Solid-State-Drive, commonly referred to as SSD, is a storage medium based on Flash memory technology. Flash memory is a so-called rewritable semiconductor memory. Unlike random access memory (RAM), its content can be modified, but without the latter being completely erased when the equipment is switched off. SSD technology is gradually replacing Hard-Drive-Disk technology, or HDD.
The SSD is now mainly used in consumer computing and is taking up more and more space in business computing. Note that there are also ranges of industrial SSD components specially designed to withstand harsh environments.
The characteristics of Solid-State-Drive
The advent of the SSD
The 2000s were a time of great change for IT. Indeed, the micro-processors on the market have seen their computing power explode, graphics cards and GPUs are more powerful every day, RAM, like RAM, offer ever greater capacities and speeds.
Among all these developments, we will also note the explosion in the capacities of mechanical hard disks, which have gone from a few GB (Giga Bytes) to several TB (Tera Bytes).
However, while many of the innovations are notable, it has not experienced a revolution. Even today, a classic HDD offers an average latency of 4.2 milliseconds (ms), coupled with an average search time between 8 and 12 ms, which gives an average access time to a data of the order of 12 to 16 ms. These access times create a bottleneck as processors grow more powerful and able to handle ever larger volumes of data in parallel.
In 2007, a revolution in the world of storage for the general public appeared: The SSD. It was a drive with a storage capacity of 2 GB, contained in an Asus laptop: the Asus Eee pc 700 SSD. SSD technology has come to provide an answer to the problem of average data access time: it is now in the order of 0.1 ms, or 100 times faster.
Solid-State-Drive technology is based on Flash memory. To store the data, the SSD has memory chips. These contain semiconductors that work with electrical charges to store data. Data processing takes place electrically (unlike the hard drive which processes it mechanically). This fundamental difference mainly explains the difference in performance.
At its inception, the technology was expensive for very low capacities, around thirty euros for a GB in 2006. Then, very quickly, manufacturers were able to offer increasingly large disks (in terms of capacity) for increasingly optimized prices. In 2011, the price of a GB reached the 1 euro mark. Then in 2013, the average price for a GB was around 50 cents. In 2015, Sandisk announced 6 Tera Bytes SSDs, for an average price of 0.33 euros per GB.
The format of SSDs is 2.5 ”. They exist in several thicknesses, we find in particular 7mm and 9.5mm. In terms of connectivity, there are several types of interfaces: the SATA / mSATA interface and the PCIe interface. Lately the arrival of the M.2 format is notable and offers excellent performance.
Comparison between Solid-State-Drive and Hard-Drive-Disk
SSDs work on electrical action where “classic” hard drives work on mechanical action. The other major differences are the write / erase speed and resistance to shock and vibration. Indeed the electrical signals are not sensitive to this type of environment, this notes an important difference with the mechanical system of the hard disks which, in addition to risking the critical failure with each vibration, generates a lot of heat with the rotation of the plate. .
An SSD also consumes less power than an HDD, despite the fact that even in standby, it continues to maintain minimal operation by performing processes like garbage collector.
Here is a table showing the performance differences between SSD and mechanical disk:
|Feature||SSD||Mechanical hard disk|
|Random access time||About 0.1 ms||2.9 to 12 ms|
|Read / write speed||From 27 MB / s to 3 GB / s||From 12 to 260 MB / s|
|Vulnerability||Sensitive to the number of write cycles. Power cuts can make the unit unrecoverable on some (old) models||Shock and vibration, sensitive to magnetic fields|
|Lifetime||Manufacturer’s warranty ranging from 1 to 10 years||Manufacturer’s warranty from 2 to 5 years. Life without limit a priori, but limited by mechanical fragility|
|Cost / capacity ratio||Around € 0.18 / GB (2019)||Around € 0.06 / GB (2019)|
|Storage capacity||up to 100 TB||up to 16 TB|
|Consommation||0.1 to 0.9 W (standby) up to 0.9 W (activity)||0.5 to 1.3 W (standby) 2 to 4 W (activity)|
The different types of SSD
Since there are different quality levels for mechanical hard drives, there are different types of SSDs:
- Consumer ranges, which are “classic” SSDs that can be obtained from all specialist stores. These SSDs use TLC type memory.
- Discs made for professional use, more specific to specific needs of intense operation. These devices generally have MLC / SLC type technology.
- Industrial IT ranges: TLC / MLC / SLC type these discs have particular resistance characteristics. Some of these industrial ssd drives even offer operating temperatures of -45 ° C to 85 ° C.
SLC type SSDs
Single-Level Cell (SLC) NAND SSDs (NAND Flash Memory Compound) store information per elementary cell. An elementary cell representing a bit, the information is reflected in whether the cell is loaded (bit at 1) or not (bit at 0). Restricting the storage of information to one bit provides more reliability and helps reduce the impact that an error that occurs within a cell could have. In addition, it reduces the amount of time that a power interruption or system bug would interrupt the data write cycle. SLC technology is therefore reliable and efficient. Note that this is the most enduring technology with around 100,000 write / erase cycles on 50 nm memories.
MLC type SSDs
Multi-Level Cell (MLC) NAND SSDs store 2 information per elementary cell. Each elementary cell is represented by 2 bits. We then manage 4 different load levels (00, 01, 10, 11). Storing 2 bits per cell results in reducing the margin between the different (electrical) states and therefore exposing yourself to a higher error rate. MLC technology tends to be inexpensive, since better storage density per cell is obtained. Memory reader software compensates for the error rate using various error correcting algorithms.
The most widely used is the Bose-Chaudhuri-Hocquenghem algorithm (code BCH).
Among the error correcting algorithms of the first generations of computers, we find the Hamming code. These SSDs remain relatively enduring with a capacity between 3,000 and 10,000 cycles per cell depending on the model.
TLC type SSDs
Triple-Level Cell (TLC) NAND SSDs store 3 pieces of information per elementary cell. Each cell is therefore represented by 3 bits, which makes it possible to manage 8 different load levels (000, 001, 010, 011, 100, 101, 110, 111). As with MLC type SSDs, the storage of 3 bits per elementary cell tends to increase the error rate in proportion to the decrease in the margin between the different states of charge. As for the MLC type, the storage density of the TLC disks is greater and their cost is lower, which is reflected in particular in a shorter life with approximately 1000 cycles per cell.
Mise à jour: Les dernières versions de NAND 3D TLC possèdent des durées de vie comparable voir meilleure suivant les marques que les versions MLC. N’hésitez pas à nous contacter à ce propos, nous pourrons vous proposer des solutions innovantes.
Major industrial devices that use SSDs
Industrial computing often requires devices that are more resistant to shock and vibration than conventional equipment. Among these solutions we generally find rugged tablets, rugged laptops, Panel PCs, Fanless PCs or even Rack PCs.
In order to improve their resistance to these various environments, these devices are generally equipped with SSDs. In addition there are SSD extended temperature hard drives specially designed for these environments.
Of course, servers can also use SSDs. They are generally of the SLC type because a server generally hosts one or more critical applications which requires speed and quality.
An aside about on-board drives
Historically, on-board systems, more specifically for automobiles, used so-called “self-propelled” mechanical hard drives. These discs were resistant to vibration, temperature and heavy use. The advent of 24/7 hard drives (called 24/7 drives), has driven the auto industries to change. The latter were more efficient than self-propelled discs. Today, SSDs are a good replacement for this category of hardware.
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