跳转至

物理存储

存储分类

  • 易失存储 volatile storage
  • 非易失存储 non-volatile storage image-20230607021440123
  • NVM（non-volatile memory）: 掉电不丢
  • primary storage: Fastest media but volatile (cache, main memory).
  • secondary storage: next level in hierarchy, non-volatile, moderately fast access time
    • also called on-line storage
    • E.g. flash memory, magnetic disks
  • tertiary storage: lowest level in hierarchy, non-volatile, slow access time
    • also called off-line storage
    • E.g. magnetic tape, optical storage

磁盘（magnetic disks）

  • 组成
    • tracks(磁道)
      • sectors(扇区)
  • 时间
    • Access Time 访问时间，毫秒级
    • Seek Time 寻道时间：找到对应磁道
    • Rotational latency 旋转延迟：转到对应扇区

  • Data-transfer rate 数据传输率

    • 数据库传输以block为单位

  • 访问模式

    • Sequential access pattern(顺序访问模式)
      • Successive requests are for successive disk blocks
      • Disk seek required only for first block
    • Random access pattern（随机访问模式）
      • Each access requires a seek
      • Transfer rates are low since a lot of time is wasted in seeks
      • 尽量转化为顺序访问模式

  • I/O operations per second (IOPS ，每秒I/O操作数)

    • 衡量磁盘访问速度
    • Number of random block reads that a disk can support per second 50 to 200 IOPS on current generation magnetic disks

  • Mean time to failure (MTTF，平均故障时间)

    • 衡量磁盘可靠性
    • the average time the disk is expected to run continuously without any failure

磁盘块访问的优化

  • Buffering 缓存:
    • in-memory buffer to cache disk blocks
  • Read-ahead(Prefetch)
    • Read extra blocks from a track in anticipation that they will be requested soon
  • Disk-arm-scheduling
    • Disk-arm-scheduling algorithms re-order block requests so that disk arm movement is minimized
  • File organization（将碎片化的文件重新整理）
    • Allocate blocks of a file in as contiguous a manner as possible
    • Allocation in units of extents(盘区）
  • Nonvolatile write buffers （非易失性写缓存）
    • speed up disk writes by writing blocks to a non-volatile RAM buffer immediately
  • Log disk（日志磁盘）
    • a disk devoted to writing a sequential log of block updates
    • Used exactly like nonvolatile RAM

Flash Storage

  • NAND flash
    • Page can only be written once
    • Must be erased to allow rewrite
  • SSD(Solid State Disks)
    • image-20230607024602936
    • SSD比磁盘快百倍，磁盘能耗高（机械运动）更新为即席写入
  • Flash storage 中的地址映射漂移，已达成磨损均衡（wear leveling)
  • image-20230607024919201

  • 对于大数据，热数据（常访问）放在SSD，冷数据放在磁盘里

  • NVM，又称Storage Class Memory

    • image-20230607025337181
    • Persistence可以看出是否易失
    • NVM和SSD与硬盘不同，用字节寻址

随堂测试

  • Which physical storage media is non-volatile?

    多选题 (1 分)

    A.cache

    B.main memory

    C.flash memory

    D.magnetic disk

    E.SSD(State Solid Disk)

    F.magnetic tapes

    G.optical disk

    正确答案: C D E F G

  • Which physical storage medias belong to the secondary storage?

    多选题 (1 分)

    A.cache

    B.main memory

    C.flash memory

    D.SSD(Solid State Disk)

    E.magnetic disk

    F.magnetic tapes

    G.optical disk

    正确答案: C D E

  • Which term represents the time that the disk controller takes to reposition the disk arm over the correct track.

    单选题 (1 分)

    A.access time

    B.seek time

    C.rotational latency

    D.data-transfer rate

    正确答案: B

  • What is the right approach to optimizing data access of disks?

    多选题 (1 分)

    A.Buffering

    B.Read-ahead

    C.defragment the file system

    D.Non-volatile write buffers

    E.Log disk

    正确答案: A B C D E

  • MTTF means 1 (注意：每个单词首字母大写).

    填空题 (1 分) (请按题目中的空缺顺序依次填写答案)

    正确答案: Mean Time To Failure

  • IOPS means 1 (注意：每个单词首字母大写).

    填空题 (1 分) (请按题目中的空缺顺序依次填写答案)

    正确答案: I/O Operations Per Second