跳转至

Physical Storage Systems

Abstract
  • 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)

Storage Hierarchy

  • 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.

主要从 speed, cost, reliability 衡量

从高层往低层走，存储设备变得更慢，更便宜和更大。

  • 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
  • tertiary storage: lowest level in hierarchy, non-volatile, slow access time
    also called off-line storage
    常用来备份

NVM (non-volatile memory) 访问和内存一样，以字节寻址，而且掉电能保持数据。

Magnetic Disks

一个磁盘有上十万个 track(磁道), 一个磁道又有上千个 sector(扇区，是计算机和磁盘交换数据的最小单位).
arm assemly 用来寻道，读写头共进退，寻找数据在哪个磁道上。
等对应扇区旋转到读写头，才开始传输数据。
同样磁道组成的柱面。对于大文件，最好存在同一个柱面上，这样可以并行读写。

  • Read-write head
  • Surface of platter divided into circular tracks（磁道）
  • Each track is divided into sectors（扇区）
  • To read/write a sector
    • disk arm swings to position head on right track
    • platter spins continually; data is read/written as sector passes under head
  • Cylinder（柱面） i consists of ith track of all the platters
  • Disk controller(磁盘控制器)– interfaces between the computer system and the disk drive hardware.

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 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.

内存传输是以块为单位的。即使是想要访问一个 byte, 也需要把这个 byte 所在的 4k 内存读进来。

  • Disk block is a logical unit for storage allocation and retrieval

    • Smaller blocks: more transfers from disk
    • Larger blocks: more space wasted due to partially filled blocks

  • Sequential access pattern(顺序访问模式)
    连续的读写请求只需要第一次访问磁盘

  • Random access pattern（随机访问模式）
    慢，希望尽量多一些顺序访问。
    可以用一个日志把要修改的数据记录下来，后面再进行修改，尽量用顺序访问替换随机访问。

  • I/O operations per second (IOPS ，每秒I/O操作数)
    Number of random block reads that a disk can support per second.
    每秒可以支持随机读的次数。

  • Mean time to failure (MTTF，平均故障时间)
    the average time the disk is expected to run continuously without any failure.

Optimization of Disk-Block Access

  • Buffering: in-memory buffer to cache disk blocks
    数据读进来就丢，比较可惜，所以我们把它放在一个地方，万一后面需要使用可以不用再读。
  • Read-ahead(Prefetch): Read extra blocks from a track in anticipation that they will be requested soon
    预取，读某块时预测邻近几块也会被访问，于是就一起取到内存中。要有依据地预取，不然无用的数据会占用缓存。

  • 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
      预先分配得到的内存是连续的
    • Files may get fragmented
      • Sequential access to a fragmented file results in increased disk arm movement
      • Some systems have utilities to defragment the file system, in order to speed up file access
  • Nonvolatile write buffers（非易失性写缓存）
    speed up disk writes by writing blocks to a non-volatile RAM buffer immediately
    把要写的数据先写到一个快速的非易失的缓存里，如 NVM. 这时上面的程序可以继续执行了, NVM 再择机将数据写回到磁盘。
  • Log disk（日志磁盘）
    a disk devoted to writing a sequential log of block updates

Flash Storage

  • NAND flas
    • requires page-at-a-time read (page: 512 bytes to 4 KB)
      顺序读写和随机读写差不多
    • Page can only be written once
      像黑板，写了数据如果要再写需要把之前的擦掉。
  • SSD(Solid State Disks)
    Use standard block-oriented disk interfaces, but store data on multiple flash storage devices internally

可能有这样的情况：我们反复读写、擦去某几个块，这会导致它们坏的很快。

  • Remapping of logical page addresses to physical page addresses avoids waiting for erase

  • Flash translation table tracks mapping

    • also stored in a label field of flash page
    • remapping carried out by flash translation layer

  • wear leveling(磨损均衡)
    evenly distributed erase operators across physical blocks

Persistence 即掉电是否能保持原数据。

评论