Solid State Drive (SSD): How rugged are these devices?
Computer technology has constantly improved over the years. Hard drives with the moving disk had dominated the tech scene for decades before this new threat called Solid State Drive (SSD) came along to ouster it out of business. Its advantages are very distinct; It is super fast thus you will not get that time lag in fetching data or that slow boot-up sequence. No moving parts means the possibility of damaging your drive in the event of a fall has been eradicated. However there are still issues with its reliability and consumers are still trying to get assurance that it is rugged while ensuring safety of data. Additionally they also need assurance that it will provide value for its apparent high cost.
Lets now take some time to look at this to see how rugged SSDs are and their life expectancy looks like.
Almost all SSDs store data in NAND flash memory cells. NAND refers to a logic gate called
"NOT AND" gate.
Flash however is a non-volatile solid state memory that retains data when power is removed.
However a limitation to flash memory is that its cell can only be programmed and erased within a limited number of times.. This can be referred to as PROGRAM/ERASE cycle (P/E cycle).
Depending on the the SSD and its specific technology, the maximum P/E Cycle can be as low as 500 and as high as 100,000 after which the flash memory cells become unusable.
Let us now examine the different types of NAND flash available
Types of NAND Flash
SLC: Early Nand flash were based on SINGLE LEVEL CELL (SLC) technology. It stores one (1) bit of data per cell.
Modern SLCs are rate at minimum of 50,000 P/E Cycles and a maximum of 100,000 P/E Cycles.
It should however be noted that SLCs are very expensive to manufacture.
MLC: This type of Nand flash stores two(2) bit of data per cell. MLC is called Multi Level Cell.
Storing more data per cell reduces the cost of SSD manufacture. It however decreases its writings speeds and also makes it wear out more rapidly.
There are 2 types of MLC drives
- eMLC - Enterprise MLC: This has low write speeds which gives the flash cells an extended life span as compared to the MLCs high write speed. They are rated at 10,000 to 30,000 P/E cycles
- MLC: This is the standard MLC drive which as said earlier has a high write speed which thus wears out the flash cells faster. Theses are rated at 3,000 to 10,000 P/E cycles
TLC: This type of Nand flash stores three (3) bit of data per cell. TLC is called Tripple Level Cell. While it drastically reduces the cost of manufacture, it also reduces is life expectancy to a P/E cycle of about 500 to 2,000
Like said earlier, there has been steady advancement in the development of the SSD.
There are two (2) technologies which complement the SLC, MLC and TLC to increase thier P/E cycles. They are:
- 3D Nand flash: This was developed by Intel and Micron and stacks 32 layers of memory cells
- V-Nand: This was developed by Samsung and it stacks 42 layer of memory cells
Ware Leveling
It should be noted that all recently manufactured SSDs use a process called wear leveling. This is used to even out the P/E cycles marked on each memory cells. This is very important as flash memory cells are erased in blocks. This means that data as to be constantly moves around in an SSD. In a hard disk, data can be written to any location on the disk at any time, and that means that data can be overwritten easily.
SSDs however can’t overwrite data directly. Instead, the SSD must first find an empty memory cells in a block and then write to that empty memory cell.
When it then comes to deletion of data an SSD will ensure that enough memory cells in a block are marked as unused. The SSD will then take the entire content of that block, commit it to memory (RAM), and erase the whole block. Then it will take the committed image and reprint it on that block.
It should also be noted that in an SSDs the lesser the free space the faster it wears out. Thus it is important to leave adequate free space in your SSD to increase its life span.
It is recommended to leave at least 10% free space to increase SSD life expectancy.
Some modern SSDs automatically keep these free space which is not accessible by the end user. It is called Over Provisioning.
Endurance rating
To allow users to predict the possible life expectancy of SSD, manufactures have come up with an endurance rating that indicates the minimum quantity of data that can be written before the SSD fails.
These are measured in the following ways:
- PBW - Petabytes written (petabyte =1024 TB). This rating is mainly for eMLC
- DWPD - Drive Writes Per Day this rating is mainly for SLC. It measures how many time a drive can be filled to full capacity and erased for every day
- TBW - Terabytes Written. This rating is mainly for MLC and TLC. The larger the drive the greater the TBW
Alternatively, how to calculate SSD life expectancy
The P/E cycle rating for NAND is only one piece of the total write endurance puzzle for an SSD.
When the host executes a write to an SSD, the controller of the drive must actually perform multiple writes to different
parts of the NAND. The number of internal writes for every host write is known as write amplification. Write amplification
can vary from 1x to 10x or higher depending on what is being written to the drive and what has previously been written to
the drive.
To calculate the life expectancy of an SSD, the following formula can be used:
However, a similar SSD with only a 500 P/E cycle NAND would have an expected life of just over
6 months.
SSD technology is getting better over time with more ideas as to how to safely store data while improving the SSDs life expectancy.
A standard had drive can sustain at least a few 100 TB at most before they fail. that means your SSD can last many years.
In my opinion, SSDs can and should be trusted. However for best practice it is wise to always have a backup or two of you data.
Thanks for reading this post. See you soon for more.
SSD pictures courtesy of:
https://www.laptopmag.com/articles/laptop-ssd-guide
http://www.itpro.co.uk/operating-systems/26152/how-to-move-windows-10-from-your-old-hard-drive-to-ssd
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