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Solid-State Storage

  • Non-volatile - Memory that retains its data after power loss.
    • ROM (Read Only Memory) - Data not rewritable after manufacture, used in BIOS chips and embedded devices.
      • PROM (Programmable Read Only Memory) - Programmed by blowing internal fuses permanently.
      • EPROM (Eletrically Programmable Read Only Memory) - Programmed and erased using ultraviolet light.
      • EEPROM (Electrically Eraseable Programmable Read Only Memory) - Can be erased more times than EPROM.
        • FRAM (Ferroelectric Random Access Memory) - Type of EEPROM with unlimited writes.
    • Flash - Easy to rewrite (like RAM), but nonvolatile (like ROM), used as a fast replacement for hard drives.
      • NOR - Flash based on NOR gates, used for code execution due to its execute-in-place (XIP) feature.
      • NAND - Flash based on NAND gates, cheaper and denser than NOR flash, used for data storage.
        • V-NAND (Vertical NAND) / 3D NAND - Stacks of memory cells to increase density.
        • SLC (Single-Level Cell) - NAND that stores only one bit per MOSFET memory cell.
        • MLC (Multi-Level Cell), TLC (Triple), QLC (Quad) - NAND that stores multiple bits per cell, slower but cheaper than SLC.
    • NVRAM (non-volatile RAM) - Memory that acts like RAM but retains its data after losing power like Flash.
      • 3D XPoint / Optane / QuantX - Balances the performance and density of DRAM and flash, uses resistance rather than charge to store bits.
      • FeRAM (Ferroelectric RAM)

nand-vs-nor-flash 1

  • Volatile - Memory that loses its data after power loss, used as a working cache to store frequently-accessed data.

    • RAM (Random Access Memory) - Fast memory for storing running programs.
      • SRAM (Static RAM) - Expensive but fast, built into CPU dies to be used as L1-L3 caches.
      • DRAM (Dynamic RAM) - Cheaper than SRAM, but slower, used as main system memory.
        • SDRAM (Synchronous DRAM) - RAM that synchronizes its clock with the CPU, most modern RAM is SDRAM.
          • DDR (Double Data Rate) - Double the transfer rate of RAM without increasing the clock.
          • GDDR (Graphics DDR) - SDRAM designed for use with GPUs.
          • HBM (High Bandwidth Memory) - 3D-stacked SDRAM for graphics and network devices.

  • How SSD storage over-provisioning works

    • The 7.37% Inherent OP (Over-Provisioning) is due to the fact that a GiB is 7.37% larger than a GB
    • The OS measures SSD size in GB, but NAND internally is measured in GiB, so the drive is actually 7.37% larger than what the OS sees
    • Factory-set OP is free space set by the manufacturer that cannot be partitioned by the OS
    • Dynamic OP is partitioned space that has not yet been used by the filesystem

Data Protocols

Storage spec Data bus Connector / form factor
NVMe PCIe M.2, mPCIe, U.2, PCIe
AHCI SATA, PATA, SAS, IDE M.2, mPCIe, U.2, mSATA, 2.5in/3.5in SATA/SAS/IDE

Units of Measurement

Name Mathematical equivalent # of bytes
Gigabyte (GB) 109 bytes 1,000,000,000
Gibibyte (GiB) 230 bytes 1,073,741,824 (1 GB * 1.07)
Gigabit (Gb) 109 bits 125,000,000 (1 GB / 8)