Storage Systems

  • HDD (Hard Disk Drive)
  • NVM (Non-Volatile Memory)

Hard Disk

  • Transfer Rate: Amount of data transferred between HDD and Computer per unit of time.
  • Random access Time: Seek Time + Rotation Latency. This is also known as positioning time.
  • Seek Time: Time required to move the head to the targeted track.
  • Rotational Latency: Time required to move the head to the targeted sector track.

NVM Storage

The NVM storage stands for Non-Volatile Memory. HDD is mechanical in nature on the other hands NVM is electric. There are numerous techniques to build NVM devices but the most common one is to build with a controller and flash-based semiconductor chip.

  • Reliable as they don’t have moving parts.
  • Faster as no seek time and rotation latency involved.
  • Consume less energy
  • Smaller in size
  • Expensive ( Although getting cheaper with time )
  • Less capacity than HDD
  • Read a page
  • Write a page (only if it’s empty)
  • Erase a block

Secondary Storage Connections

We have talked about the popular drives used for secondary storage. Now they are connected to the computer either by a system bus or I/O bus.

  • ATA: Advanced Technology Attachment
  • SATA: Serial ATA
  • eSATA
  • SAS: Serial Attached SCSI
  • USB: Universal Serial Bus
  • FC: Fiber Channel
  • NVMe
  • Host controller (Controller in the computer)
  • Device controller (Controller in a storage device)

Address Mapping

The operating system refers to storage devices as the logical blocks irrespective of the physical storage being used underneath. OS considers storage as a 1D array. Logical Block is the atomic unit of data transfer for OS and maps to physical smallest unit eg. track sector (in case of HDD) and to page (in case of NVM).

  • Disk many have defective sectors but mapping doesn’t address it.
  • Physical address space is not sequential which is assumed by the logical addresses.
  • The disk manufacturer also uses the LBA concept to internally map to the physical address space. Hence OS LBA is not directly pointing to the physical address space


CLV stands for Constant Linear Velocity


CAV stands for Constant Angular Velocity



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Ajay Yadav

Ajay Yadav

Believer of Distributed Systems