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Physical Storage Systems

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Outline:

  • Classification of Physical Storage Media
  • Storage Hierarchy
  • Magnetic Disks
    • Disk Interface Standards
    • Performance Measures of Disks
    • Optimization of Disk-Block Access
  • Flash Storage & SSD
  • Storage Class Memory(NVM)

Classification of Physical Storage Media

可以把存储分为两类：

  • volatile storage(易失存储): loses contents when power is switched off
  • non-volatile storage(非易失存储):
    • Contents persist even when power is switched off
    • Includes secondary and tertiary storage, as well as batter-backed up main-memory

评价存储的标准：

  • Speed with which data can be accessed
  • Cost per unit of data
  • Reliability
    • data loss on power failure or system crash
    • physical failure of the storage device

Storage Hierarchy

Storage Hierarchy

从上图可以看出，易失存储包括： cache, main memory

非易失存储包括：flash memory, magnetic disk, optical disk, magnetic tapes

还可以把存储分为三级：

  • primary storage(一级存储): cache, main memory
    • Fastest media but volatile
  • secondary storage(二级存储, online storage): flash memory, magnetic disk
    • non-volatile, moderately fast access time
  • tertiary storage(三级存储, offline storage): optical disk, magnetic tapes
    • non-volatile, slow access time

Magnetic Disks

磁头(Read-write head)

  • 磁头非常靠近磁盘表面，几乎接触到磁盘表面
  • 用于读写数据

磁盘表面被划分为多个同心圆，称为track(磁道)

  • 通常一个磁盘每个盘片上有 50K - 100K 个磁道

每个磁道被划分为多个扇区(sector)

  • 扇区是读写数据的最小单位
  • 每个扇区通常为512字节
  • 内圈磁道通常有 500 - 1000 个扇区
  • 外圈磁道通常有 1000 - 2000 个扇区

读写扇区的时候

  • 磁臂(disk arm)会移动以将磁头定位到正确的磁道
  • 盘片持续旋转，扇区从磁头下面经过时，读写数据

Head-disk assemblies

  • 在主轴上通常有 1 到 5 个盘片
  • 一个盘片对应一个磁头

柱面(cylinder) \(i\) 包含所有盘片的第 \(i\) 个磁道

磁盘控制器(disk controller) - interfaces between the computer system and the disk drive hardware

  • 接受高层次的读写扇区的命令
  • 执行例如移动磁头到指定的磁道，读写扇区等动作
  • 计算校验和，并将其附加到每个扇区，以验证数据是否正确读回
    • 如果数据损坏了，有很大的概率校验和会匹配不上
  • 通过在写入扇区之后再读取一遍来确保正确的写入操作
  • 对于损坏的扇区，执行重新映射(remapping)

Disk Interface Standards

  • SATA(Serial ATA)
    • SATA 3 supports data transfer speeds of up to 6 gigabits/second
  • SAS(Serial Attached SCSI)
    • SAS Version 3 supports 12 gigabits/second
  • NVMe(Non-Volatile Memory Express) interface
    • Works with PCIe connectors to support lower latency and higher transfer rates
    • Supports data transfer speeds of up to 24 gigabits/second

Performance Measures of Disks

  • Access time(访问时间) - the time it takes from when a read or write request is issued to when data transfer begins. Consists of:
    • Seek time(寻道时间) - time it takes to reposition the arm over the correct track
      • Average seek time is ½ the worst case seek time
      • 4 to 10 milliseconds on typical disks
    • Rotational latency(旋转延迟) - time it takes for the sector to be accessed to appear under the head
      • Average rotational latency is ½ of the worst case latency
      • 4 to 11 milliseconds on typical disks(5400 to 15000 r.p.m)
  • Data-transfer rate(数据传输率) - 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 tracks
    • Multiple disks may share a controller, so rate the controller can handle is also important
      • E.g. SATA: 150 MB/sec, SATA-II 3Gb(300 MB/sec)
      • Ultra 320 SCSI: 320 MB/sec, SAS (3 to 6 Gb/sec)
      • Fiber Channel(FC2Gb or 4Gb): 256 to 512 MB/sec

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)

  • Successive requests are for blocks that can be anywhere on disk
  • Each access requires a seek
  • Transfer rates are low since a lot of time is wasted in seeks

Note

I/O operations per second, 每秒 I/O 操作数

  • Number of random block reads that a disk can support per second. 磁盘每秒可以支持的随机块读取数
  • 50 to 200 IOPS on current generation magnetic disks

Mean time to failure, 平均故障时间

  • The average time the disk is expected to run continuously without any failure. 磁盘在没有任何故障的情况下，预计可以连续运行的平均时间
  • Typically 3 to 5 years
  • 新磁盘的平均故障时间很低，大约在 57 到 136年之间
  • 随着时间的推移，MTTF 会减少（故障频率上升）

Optimization of Disk-Block Access

Buffering: 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: algorithms re-order block requests so that disk arm movement is minimized

  • Elevator algorithm

File organization:

  • Allocate blocks of a file in as contiguous a manner as possible
  • Allocation in units of extents(盘区)
  • Files may get fragmented

Nonvolatile write buffers(非易失性写缓存) - 通过把块写入非易失性 RAM 缓冲区来加速磁盘写入

  • 非易失性 RAM: 电池供电的 RAM 或者闪存
  • 当磁盘没有别的请求或者请求已经等待了一段时间后，控制器才会写入磁盘
  • 数据库中的操作不需要等到数据写入磁盘后在执行
  • 写入操作可以重新调整顺序来最小化磁臂的移动

Log disk(日志磁盘) - 一块用来写入块更新日志的磁盘

  • 写入非常快，因为不需要寻道
  • 不需要特殊硬件(NV-RAM)

Flash Storage

NAND flash: 广泛应用于存储，比 NOR flash 更便宜

  • 需要整页读取（一页大概是 512 byte 到 4 KB）
    • 顺序访问和随机访问没有太大的区别
  • 页只能被写入一次，如果需要重新写入，需要把整页擦除

SSD(Solid State Disks): use standard block-oriented disk interfaces, but store data on multiple flash storage devices internally

Storage Class Memory(NVM)

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