2 - Solid State Drives 101

Solid State Drives 101

Before we go deeper into the review, I wanted to chat with you a little about what precisely the difference between the traditional HDD (disk drive) and all new SDD precisely is. When we look at the HDD we receive a big bulky mechanical chunk of technology, they have spindle motors, rotating magnetic platters and read/write heads. Solid-state drives however consist of flash memory chips and have no moving parts whatsoever, which is a tremendous gain.

SSD drives are also known under the name Fibre Channel hard disks. In this year and age Solid state disks (SSDs) are typically faster than regular disks because there is zero latency (the read/write head does not need to move anymore). Next to that flash memory can be addressed in multiple channels, look at that as internal RAID. Improving speed tremendously. This year we'll already have SSD drives topping 200 MB/s read speeds and 120 MB/s write speeds.

SSD's are also more sturdy than hard disks and offer greater protection in hostile environments. As a result, they may also be used in desktop computers and even servers in dangerous locations.

All flash memory is not created equal. There are 2 main types:
SLC and MLC (single and multi level cells) SLC stores one bit per cell, MLC doubles it. MLC we see a lot in flash-memories used for example in your digital camera. If we break it down:

• SLC is faster, and has around 1-5 million write cycles (similar to hard disks).
• MLC is somewhat slower but cheaper, unfortunately also less reliable (sometimes as few as 10,000 write cycles).

And that's where we land at limited write cycles. The  reality is that you purchase an SSD drive on borrowed time. By its very nature, flash memory cells can resist only so many write cycles before they are prone to failure.

Typically SLC Flash storage will wear out after hundreds of thousands of write cycles, while high endurance Flash storage is often marketed with endurance of 1-5 million write cycles. Special file systems or firmware designs mitigate this problem by spreading writes over the entire device (so-called wear leveling), rather than rewriting files in place. Wear leveling is designed to prevent rewriting the same cell constantly. Pretty much data is constantly spread out over the entire drive so that all cells will be written equally over time. Wear leveling isn't done in hardware, it is done in firmware on the SSD. It will spread the writes over the whole range of the NAND storage, so you can't really wear out one area while the rest stayed untouched.

At this very moment typically the life-span of an SSD drive MTBF (Mean Time Between Failures) is 1,500,000 to 2,000,000 hours.

Mind you that the limited write cycles is per cell, not per disk, and all controllers have wear leveling (using different cells) for a few years now which actually explains slower MLC writes. Any "bad" cell can be mapped out just like a bad block on a hard drive (transparently).

MLC then what ? Well, only the  future will tell ...  However, with modern flash technology and error correction, the reliability of the drive in a PC definitely exceeds 10 years.