What is a solid state drive really? Just the words suggest a device that is a secondary storage device built of solid state devices.
There's one device we are all familiar with. This is the memory card we use all the time to augment the storage capacity of our cell phone, music players, media players, digital cameras and so on. These are Flash memory devices with a specific interface. SDHC is a popular one. SSD as it has come to be known is also a card with flash memory device built with HDD (the hard disk drive) like interface standards; SATA, for example.
The hard disk today has huge capacities. 160 GB is almost obsolete; 500 GB and TB drives are affordable at $150 or less. Capacity wise it is hard to beat. Technologically, the capacity is likely to grow for some more time.
Limitations of a Hard Drive
Because of the very construction, the hard drives are slow compared to things that happen within a processor. The access time, that measures this delay, is of the order of a few milliseconds even with the faster drives that rotate at 7200 rpm or even 10,000 rpm. As of now this cannot be improved much whatever you do. Being an electromechanical drive it consumes a lot of power and are less reliable than semiconductor memory units. Capacities have been improving dramatically recently with the vertical recording technology coming along. However, even this is likely to hit a limit soon.
Promise of SSD
SSD units are all solid state; a block of memory along with a storage controller housed in a box with a HDD interface that can be connected to a computer with the same HDD interface. Since that does not require any electromechanical parts it is going to be more reliable than HDD's. Access times are much faster and the device consumes less power. Those were the promises product developers have been chasing ever since the idea caught on. Faster access, better reliability, lower power budget were the dimensions of the Holy Grail. What it lacked was the capacity. SSD's seem to be catching up there.
What's holding it back?
Flash devices were originally used for semi permanent storage. Data that would be held for a reasonably long time, changed infrequently. Unlike RAM devices individual cells cannot be written to. You have to erase a block of memory and then the writing can be done cell after a cell in that block. There is a limit to these cycles of write-erase-write cycles. Capacities are anywhere near the capacities offered by the hard disks. While 32 GB/64 GB products are available in SSD version, HDD versions already have stable products at 1TB/2 TB kind of capacities. For a comparable capacity it is still a lot more expensive.
What's likely to happen?
The capacity anomaly will probably be straightened out first. The dramatic improvements in capacities seen in HDD devices in recent times have been due to the vertical recording technology. The areal density of recording cannot be increasing in such dramatic leaps for long. Whereas The Moor's law would help push the capacity of solid state devices used for SSDs. Though not mass produced, devices of 1 TB capacity has been achieved with SSD devices.
The write limitations mentioned in the earlier section is already being compensated by techniques such as wear leveling is being used. What this does is to direct writing to block which has been used less. So that all the blocks in the drive face similar number of cycles and thus the life is maximized. The number of cycles these devices can face is very large already. 100,000 or more cycles are the typical specs today and with common usage as a disk drive this may mean a usage period of years.
While the hard disk costs about 40 cents per GB of capacity, the SSD's manage this at about 2 to 3 dollar per GB. That is a huge disparity. Until this gap is covered it is very difficult to see SSD taking over from HDD's any time soon.
Western Digital was due to release WD20EADS Caviar Green Series in Feb 2009 that has 2 TB capacity. This is supported by 32 MB cache; seek time of 8.9 ms and speeds of 5400 /7200 rpm and 3.9 w power consumption. This was to be built out of 4x500 GB platter. According to a TechReport report, the street price is $299 (http://techreport.com/articles.x/16393). The 500 GB platter recording density is an increase of 50% of the areal density of earlier products.
According to an announcement in Gizmode Jan 10, 2009 the 1 TB SSD, 3.5" is going to be available this year from Pure Silicon (http://gizmodo.com/5128280/worlds-first-1-tb-25-ssd-from-puresilicon-is-the-dream-drive). Price data is not known. So capacity gap is not different by an order or more now. They are in comparable range. It is the price gap that will decide when the SSD becomes a consumer device.
Price gap is still high at current prices. A 126 GB SSD costs about $460 for a $3.58 per GB of storage. A 160 GB hard drive would cost about $60 or 37.5 cents per GB. For this gap to close it is going to take a few years.