The difference between SLC and MLC, the hottest so

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The difference between SLC and MLC

paging Title solid state disk (solid state disk or solid state drive), also known as electronic hard disk or solid-state electronic disk, is a hard disk composed of control unit and solid-state storage unit (DRAM or flash chip)

the interface specification, definition, function and use method of solid-state disk are the same as those of ordinary hard disk, and the product shape and size are also the same as those of ordinary hard disk-2 Size of bonding specimen with EPS board: 100x100x50mm, 5 pieces. Solid state disk has excellent shock resistance because it does not have the rotating medium of ordinary hard disk. The working temperature range of the chip is very wide (-40~85 ℃). At present, it is widely used in military, vehicle mounted, industrial control, video monitoring, network monitoring, network terminal, electric power, medical treatment, aviation, navigation equipment and other fields

ssd working principle

the following is a schematic tower diagram of NAND flash disk. We can see from the figure below that each "unit" is composed of such a transistor

the channel writes data to the unit; Apply a high enough voltage to both ends to create a strong enough electric field, and the electrons will penetrate the oxide and enter the floating gate. Eliminate the voltage, and the electrons will continue to stay in the floating door. By passing the voltage through the channel without applying it to the door, and twisting the bias voltage, the electrons will move in the other direction

simply put, this is the working principle of the flash disk - you have two states, 0 and 1. Even if the unit has no power, the state will be maintained, so it is very ideal for storage devices

programming flash memory is an iterative process. The controller applies voltage to the door (or channel), allowing some electrons of organic solvent to pass through and checking the critical voltage of the unit. When the critical voltage has reached some predetermined values, it begins to program and store data in the medium and high-end market

difference between MLC and SLC

at present, SSD hard drives use two forms of NAND flash memory: single level unit (SLC) and multi-level unit (MLC). The difference between the two is the amount of data stored per unit. SLC stores 1 bit per unit and MLC stores 2 bits per unit. The key is that SLC and MLC occupy the same chip area. Therefore, at the same price, MLC can double the capacity

Intel's SLC and MLC flash memory actually use the same transistors, with the difference in how to read/write these two flash memories. SLC has only two voltage symbols because it has only two states (0 or 1). MLC has four states (00, 01, 10, 11), so it takes a long time to access, because you don't want to accidentally write wrong data; You have the same maximum and minimum voltage, and now you only have more scales between them:

here is a table about the basic data of SLC and MLC performance:

the erasure performance of SLC and MLC is the same, and the reading performance of MLC flash memory takes twice as long, and the writing performance takes four times as long. If you've heard people complain about MLC write speed before, that's part of the reason. However, it must be remembered that the numbers we are talking about here are ridiculously low - even the speed of writing to MLC flash memory at 900 microseconds is much faster than that to ordinary mechanical hard disk

slc's biggest advantage lies not in its good performance but in its long service life. To understand the durability of flash memory, we first need to look at the internal structure of the storage device

flash memory hierarchy and data loss

we have confirmed that whether a flash memory unit can store one or two bits depends on whether it is an SLC or MLC device. Put a group of units together to get a page. Page is the smallest structure that you can program (write) NAND flash memory devices

each page of most MLC NAND flash memory is 4KB. A block is composed of many pages, In Intel's MLC SSD, a block contains 128 pages (128 pages x 4kb/page = 512kb/block = 0.5Mb. Block is the smallest structure you can erase.

therefore, when you write to SSD, you can write 4KB data at a time; but when you erase data from SSD, you have to delete 512KB at a time. Later, I will explore this further, but now let's see what happens when you delete data from an SSD.

whenever you write data to flash memory, We all go through the same programming process over and over again. Creating an electric field, electrons penetrate peroxides and store charges. Erasing data will cause the same thing to happen, but in the opposite direction. The problem is that the more times an electron penetrates the peroxide, the weaker it will become. Finally, the voltage can no longer prevent the free movement of electrons. At this time, the SSD unit fails

after about 10000 erasure/programming cycles, MLC flash memory will finally have that problem. SLC can be used 100000 times, thanks to its simple design

due to the limited service life, SSD must choose the time and method of erasing/programming each unit very carefully. Please note that you can read data from a cell as many times as you like, which does not reduce the capacity of the cell to store data. Only the erase/program cycle will reduce the life

in addition, it should be noted that since SSD does not have the concept of erasing blocks, the only time to erase blocks is when writing new data. If you delete a file in windows without creating a new file, SSD does not actually remove the data from flash memory unless you are ready to write new data

how to ensure the service life of SSD

now go back to the difference between programming and erasing SSD data. You program on page and erase data on block. Suppose you save an 8KB file and then decide to delete it. This may be a simple note you wrote to yourself, indicating that you no longer need it

when you save the file, it will be saved in flash memory in the form of 2 pages; When you delete it, SSD will mark pages as invalid, but in fact, it will not delete the block. SSD will not copy some valid data to the new pages and erase the block until a certain proportion of pages in the block are marked as invalid. SSD does this to limit the number of erasures per block, thereby extending the life of the drive

however, not all SSDs handle delete requests in the same way. Therefore, in principle, how to handle delete requests has a great impact on SSD security

we have introduced the differences between SLC SSD and MLC SSD from the perspective of overview, and described how the designers of general SSDs can ensure the performance and security of SSDs through programming and algorithms. Below, we will elaborate on how Intel x-25m SSD hard disk can cope with data loss. (end)

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