electronica Nov 2014
12
E
MBEDDED
C
OMPUTING
Solid state drives are replacing
hard disks in industrial applications
By Robert Herth,
MSC
The potential advantages
of solid state over hard disk
drives are now becoming apparent
in the marketplace. This article
discusses the technological
background to this trend.
A few years ago, solid state drives (SSDs)
still had some difficulties compared with
hard drives. Today, SSDs celebrate consid-
erable success and over the next few years,
the market for SSDs will continue to grow
very rapidly. The main reasons for this are
improved technical characteristics, especially
in terms of lifetime, performance and security,
which determine the selection of SSDs today.
Whereas the consumer market is dominated
by price per gigabyte (GB), performance data
and lifetime per GB are what count in the
industrial market.
The market for industrial applications is
divided into embedded customers – which
use a flash card or SSD in their system, for
example, industrial PC (IPC), data logger or
measurement device – and enterprise custom-
ers in the field of networking and hosting. For
customers with embedded applications, SSDs
with smaller capacities such as, for example,
32 GB or 64 GB are sufficient in most cases. In
the enterprise sector, there is a much greater
need for storage, therefore large SSDs with 480
GB and 960 GB are used, and the demand for
storage capacity is increasing rapidly. We can
expect to be talking next year about 1.5 tera-
byte (TB) and 2 TB. Industrial customers are
focusing more and more on data storage with
NAND flash memory chips that are based on
multi-level cell (MLC) instead of single-level
cell (SLC). In a MLC construction with com-
mon 19nm structures today, 2 bits of data per
cell can be stored. With triple-level cell (TLC),
3 bits of data per cell can be stored, the high-
est cell density. However, the requirements in
regard to the controller and handling of the
SSD for ensuring performance and lifetime
are more demanding. In assessing lifetime,
the SSD must be considered as a whole system
and not just the lifetime of an individual flash
cell. Only approximately 3000 program-erase
(P/E) cycles are allowed with the 0 or 1 pro-
grammed cells. However, by the implementa-
tion of diverse technologies that have evolved
during recent years, the lifetime of the SSD
can be extended considerably. Examples of
this are: wear leveling, EDC/ECC, bad block
management, write amplification, garbage
collection, and over-provisioning.
Wear leveling is the intelligent distribution
of data written in the SSD. The objective is
to achieve even wear of the flash cells and
thus increase lifetime of the SSD. With static
wear leveling, wear of the flash cells over the
entire capacity can be reduced through auto-
matic transfer of data to less used flash blocks.
Without wear leveling, individual sectors on
the SSD would be hardly or heavily used to a
varying extent. To maximize performance, the
data buffering function catches data in a cache
in order then to write this in a single opera-
tion. The error detection code/error correc-
tion code (EDC/ECC) algorithm for detecting
and correcting errors at the bit level has an
influence on the maximum number of read/
write cycles. When a flash block approaches
the end of its lifetime, the probability of a
bit error increases. By checking the cell, if it
is defective or not, the maximum number of
cycles can be considerably increased.
Generally, in SSDs with MLC technology,
approximately 7% of the flash memory chips
are reserved as reserve blocks (spare blocks)
for data storage. If a flash cell is worn out or
defective, the bad block management feature
will mark it with a flag and replace it with a
cell from the reserve block. This increases
the lifetime and reliability of the SSD. Write
amplification is a criterion of how intelligent
the manufacturer has written its firmware and
how optimally the controller can manage the
data. The garbage collection (GC) is respon-
sible for efficient deletion of data in the flash
blocks.
The data management requires that a SSD has
enough storage space available for manage-
ment and transfer of temporary data. In mod-
ern SSDs, over-provisioning (OP) is currently
used in order to guarantee this, even as the
drive is progressively filled with data. OP is a
way to set aside a certain amount of storage
Robert Herth, BDM Memory
&
Storage, MSC Technologies
