پاورپوینت کامل Chapter 11: Storage and File Structure 92 اسلاید در PowerPoint

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پاورپوینت کامل Chapter 11: Storage and File Structure 92 اسلاید در PowerPoint

اسلاید ۴: Physical Storage MediaCache – fastest and most costly form of storage; volatile; managed by the computer system hardware.Main memory:fast access (10s to 100s of nanoseconds; 1 nanosecond = 10–۹ seconds)generally too small (or too expensive) to store the entire databasecapacities of up to a few Gigabytes widely used currentlyCapacities have gone up and per-byte costs have decreased steadily and rapidly (roughly factor of 2 every 2 to 3 years)Volatile — contents of main memory are usually lost if a power failure or system crash occurs.

اسلاید ۵: Physical Storage Media (Cont.)Flash memory Data survives power failureData can be written at a location only once, but location can be erased and written to again Can support only a limited number (10K – ۱M) of write/erase cycles.Erasing of memory has to be done to an entire bank of memory Reads are roughly as fast as main memoryBut writes are slow (few microseconds), erase is slowerCost per unit of storage roughly similar to main memory Widely used in embedded devices such as digital camerasIs a type of EEPROM (Electrically Erasable Programmable Read-Only Memory)

اسلاید ۶: Physical Storage Media (Cont.)Magnetic-diskData is stored on spinning disk, and read/written magneticallyPrimary medium for the long-term storage of data; typically stores entire database.Data must be moved from disk to main memory for access, and written back for storageMuch slower access than main memory (more on this later)direct-access – possible to read data on disk in any order, unlike magnetic tapeHard disks vs floppy disksCapacities range up to roughly 400 GB currentlyMuch larger capacity and cost/byte than main memory/flash memoryGrowing constantly and rapidly with technology improvements (factor of 2 to 3 every 2 years)Survives power failures and system crashesdisk failure can destroy data, but is rare

اسلاید ۷: Physical Storage Media (Cont.)Optical storage non-volatile, data is read optically from a spinning disk using a laser CD-ROM (640 MB) and DVD (4.7 to 17 GB) most popular formsWrite-one, read-many (WORM) optical disks used for archival storage (CD-R, DVD-R, DVD+R)Multiple write versions also available (CD-RW, DVD-RW, DVD+RW, and DVD-RAM)Reads and writes are slower than with magnetic disk Juke-box systems, with large numbers of removable disks, a few drives, and a mechanism for automatic loading/unloading of disks available for storing large volumes of data

اسلاید ۸: Physical Storage Media (Cont.)Tape storage non-volatile, used primarily for backup (to recover from disk failure), and for archival datasequential-access – much slower than disk very high capacity (40 to 300 GB tapes available)tape can be removed from drive storage costs much cheaper than disk, but drives are expensiveTape jukeboxes available for storing massive amounts of data hundreds of terabytes (1 terabyte = 1) to even a petabyte (1 petabyte = 1012 bytes)

اسلاید ۹: Storage Hierarchy

اسلاید ۱۰: Storage Hierarchy (Cont.)primary storage: Fastest media but volatile (cache, main memory).secondary storage: next level in hierarchy, non-volatile, moderately fast access timealso called on-line storage E.g. flash memory, magnetic diskstertiary storage: lowest level in hierarchy, non-volatile, slow access timealso called off-line storage E.g. magnetic tape, optical storage

اسلاید ۱۱: Magnetic Hard Disk MechanismNOTE: Diagram is schematic, and simplifies the structure of actual disk drives

اسلاید ۱۲: Magnetic DisksRead-write head Positioned very close to the platter surface (almost touching it)Reads or writes magnetically encoded information.Surface of platter divided into circular tracksOver 50K-100K tracks per platter on typical hard disksEach track is divided into sectors. A sector is the smallest unit of data that can be read or written.Sector size typically 512 bytesTypical sectors per track: 500 (on inner tracks) to 1000 (on outer tracks)To read/write a sectordisk arm swings to position head on right trackplatter spins continually; data is read/written as sector passes under headHead-disk assemblies multiple disk platters on a single spindle (1 to 5 usually)one head per platter, mounted on a common arm.Cylinder i consists of ith track of all the platters

اسلاید ۱۳: Magnetic Disks (Cont.)Earlier generation disks were susceptible to head-crashesSurface of earlier generation disks had metal-oxide coatings which would disintegrate on head crash and damage all data on diskCurrent generation disks are less susceptible to such disastrous failures, although individual sectors may get corruptedDisk controller – interfaces between the computer system and the disk drive hardware.accepts high-level commands to read or write a sector initiates actions such as moving the disk arm to the right track and actually reading or writing the dataComputes and attaches checksums to each sector to verify that data is read back correctlyIf data is corrupted, with very high probability stored checksum won’t match recomputed checksumEnsures successful writing by reading back sector after writing itPerforms remapping of bad sectors

اسلاید ۱۴: Disk SubsystemMultiple disks connected to a computer system through a controllerControllers functionality (checksum, bad sector remapping) often carried out by individual disks; reduces load on controllerDisk interface standards familiesATA (AT adaptor) range of standardsSATA (Serial ATA) SCSI (Small Computer System Interconnect) range of standardsSeveral variants of each standard (different speeds and capabilities)

اسلاید ۱۵: Performance Measures of DisksAccess time – the time it takes from when a read or write request is issued to when data transfer begins. Consists of: Seek time – time it takes to reposition the arm over the correct track. Average seek time is 1/2 the worst case seek time.Would be 1/3 if all tracks had the same number of sectors, and we ignore the time to start and stop arm movement4 to 10 milliseconds on typical disksRotational latency – time it takes for the sector to be accessed to appear under the head. Average latency is 1/2 of the worst case latency.4 to 11 milliseconds on typical disks (5400 to 15000 r.p.m.)Data-transfer rate – the rate at which data can be retrieved from or stored to the disk.25 to 100 MB per second max rate, lower for inner tracksMultiple disks may share a controller, so rate that controller can handle is also importantE.g. ATA-5: 66 MB/sec, SATA: 150 MB/sec, Ultra 320 SCSI: 320 MB/sFiber Channel (FC2Gb): 256 MB/s

اسلاید ۱۶: Performance Measures (Cont.)Mean time to failure (MTTF) – the average time the disk is expected to run continuously without any failure.Typically 3 to 5 yearsProbability of failure of new disks is quite low, corresponding to a “theoretical MTTF” of 500,000 to 1,200,000 hours for a new diskE.g., an MTTF of 1,200,000 hours for a new disk means that given 1000 relatively new disks, on an average one will fail every 1200 hoursMTTF decreases as disk ages

اسلاید ۱۷: Optimization of Disk-Block AccessBlock – a contiguous sequence of sectors from a single track data is transferred between disk and main memory in blocks sizes range from 512 bytes to several kilobytesSmaller blocks: more transfers from diskLarger blocks: more space wasted due to partially filled blocksTypical block sizes today range from 4 to 16 kilobytesDisk-arm-scheduling algorithms order pending accesses to tracks so that disk arm movement is minimized elevator algorithm : move disk arm in one direction (from outer to inner tracks or vice versa), processing next request in that direction, till no more requests in that direction, then reverse direction and repeat

اسلاید ۱۸: Optimization of Disk Block Access (Cont.)File organization – optimize block access time by organizing the blocks to correspond to how data will be accessedE.g. Store related information on the same or nearby cylinders.Files may get fragmented over timeE.g. if data is inserted to/deleted from the fileOr free blocks on disk are scattered, and newly created file has its blocks scattered over the diskSequential access to a fragmented file results in increased disk arm movementSome systems have utilities to defragment the file system, in order to speed up file access

اسلاید ۱۹: Nonvolatile write buffers speed up disk writes by writing blocks to a non-volatile RAM buffer immediatelyNon-volatile RAM: battery backed up RAM or flash memoryEven if power fails, the data is safe and will be written to disk when power returnsController then writes to disk whenever the disk has no other requests or request has been pending for some timeDatabase operations that require data to be safely stored before continuing can continue without waiting for data to be written to diskWrites can be reordered to minimize disk arm movementLog disk – a disk devoted to writing a sequential log of block updates Used exactly like nonvolatile RAMWrite to log disk is very fast since no seeks are requiredNo need for special hardware (NV-RAM)File systems typically reorder writes to disk to improve performanceJournaling file systems write data in safe order to NV-RAM or log diskReordering without journaling: risk of corruption of file system dataOptimization of Disk Block Access (Cont.)

اسلاید ۲۰: RAIDRAID: Redundant Arrays of Independent Disks disk organization techniques that manage a large numbers of disks, providing a view of a single disk of high capacity and high speed by using multiple disks in parallel, and high reliability by storing data redundantly, so that data can be recovered even if a disk fails The chance that some disk out of a set of N disks will fail is much higher than the chance that a specific single disk will fail. E.g., a system with 100 disks, each with MTTF of 100,000 hours (approx. 11 years), will have a system MTTF of 1000 hours (approx. 41 days)Techniques for using redundancy to avoid data loss are critical with large numbers of disksOriginally a cost-effective alternative to large, expensive disksI in RAID originally stood for “inexpensive’’Today RAIDs are used for their higher reliability and bandwidth. The “I” is interpreted as independent

اسلاید ۲۱: Improvement of Reliability via RedundancyRedundancy – store extra information that can be used to rebuild information lost in a disk failureE.g., Mirroring (or shadowing)Duplicate every disk. Logical disk consists of two physical disks.Every write is carried out on both disksReads can take place from either diskIf one disk in a pair fails, data still available in the otherData loss would occur only if a disk fails, and its mirror disk also fails before the system is repairedProbability of combined event is very small Except for dependent failure modes such as fire or building collapse or electrical power surgesMean time to data loss depends on mean time to failure, and mean time to repairE.g. MTTF of 100,000 hours, mean time to repair of 10 hours gives mean time to data loss of 500*106 hours (or 57,000 years) for a mirrored pair of disks (ignoring dependent failure modes)

اسلاید ۲۲: Improvement in Performance via ParallelismTwo main goals of parallelism in a disk system: 1.Load balance multiple small accesses to increase throughput2.Parallelize large accesses to reduce response time.Improve transfer rate by striping data across multiple disks.Bit-level striping – split the bits of each byte across multiple disksIn an array of eight disks, write bit i of each byte to disk i.Each access can read data at eight times the rate of a single disk.But seek/access time worse than for a single diskBit level striping is not used much any moreBlock-level striping – with n disks, block i of a file goes to disk (i mod n) + 1Requests for different blocks can run in parallel if the blocks reside on different disksA request for a long sequence of blocks can utilize all disks in parallel

اسلاید ۲۳: RAID LevelsSchemes to provide redundancy at lower cost by using disk striping combined with parity bitsDifferent RAID organizations, or RAID levels, have differing cost, performance and reliability characteristicsRAID Level 1: Mirrored disks with block stripingOffers best write performance. Popular for applications such as storing log files in a database system.RAID Level 0: Block striping; non-redundant. Used in high-