Ram dd3 8gb SSD Popular Principle Introduction
1TB SSD drive SSD OEM ram dd3 8gbSSD series Solid State Driver dd3 8gb+8618750919058, which is the abbreviation of SSD. At present, the mainstream SSD series uses semiconductor flash memory (Flash) as a medium storage device ram dd3 8GB + 8618750919058, SSD is different from HDD (Hard Disk Drive) mechanical hard disk.
SSD was born in the 1970s, the earliest SSD uses RAM, RAM power loss data will be lost, and the price is also particularly expensive. After the flash memory appears based on the flash memory SSD, the data is lost after the flash power is down, and the flash SSD slowly replaces the flash RAM SSD. At this time, HDD disks have occupied most of the storage market. By the beginning of this century, due to the progress of the process, SSD has developed greatly, capacity and performance have been improved, and the price has also declined. HDD ciliary process and technology has been difficult to have breakthrough progress, SSD performance and capacity is still a breakthrough, compared to the HDD market shrinkage, whether the enterprise market or the consumer market SSD rapid popularization, SSD market share has been expanding. It is believed that the SSD Coraline storage field will replace HDD as the mainstream storage device and become the mainstream device for the definition of the bulking storage device.
In the way it works, HDD uses disks, that is, magnetic media as the data storage medium, reads and writes data, and uses the magnetic head + motor for mechanical addressing. Because the mechanical hard disk relies on a mechanical drive to read and write data restrictions, resulting in the performance improvement of the mechanical hard disk encounters a bottleneck. In particular, the random reading and writing ability of HDD disks is limited by their mechanical characteristics, which is a huge bottleneck. SSD uses Flash as a storage medium, data reads and writes are addressed through the SSD controller, which requires mechanical operation and excellent random access capabilities.
The figure below is composed of HDD and SSD, respectively. HDD uses mechanical SSDs consisting of master control, flash memory, DRAM (optional), PCB (power chip, resistor, capacitor, etc.), and interface (SATA, SAS, PCIe, etc.).
SSDs are divided into front-end, intermediate, and back-end. The front-end system interface and related protocol modules (STAT/SAS/PCIe, etc.), the intermediate system FTL (Flash Translation Layer) modules, and the back-end system and flash communication modules (flash memory follows the ONFI or Toggle protocol).
Flash basic storage unit system – floating gate transistors.
The above figure is a floating gate transistor structure, and the bottom line substrate, source, and drain. Above the substrate, there is a tunnel oxide layer, a Floating Gate, an oxide layer, and a control gate. Surrounded by an insulating layer, electrons are easy to get in and out of, electrons are charged and discharged through the Flat Gate, and the transistors are written and erased.
The current between the source (Source) and the drain (Drain) between the current unidirectional conduction semiconductor forms a storage electron floating gate, the floating gate is surrounded by an insulating layer, and the storage electrons inside the storage will disappear due to power failure, so the flash memory is non-volatile.
The following figure is the principle of floating gate transistor writing and erasure.
The write operation is shown in the figure on the left, and the upper control gate is added to the positive voltage Vpp so that the electrons enter the floating gate through the insulating layer. The erasure operation is just the opposite, as shown in the picture on the right, the tie substrate is added to the positive voltage Vpp, and the electrons are sucked out of the floating grid. The writing process is a charging sub-process, if it has been written before writing the page, before writing, it must first be wiped 0 by flash, and the electrons in the floating grid must be cleared.
The data system is stored in binary 0 and 1, according to the floating gate there are two states of electrons, the data 0 and 1 can be represented, and the data must be stored. Generally speaking, the state of electrons is recorded as 0, and the state of electrons is recorded as 1.
According to the production process, the flash memory score is NOR type and NAND type.
Nor type is designed to replace EEPROM, can be accessed bitwise or byte, NOR type flash memory chip has the advantages of high reliability, fast random read speed, but erase and programming speed is slower, small capacity, mainly used to store executable Chengdu code.
NAND flash memory has a large capacity, read and write by page, and has a large capacity, which is suitable for data storage. The introductions in this article are based on NAND flash.
The following figure shows the internal organizational structure of the flash memory:
The traditional Flash is composed of a two-dimensional line( WL) and a bit line (BL), as shown in the following figure. A word line represents a page, and when reading and writing, the bit line controls each bit on the word line. The word line and the bit line intersect to form a block. Block tiles form a flash.
With the development of the flash memory technology manufacturing process, the size of the two-dimensional flash memory is reduced, and the interference between the cells becomes larger with the reduction of the size. 3D Nand Flash technology has emerged, which has the effect of solving the problem of interference of the unit.
The following figure is a 3D flash memory stereogram, which is stacked in this three-dimensional flash memory. If 2D Nand Flash is compared to a bungalow, 3D Nand Flash has to be looked at as a building, and 3D Nand Flash has to pile more transistors per unit area by increasing the number of flash layers. 3D Nand Flash stacks more storage units per bit, which has an advantage in reducing the cost per bit.
The reading speed is so fast; When writing data, it is slightly slower because it needs to be electronically filled by a pressurized storage unit; The erase speed is the slowest, and the erasing block is in the ms level. When using SSD, it is necessary to take into account the characteristics of SSD read and write balance.
Nand Flash is written on the page and erased in blocks. Before the page is written, the block of the page must be erased. The system is determined by the working principle of Nand Flash.
A Worldline should have one or more Pages, depending on the SLC, MLC, or TLC. SLC said that a Word line should be a Page; MLC should have 2 Pages, and the two Pages should be one (Lower Page and Upper Page); TLC should have 3 Pages (Lower Page, Upper Page, and Extra Page, as flash memory manufacturers call it). How big a Page is, there are more than a few storage units on the Wordline, and there are more than a few bitlines. Writes are in page units.
A block in which all of these storage units are tied to a common substrate. When the substrate is reinforced, all the floating barrier electrons above will be sucked out. So the erasure system is in blocks.
Each NAND Block has a limit on the number of erases to be written, and when the number of times is exceeded, the Block may not be able to use it: the floating gate electrode is charged with an electron (write failure), or the floating gate electrode is easy to run out of the electron (bit flipping, 0-> 1), or the floating gate pole inside the electron runs out of the tube (erase failure). The maximum number of erasures is reduced by SLC, MLC, and TLC in turn: the number of SLC erases can reach 100,000 times, MLC is generally thousands to tens of thousands, and TLC is reduced to hundreds to thousands.
According to how many bits of data can be stored in a storage unit, the flash cell score is SLC (Single Level Cell), MLC (Multiple Level Cell), and TLC (Triple Level Cell).
The above figure is a threshold voltage distribution function of the memory unit in the flash memory chip, the threshold voltage of the horizontal value system, and the number of storage units in the vertical axis system.
The more electronic division of a memory unit, the more delicate the number of electrons controlled to enter the floating gate, so the longer the writing time; Similarly, when reading, you need to try to read it with the reference voltage of the same as the one, and to a certain extent, the reading time is lengthened. In terms of performance, TLC is like MLC, MLC is like SLC. In terms of lifetime, SLC > MLC > TLC. Qualm price on SLC > MLC > TLC. The following table is a comparison of SLC, MLC, and TLC purine parameters, the data is older, but the trend of the system ratio does not change. Now there is also a QLC, a flash memory unit that has to store 4 bits of data.
Flash type SLCMLCTLC bit number per cell 123 per cell indicates the number of erases (k) 10031 read time (us) 305075 write time (us) 3006001000 erase time (us) 150030004500
FTL flash translation layer is an important part of the SSD, to achieve the following functions:
In the next article, “Take You to SSD(2)-FTL”, the functions of FTL will be introduced in detail.
Notes
Author: NetEase Storage Team Siege Lion Chen Wei
If there are omissions or mistakes in understanding and description, we are welcome to communicate together; References have been indicated in the references, but there may still be omissions, any infringement or clarification, welcome to point out, must be corrected or deleted promptly; The article is for learning and exchange, and the reprint indicates the source
SSDFans.SSD: Core Technology, Principle and Practice of Solid-State Storage[M].Beijing: China Machine Press,2018.6.
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