Solid State Drives

I have a distrust of anything with moving parts. My favorite example of this is the incredibly inefficient,noisy and smelly internal combustion engine. It has so many moving parts flying around at high speed in different directions and containing multiple explosions it’s pretty unbelievable it works at all. A close second to this would be hard drives.

Spinning Hard Drives
The tolerances a hard drive has to conform to is so incredibly slim that it’s amazing it works outside of a lab in ideal conditions. However work they do,and for the most part extremely well,lasting many years and storing very large amounts of data. I still think placing them in mobile devices such as iPods and laptops is asking for trouble. My laptop has a built in accelerometer that detects when it’s moving and this ‘parks’the disks head to a ‘Landing Zone’where any sudden jolt won’t damage the disk (platter) and trash your data.  Most mobile devices don’t have this feature,which is probably why so many iPods die.

So why do I think they are so fragile? Well each drive is a series of disks,or platters,one on top of the other spinning at 5,400~10,000 RPM this roughly means that the disk is passing the read/write head at a speed of 270 KPH. Imagine your old tape deck working at this speed!

There is a single arm attached to a read/write head,one for each side for every platter. So if you have 4 platters thats 8 heads. This arm can move from the edge of the disk to the center at something like 50 times a second. If any of these heads make contact with the platter this is called a head crash and can destroy that section of the platter or the head itself. So this is a bad thing. How much of a gap,or ‘Flying Height’is there between the platter and head I hear you ask,would you believe 0.0127 microns! 1mm = 1,000 microns,another way of putting it into perspective is to say that a human hair is 50.8 microns thick.

So we have multiple heads speeding at 270 KPH flying at an altitude of 0.0127 Microns lurching sideways 50 times a second being carried around by some bumbling human and for the most part these drives don’t fail. Now do you see why I find it amazing these devices work at all?

Well in matter of fact these drives are failing all the time but due to something called SMART,internal error correction and sector swapping the user is not aware. As a sector of the drive starts to fail the drive will swap this data to a good section of the disk and then shut down the bad sector. Sometimes the damage can’t be corrected and the drive dies. Generally the larger the drive the higher the density and the more errors. So I have always shied away from large capacity hard drives.

Solid State Drives
Which is why the news of new Solid State Drives (SSD) are so interesting. As they have no moving parts they are much more durable,consume less power,no noise,much less heat to name just some advantages. The one big drawback is cost.

For about £70 you can get a 320Gig HD. For a SSD your £70 gets you 8Gig. OK but this is the price of a more reliable/faster/efficient storage medium right. Er I guess,but hang on,I have an 8Gig card in my cell phone that cost £20 almost a quarter the price of SSD media,are they not the same thing?

Memory cards and Thumb drives use NAND Flash memory. This works best with larger amounts of data as it reads data in large chunks.  NAND is fast to erase and write,but slow to read non-sequential data through its serial interface. NAND is also prone to single-bit errors,requiring rigorous algorithms for error detection and correction,slowing the operation down. NAND memory can fail,failing sectors are swapped out automatically exactly like the SMART feature in HDs,this is called Wear Leveling. NAND is cheap to produce.

SSDs use NOR Flash memory. It’s faster than NAND and has better random access capabilities. It’s ideal for storing firmware,boot code,operating systems,and other data that changes infrequently. NOR is fast to read current data but markedly slower to erase it and write new data. NOR also suffers from failing sectors and also uses Wear Leveling. NOR memory also comes in two variants,SLC and MLC. SLC or Single Level Cell means that each cell stores one bit,1 or 0. MLC or Multi Level Cell stores more than one bit,ie 0~3. This means that it can store more data,however this makes it less reliable.

Longevity
HDs will generally last 5~10 years,SSD perhaps 2~5. It’s not a good idea to defrag a SSD,it’s seek times are so low that it’s not really effected by de-fragmentation and running one will create a whole lot of write cycles,reducing it’s life.
The write endurance of SLC NOR flash is typically equal or greater than that of NAND flash,while MLC NOR&NAND Flash have similar Endurance capabilities.

• NAND Flash is typically rated at about 100K cycles
• SLC Floating Gate NOR Flash has typical Endurance rating of 100K to 1,000K cycles
• MLC Floating Gate NOR has typical Endurance rating of 100K cycles

I will build a new PC next year and will almost certainly get a small SSD for the operating system and a traditional HD for media and games etc. I will also use a 64bit OS enabling me to address more than 3.2gig and load it up with 8gig of RAM thus enabling me to turn of virtual memory. This should dramatically reduce the wear on the SSD.

I am hoping that in 2~5 years time when my SSD dies the next gen SSDs will be up there with old spinning media devices in lifetime and price per Gig.

The laptop on the left has an SSD,the right a traditional HD.

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