OS Lesson 5

• Storage Management is defined as it refers to the management of the data storage equipment's that are used to store the user/computer generated data

  
• Storage management


  A tool or set of processes used by an administrator to keep your data and storage equipment's safe
• Key attributes of storage management

  • Performance
  • Reliability
  • Recoverability
  • Capacity
• Hard Disk Drives (HDDs)


  • Conceptually simple
  • Flat circular platters covered with magnetic material
  • Read-write heads fly above platter surfaces
  • Platters logically divided into circular tracks and sectors
  • Disk drive motor spins at high speed (60-250 RPM)

  
• Seek time


  Time necessary to move the disk arm to the desired cylinder
• Rotational latency


  Time necessary for the desired sector to rotate to the disk head
• Transfer rate


  Rate at which data flows between the drive and the computer
• Head crash


  Accident where the head makes contact with the disk surface, damaging the magnetic surface
• HDDs are sealed units, some chassis allow removal without shutting down
• The IBM Model 350 disk storage system from 1956 had 50 24-inch platters and an access time of less than 1 second
• Theoretical hard disk transfer rate is 6 Gb/sec, effective rate is 1 Gb/sec

  
• Seek time ranges from 3ms to 12ms, 9ms is common for desktop drives
• Average latency

  1/(2*RPM) seconds
• Access latency


  Average seek time + average latency
• Average I/O time


  Average access time + (data to transfer / transfer rate) + controller overhead
• Nonvolatile Memory (NVM) Devices


  • Electrical rather than mechanical
  • Composed of controller and flash NAND chips
  • More reliable than HDDs due to no moving parts
  • Faster than HDDs due to no seek time or rotational latency
  • Consume less power
  • More expensive per megabyte than HDDs
  • Capacity increasing faster than HDDs
• NAND semiconductor characteristics

  • Read and written in "page" increments, can't overwrite in place
  • Data must be erased in larger "block" increments
  • Limited number of erase cycles before worn out (~100,000)
• NVM lifespan


  Measured in Drive Writes Per Day (DWPD), how many times the drive capacity can be written per day before failure
• RAM drives


  • DRAM used as temporary high-speed storage, not technically secondary storage since volatile
  • Allow users/programmers to place data in memory for temporary safekeeping using standard file operations

  
• RAM drives are found in all major operating systems (Linux, macOS, Windows, Solaris)
• Secondary storage connection methods

  • ATA, SATA, eSATA, SAS, USB, Fibre Channel
  • SATA is the most common
  • NVMe directly connects NVM devices to the system PCI bus for higher throughput and lower latency
• Host controller and device controller


  • Host controller is at the computer end, device controller is built into each storage device
  • Host controller sends commands to device controller to operate the drive hardware
• Magnetic tape was used as an early secondary storage medium
• PCI bus

  System that increases throughput and decreases latency compared with other connection methods
• Controllers or host-bus adapters (HBA)


  Special electronic processors that carry out data transfers on a bus
• Host controller


  The controller at the computer end of the bus
• Device controller


  Built into each storage device to operate the drive hardware
• Performing a mass storage I/O operation

  1. Computer places a command into the host controller

  2. Host controller sends the command via messages to the device controller

  3. Device controller operates the drive hardware to carry out the command
• Device controllers


  Usually have a built-in cache
• Data transfer at the drive

  1. Happens between the cache and the storage media

  2. Data transfer to the host occurs between the cache and host DRAM via DMA
• Magnetic tape was used as an early secondary-storage medium
• Magnetic tape


  • Nonvolatile and can hold large quantities of data
  • Access time is slow compared with main memory and drives
  • Random access is about a thousand times slower than HDDs and about a hundred thousand times slower than SSDs
• Tapes


  Used mainly for backup, for storage of infrequently used information, and as a medium for transferring information from one system to another
• Using a tape

  1. Kept in a spool and is wound or rewound past a read–write head

  2. Moving to the correct spot can take minutes

  3. Once positioned, tape drives can read and write data at speeds comparable to HDDs
• Tape capacities


  • Vary greatly, depending on the particular kind of tape drive, with current capacities exceeding several terabytes
  • Some tapes have built-in compression that can more than double the effective storage
• Tape categorization


  • By width, including 4, 8, and 19 millimeters and 1/4 and 1/2 inch
  • Named according to technology, such as LTO-6
• HDD scheduling


  Responsibility of the operating system to use the hardware efficiently by minimizing access time and maximizing data transfer bandwidth
• HDD access time


  • Seek time is the time for the device arm to move the heads to the cylinder containing the desired sector
  • Rotational latency is the additional time for the platter to rotate the desired sector to the head
• Device bandwidth


  Total number of bytes transferred, divided by the total time between the first request for service and the completion of the last transfer
• Servicing a storage I/O request

  1. Process issues a system call to the operating system

  2. Request specifies: input/output, open file handle, memory address, amount of data to transfer

  3. If drive and controller are available, request can be serviced immediately

  4. If drive or controller is busy, new requests are placed in the queue of pending requests