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
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