[ale] musings on the insides of an ssd - part 1

[Ron Frazier (ALE)]

Hi all,

This didn't look like it posted before, so I'm reposting it in two parts.

Today I'm musing on the insides of an ssd. Why am I doing this? There are three reasons. 1) Maximum PC magazine has a nice overview in the August issue of all the major components of a pc, including ssd's. 2) I have a couple of hybrid drives, with a small amount of ssd cache memory, and I wonder about their longevity. 3) I want an ssd for my main desktop drive, and don't have one. That's not likely to happen though, since I just upgraded that to a 1 TB spinning drive.

In any case, I feel that too little attention is paid to what's inside the ssd drive, and too much to the benchmarks. I want to share what I'm learning.

Here are some random observations based on my reading. Some of this has been addressed here before, but it's good to update things on occasion.

What the heck stores the data? In essence, your data is stored in a memory cell, which is insanely small, maybe 20 nm or less, which is a capacitor that stores an electrical charge. You put an electrical charge of voltage in the cell, that's a binary 1 (or possibly the other way around). You take it away, the binary bit changes. To write the charge to the cell, you apply voltage through an insulator whose job it is to keep the charge in the cell, so your data won't vanish. In so doing, you partially destroy the insulator. Hence, the cells have a finite number of times they can be written to reliably. This is not a significant concern with writing magnetic domains on a hdd. Sometimes magnetic coatings on hdd's fail, but many times, mechanical things kill hdd's. Just using an ssd normally will use it up, over time. In a sense, you do the same to a hdd. You use up the mechanism, primarily. Hdd failures are probably less predictable than ssd failures, but both will fail.

Flash memory cells are rated according to the number of program / erase cycles they can endure. The type of cell I described, with a full state and an empty state, is called an SLC, or single level cell. It stores 1 binary bit. It is the most reliable and has the most endurance. It is also the most expensive technology, and is usually only used for enterprise drives.

See these websites for some very good information:

https://en.wikipedia.org/wiki/Single-level_cell http://www.centon.com/flash-products/chiptype https://en.wikipedia.org/wiki/Solid-state_drive https://en.wikipedia.org/wiki/Flash_memory

The first two, in particular, explain cell types.

What bugs me about many articles and ads is that they don't tell you what's inside the drive. They just look at benchmarks. But, in my opinion, what's inside is important. According to the last article, SLC NAND flash cells are rated at about 100K cycles. If you can afford that technology, it's the best thing to have. Depending on your usage, it may not be the most cost effective for you.

The flash memory article mentions Samsung OneNAND KFW4G16Q2M as an example of SLC. I don't know if that's a flash memory part number or an SSD part number which uses this flash.

The next type of memory cell you'll hear bandied about is MLC, or multi level cell. Most of these hold 4 voltage states and store 2 binary bits So, it takes more or less 1/2 as many cells for a given capacity of drive. However, they're more prone to errors. So more error correction logic and spare cells must be provided. An MLC drive will not necessarily cost 1/2 of what an SLC drive does, but it will be much cheaper. More of a concern, to me, is the fact that, according to the last article, these cells only have 1/10 - 1/20 of the endurance of the SLC, or 10K - 5K cycles. So, if an SLC drive was able to last 5 years, or 1825 days AT A CERTAIN DATA RATE of programming and erasing, then an MLC drive, used in the same conditions, might only last 1825 / 20 = 91 days until the cells are no longer guaranteed to perform properly. Things like over provisioning affect this which I'll mention below.

The flash memory article mentions this as an example of MLC: Samsung K9G8G08U0M.

There is a new player in this game, which I personally recommend avoiding like the plague. It's called TLC, and I haven't found what the acronym means. However, it stores 8 voltage levels, or 3 binary bits. This is the most error prone, the quickest to wear out, and the least expensive. The endurance rating of TLC, according to the flash memory article, is 1/100 of that of SLC, or 1K cycles.

So, the same SSD that I said would last 5 years or 1825 days at a certain data rate if it had SLC cells, might only last 18 days with TLC memory.

The article gives Samsung 840 as an example of a TLC device. Samsung generally has a very good reputation in this industry. However, I would NOT buy this device if it has TLC in it.

Not that, after the endurance rating is over, the drive doesn't just die, but the data stored in the cells would become less and less reliable. The ability of the cells to hold the data will become less and less sure. Freshly written data might be readable now but not months later. More and more error correction kicks in. More and more over provisioning space is used up, if any is available. Eventually, you get unrecoverable read and write errors which grow in volume.

So, whether an ssd can work for you depends on your data usage and pattern. SLC is the best option, but may be overly expensive. MLC may be a good option, if you don't over tax it. I personally would say avoid TLC. The general rule I try to go by is not to buy the cheapest of anything I depend on or which is under stress.

Here's the proper way to review and rate and spec an SSD. This is from Computer Power User magazine, April 2013, an article about the Intel SSD 520 180 GB.

<quote>

... that memory is 25 nm MLC NAND manufactured by Intel. ... The drive has a 5 year warranty and a 5 year endurance rating (at 20 GB of writes each day), not that we expect to have to worry about either.

</quote>

(Continued in part 2.)

Sincerely,

Ron



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Ron Frazier
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