What’s the difference between Hard Disk Drives (HDD) and Solid State Drives (SSD)?
Platters with the ability to store data magnetically in concentric circles (tracks) are spun by a spindle motor. A “head” on the end of an “arm” is used to read/write data on these “tracks”. This arm is attached to an actuator, or stepper motor for older hard drives, which moves the head to the track to be read or written.
Hard disk drives mechanical ability and incredibly high tolerances are amazing. The spindle motor in common HDDs spins at 5400 or 7200 revolutions per minute (RPM), with higher end server drives up to 15,000 RPM.
While the platters are spinning, the read/write head is flying over them at 3 nanometers (billionths of a meter). At this height, a dust or smoke particle can collide with the head and disrupt operation. Due to this, the head/disk assembly (HDA) of the HDD is sealed in a clean room to avoid contaminants.
Since the head needs an “air bearing” to fly over, the HDA is usually ventilated with a filter to equalize pressure, while barring the finest particulate matter. Specifically because of the need for this air bearing, unpressurized, high altitude applications are an issue for HDD.
How do Solid State Drives (SSD) differ?
Both HDD and SSD have unlimited number of read cycles. HDD also have unlimited writes.
Due to limitations with the NAND memory, SSD have a finite number of writes. The number of endurance cycles is based significantly on the type of SSD (Consumer, Industrial), the NAND type (SLC, MLC, TLC) and the trace width/geometry (4x, 3x, 2x, 1x nm) of the NAND components. See this white paper for more details.
This write cycle limitation of SSDs is usually rated as Endurance cycles per logical block or more recently several Commercial/Enterprise SSDs are rating this as TBW (Terabytes Written) for the life of the SSD.
Other benefits of SSD over HDD are zero noise, better battery life, lower system temperatures and quicker boot and application load times.
So why doesn’t everyone buy SSD instead of HDD?
The main answer - - cost and capacity.
HDDs are being produced in the 3 Terabyte range today. For the same amount of money as the 3TB HDD, you would only get a 120GB SSD based on MLC NAND. If you choose to go the higher reliability route of Industrial Grade SSD, the difference becomes much wider.
Over time, HDDs will continue to increase capacity while selling for the same or similar costs. SSDs will continue to lower costs for the same capacity. For example, a mainstream 240GB MLC SSD may have a similar selling price as the 3TB HDD in 2015, but a 4TB HDD may be available for the same price as well.