SSDs versus spinning-rust drives

[Ryan Petris]

On 2019-12-01 22:32, Jim wrote:
> I've seen the limited write cycles mentioned here.  I can't help but
> wonder if it's possible to make SSDs with more write cycles.  If it is
> possible, I'm guessing it becomes much more expensive to do so.

It depends on the type of SSD. SLC SSDs are the simplest and least-dense 
type, though because each cell only stores a single bit, they can handle 
much more degradation and therefore have many more write cycles than 
others. I believe this is what most Intel Optane SSDs use, which 
explains their low capacity. MLC is next, storing two bits per cell, and 
is what I believe Samsung Evo drives use; they generally have a much 
higher TBW specification than other SSDs on the market (but lower than 
Optane) though are generally much more expensive. TLC SSDs, storing 
three bits per cell, is the majority of what you're going to find on the 
market right now. QLC SSDs, storing four bits per cell, are few and far 
between, though there are some such as the Intel 660p which use it.

As more bits are stored per cell, write endurance, price, and write 
performance generally go down, however you get a better cost/gb ratio. 
Good drives will mitigate any write performance issues by having a DRAM 
cache and/or have a portion of the SSD act as an SLC ssd, moving it to 
the TLC/QLC portion in the background after being committed to disk. 
I've even seen a few disks apply compression to the data prior to 
committing to disk, reducing the amount of space actually used and 
therefore prolonging the life of the SSD (though this won't help if the 
disk is encrypted).

That said, the TBW ratings are what the manufacturer is comfortable 
warrantying the drive at; it isn't necessarily going to burst into 
flames after writing 1 bit over the TBW rating. It will probably last 
many many more write cycles after that. Even so, most will never run 
into the TBW rating anyway, even for the minuscule TBW ratings of QLC 
drives.