SSD-Solid State Disk Drives


What you see above is a 256GB SSD – Solid State Drive. This is something that has been promised in Byte magazine over a dozen years ago. The advanatge of SSD over traditional HDD-Hard Disk Drives is speed, weight, size, plus less power consumption. But holding back SSD has been hard disks drives uncanny ability to constantly improve capacity, speed, and particularly cost to keep the SSDs at bay. But new application requirements and improvements in underlying SSD technology using Flash memory apears to have crossed an important tipping point.

The drive above has a capacity of 256GB and has an average access speed of 0.1ms. I have highlighted in yellow some of its other important operating specs. Lets compare this SSD to a regular hard disk drive-HDD used in most PCs, and particularly notebooks. Both drives will use a standard SATA connection to the device. The HDD-Hard Disk Drive will be a 7200RPM, 3.5″ drive used in many laptops(better performer is placed first):
Maximum capacity – HDD 1000GB, SSD-850GB – not much difference here
Average Access Time – SSD 0.1ms, HDD-7MS – huge speed advantage to SSD
Transfer Rate – HDD-75MB/sec, SSD-50-65/MB/sec – but once found HDD has 15-20% faster data movement rate
MTBF – SSD 1Million hours, HDD – .6Million – advantage to SSD but note failure cautions below
Cost – HDD $1/GB, SSD $10-15/GB – decided advantage to HDD
So in just these five factors one can see the major trade-offs between HDD-Hard Disk Drives and SSD as used in lpaptops and other mobile devices.

However there are some secondary factors to take into account. First, the technologies in SSD using Flash memory are relatively new where as HDD technology is at least 40 years old. Second there are some performance characteristics harder to quantify comparatively which give additional advantages to SSD. First, power consumption for SSD drives is 30-80% lower than HDD giving longer battery life and less heat. The SSD has virtually no noise. And finally the SSD is much better able to stand falls and high G force shocks than a hard disk. That is why SSD are used in military and industrial notebooks and other devices.

On the downside for Flash-memory based SSD (which have been used throughout this note as the point of comparison – not the even faster DRAM based SSDs) is the fact that repeated writes to a memory cell in a Flash memory device can lead to write failures. Typically, the failure occurs after 1–5 million write cycles (for high usage files such as log files, file allocation tables, and other commonly used parts of the file system exceed this over the lifetime of a typical computer). However, new write leveling algorithms are improving this factor.

So HDD disk drives are already appearing in laptops (new MacBook air and the OLPC-One Laptop Per Child) initiative. Likewise in photo equipment both highend camcorders and SLR digital cameras are using SSD. It appears the constraint for wider usage of SSD will be the cost per GB and some other competing technologies like compact flash cards and more importantly SDHC cards popular in many point and shoot cameras, digital SLRs, camcorders and laptops. In sum, SSDs will appear and add advanatge to consumer oriented laptops and specialty devices, while traditional HDD will prevail in super low cost PCs and devices plus large disk capacity oriented servers. But SSD now is a big and growing part of the IT scene just like Byte said it would be.