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by Zsolt Kerekes,
editor - first published in 2000
A solid state disk / drive
(SSD) - is electrically, mechanically and software compatible with a
conventional (magnetic)
hard disk.
The
difference is that the storage medium is not magnetic (like a hard disk) or
optical (like a CD) but
solid state
semiconductor such as
battery backed RAM, RRAM,
PRAM or other electrically erasable RAMlike non volatile memory chip such as
flash.
This provides faster access time than a hard disk, because the SSD
data can be randomly accessed in the same time whatever the storage location.
The SSD access time does not depend on a read/write interface head
synchronising with a data sector on a rotating disk. The SSD also provides
greater physical resilience to physical vibration, shock and extreme temperature
fluctuations. SSDs are also imune to strong magnetic fields which could
sanitize a hard
drive.
The only downside to SSDs is a higher
cost per megabyte of
storage - although in some applications the higher
reliability of SSDs
makes them cheaper to own than replacing multiple failing hard disks. When the
storage capacity needed by the application is small (as in some embedded
systems) the SSD can actually be cheaper to buy because hard disk oems no
longer make low capacity drives. Also in enterprise server acceleration
applications - the benefit of the SSD is that it reduces the number of servers
needed compared to using hard disk based
RAID on its own.
Historically
RAM based SSDs were
faster than
flash based products
- but in recent years
the performance of the fastest flash SSDs has been more than fast enough to
replace RAM based systems in 97% of server acceleration applications.
Both
types of SSDs have been available in a wide range of form factors and supporting
traditional disk interfaces. A complete list of manufacturers with tables by
form factor, technology type and interface type is updated in real-time in
the Solid State
Disks Buyers Guide
The reasons that users might benefit from buying
SSDs are listed in the
SSD Market
Adoption Model
Take the case of SSD speedup in servers. One way
of thinking about this concept in computer architecture is - SSD CPU
Equivalency. For a wide range of applications if you take a black box
approach and analyze the overall application performance of a computer system -
you would not know whether that system had more CPUs with hard disks or less
CPUs with more SSDs.
Implicit in all my usages of the term "flash SSD" is the
assumption that the device includes some form of
controller which
performs wear-leveling - as opposed to less smart flash memory storage which
doesn't.
News about SSDs and a directory of all market active SSD oems can be
seen on STORAGEsearch.com's main
SSD page.
For
other SSD related terms see -
flash SSD Jargon.
Most of this definition (above) by STORAGEsearch.com was first
published in 2000. |
