NAND Flash memory

SSD

Hard disk drives (HDD).

• few manufactures:

  • concentration of manufacturers through purchases and mergers over the years.

• mature technology, with many aspects in common:

  • between disk models and sizes.

  • between manufacturers.

• digital research in hard drives is almost the same in all models and brands.

Solid-State Drives (SSD).

• basic components are the same or very similar.

  • between manufacturers.

  • between flash memory and SSDs.

• but there are important differences:

  • flash memory requires driver software – uses CPU.

  • SSD has its processing unit – doesn’t use CPU.

  • firmware between models or manufacturers can be very different.

Advantages

• are mechanically more reliable.

  • have no moving parts and are more resistant to falls.

• read speed is independent of the data location (which doesn’t happen with HDD).

• power consumption is lower (1h to 2h of increased battery autonomy on a laptop).

• emits no noise or vibrations.

• heat less than HDD – HDD can reach very high temperatures.

• are lighter – don’t require a metallic structure as HDDs.

Different kinds of SSD

DRAM

• older solid state disk (they have existed for more than 30 years).

• based on volatile DRAM memory.

• require battery or other power source to ensure redundancy.

• need of a traditional drive to store data permanently.

• used in high-performance systems such as banks, stock exchanges, military assets, ...

• the cost of flash memory is falling more than DRAM → the crossing point was reached in 2004.

Flash memory

• non-volatile.

• there are 2 categories:

  • NOR gates, NAND gates.

Flash memory

With NOR gates.

• used for small amounts of memory (< 16MB), e. g. BIOS.

• allows very fast readings, but is slow to write and erase (up to 5 seconds).

• supports fewer write cycles (10× less than NAND gates).

• allows one to read or write a single byte at a time.

• allows local execution, without having to use RAM.

  • uses an SRAM interface that enables it to address all bytes.

With NAND gates.

• provides large bit density → ideal for replacing HDD.

• erase and write faster than NOR (up to 4 ms), but slightly slower readings.

• reads and writes are made in large blocks of bytes.

• disadvantages:

  • internal management complexity.

  • serial access to data, wear leveling, garbage collection, ...

NAND Flash memory – is the most common type of flash.

• USB pen drives.

• Solid State Drives (SSD).

Management of bad blocks.

• all devices have bad blocks.

• an initial test to identify bad blacks is required.

  • the cost of creating chips without defects does not pay off.

  • it is preferable to put capacity in excess and then remove the addresses with bad blocks.

Inner working of a NAND chip.

• at rest = 1 (stores the value 1) at load = 0 (stores the value zero).

• to increase density, they can be produced in layers: MLC (multi-layer chip).

  • several bits have to be read/written simultaneously.

  • allows more capacity, but has lower performance than the single-layer chip (SLC).

  • cheaper.

Data access.

• data access in a grid with word lines (16 bits).

• minimum writing unit is a sector with a size multiple of word lines.

  • HDD: 1 sector = 512 Bytes → minimum read and write unit.

  • SSD: 1 sector = [512, 2048] Bytes.

    • depends on several factors, such as a manufacturer and disk capacity.

    • minimum writing unit differs from the minimum erasing unit.

    • it is not possible to erase a single sector.

    • data as to be erased by blocks – the electric charge to erase is similar to a photographic flash.

Structure