Page fault handling.
  • Page replacement algorithms:
    • FIFO
    • MIN
    • RANDOM
    • LRU
  • LRU Approximations
  • Allocation of the frames: local vs. global
  • Thrash with working set
  • Memory-Mapped Files

Objectives

  • Describe the physical structure of secondary and tertiary storage devices and their effects.
  • Explain performance characteristics of mass-storage devices.
  • Discuss OS services for mass storage, including RAID and HSM.

Device Controller

  • CPU interacts with a controller.
  • Controller contains registers for reading and writing.
  • May contain memory for request queues or bit-mapped images.
  • Processor accesses registers via:
    • I/O instructions: in/out instructions
      • Example (Intel): out 0x21, AL
    • Memory mapped I/O: load/store instructions
      • Registers/memory appear in physical address space
      • I/O accomplished with load and store instructions

Chapter 11: Mass-Storage 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 Issues

Magnetic Disks

  • Provide bulk of secondary storage.
  • Drives rotate at 60 to 200 times per second.
  • Transfer rate: data flow rate between drive and computer.
  • Positioning time (random-access time):
    • Seek time: time to move disk arm to desired cylinder.
    • Rotational latency: time for desired sector to rotate under the disk head.
  • Head crash: disk head makes contact with the disk surface.

Magnetic Disks (Cont)

  • Disks can be removable.
  • Drive attached to computer via I/O bus.
  • Buses: EIDE, ATA, SATA, USB, Fibre Channel, SCSI, FC.
  • Host controller in computer uses bus to talk to disk controller.

Properties of a Hard Magnetic Disk

  • Properties:
    • Independently addressable element: sector.
    • OS transfers groups of sectors together— ”blocks”.
    • Disk can directly access any given block of information (random access).
    • Disk can be rewritten in place: read/modify/write a block.

Magnetic Tape

  • Early secondary-storage medium.
  • Relatively permanent; holds large quantities of data (20-200GB typical).
  • Access time slow; random access ≈1000 times slower than disk.
  • Mainly used for backup, infrequent data, transfer medium between systems.
  • Data transfer rates comparable to disk once data is under head.
  • Common technologies: 4mm, 8mm, 19mm, LTO-2 and SDLT.

Disk Attachment

  • Host-attached storage accessed through I/O ports talking to I/O busses.
  • SCSI:
    • Bus with up to 16 devices on one cable.
    • SCSI initiator requests operation; SCSI targets perform tasks.
    • Each target can have up to 8 logical units (disks attached to device controller).
  • FC (Fibre Channel):
    • High-speed serial architecture.
    • Can be switched fabric with 24-bit address space – basis of storage area networks (SANs).
    • Can be arbitrated loop (FC-AL) of 126 devices.

Network-Attached Storage

  • Network-attached storage (NAS) is storage made available over a network rather than over a local connection.
  • 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 Network

  • Common in large storage environments.
  • Multiple hosts attached to multiple storage arrays - flexible

Disk I/O Performance

  • Response Time = Queue + Disk Service Time
  • Performance metrics: Response Time, Throughput
  • Contributing factors to latency:
    • Software paths
    • Hardware controller
    • Physical disk media
  • Queuing behavior:
    • Can lead to big increases of latency as utilization approaches 100%

Disk Latency

  • Cylinder: all the tracks under the head at a given point on all surfaces.
  • Read/write data is a three-stage process:
    • Seek time: position the head/arm over the proper track (into proper cylinder).
    • Rotational latency: wait for the desired sector to rotate under the read/write head.
    • Transfer time: transfer a block of bits (sector) under the read-write head.
  • Disk Latency = Queueing Time + Controller time + Seek Time + Rotation Time + Xfer Time

Disk Bandwidth

  • 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.
  • Highest Bandwidth: Transfer large group of blocks sequentially from one track

Disk Scheduling Algorithms

  • Several algorithms exist to schedule the servicing of disk I/O requests.
  • Example request queue (0-199): 98, 183, 37, 122, 14, 124, 65, 67, Head pointer 53

FCFS Scheduling

  • Illustration shows total head movement of 640 cylinders.
  • Sequence: 53, 98, 183, 37, 122, 14, 124, 65, 67

Shortest Seek Time First (SSTF) Scheduling

  • 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.

SCAN Scheduling

  • 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.

C-SCAN Scheduling

  • 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.

C-LOOK

  • Version of C-SCAN
  • Arm 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.

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.

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.

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, 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 memory, not when the virtual memory page is first created.

RAID

  • RAID (Redundant Array of Independent Disks) was invented to address problems of disk reliability, cost, and performance.
  • In RAID, data is stored across many disks, with extra disks added to the array to provide error correction (redundancy).
  • The inventors of RAID: David Patterson, Garth Gibson, and Randy Katz.

RAID Levels

  • RAID Level 0:
    • Improved performance, but no redundancy.
    • Data is written in blocks across the entire array.
    • Disadvantage: low reliability.
  • RAID Level 1:
    • 100% redundancy, and good performance.
    • Two matched sets of disks contain the same data.
    • Disadvantage: cost.
  • RAID Level 2:
    • Data drives and Hamming code drives.
    • Hamming code drives provide error correction for the data drives.
    • RAID 2 performance is poor and the cost is high.
  • RAID Level 3:
    • Stripes bits across a set of data drives and provides a separate disk for parity.
    • Parity is the XOR of the data bits.
    • RAID 3 is not suitable for commercial applications, but is good for personal systems.
  • RAID Level 4:
    • Like adding parity disks to RAID 0.
    • Data is written in blocks across the data disks, and a parity block is written to the redundant drive.
    • RAID 4 would be feasible if all record blocks were the same size.
  • RAID Level 5:
    • RAID 4 with distributed parity.
    • With distributed parity, some accesses can be serviced concurrently, giving good performance and high reliability.
    • RAID 5 is used in many commercial systems.
  • RAID Level 6:
    • Carries two levels of error protection over striped data: Reed-Soloman and parity.
    • It can tolerate the loss of two disks.
    • RAID 6 is write-intensive, but highly fault-tolerant.
  • Double parity RAID (RAID DP):
    • Employs pairs of overlapping parity blocks that provide linearly independent parity functions.
    • Like RAID 6, RAID DP can tolerate the loss of two disks.
    • The use of simple parity functions provides RAID DP with better performance than RAID 6.
    • RAID DP’s performance is somewhat degraded from that of RAID 5.
    • RAID DP is also known as EVENODD, diagonal parity RAID, RAID 5DP, advanced data guarding RAID (RAID ADG) and-- erroneously-- RAID 6.
  • Hybrid RAID System
    • Large systems consisting of many drive arrays may employ various RAID levels, depending on the criticality of the data on the drives.

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.

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.

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.
  • Magneto-optic disk:
    • Records data on a rigid platter coated with magnetic material.
    • Laser heat is used to amplify 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.
  • Optical 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.
  • WORM (“Write Once, Read Many Times”) disks can be written only once.
    • Thin aluminum film sandwiched between two glass 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 by not altered.
    • Very durable and reliable.
  • Read Only disks, such ad CD-ROM and DVD, com from the factory with the data pre-recorded.

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.

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.

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 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.

Tape Drives

  • The basic operations 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-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.

File Naming

  • The issue of naming files on removable media is especially difficult when we want to write data on a removable cartridge on one computer, and then use the cartridge in another computer.
  • Contemporary 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 data.
  • Some kinds 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 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.

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.

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.

Reliability

  • A fixed disk drive is likely to be more reliable than a removable disk or tape drive.
  • An optical 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 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.

Summary

  • Disk drives are the major secondary-storage I/O devices on most computers.
  • Disk scheduling algorithms can improve the effective bandwidth, the average response time, and the variance in response time.
  • Disks are frequently make redundant via RAID algorithms for the amount of storage required on large systems.
  • Tertiary storage is built from disk and tape drives that uses removable media.