How do SSDs Really Work?

SSDs (Solid State Drives) work very differently to older HDDs (Hard Disk Drives). This gives them several advantages over HDDs, but also a few drawbacks.

SSDs are storage devices that use integrated circuit assemblies as memory. They are able to store data persistently, which means that the data is not lost when the RAM loses power. They differ from traditional HDDs in several ways, making them more efficient in some ways, but also more costly. Here we look at some of the major differences between the two.

HDDs are electromechanical storage devices that include a spinning disk and moveable read/write heads which store and retrieve data. SSDs have no moving parts, and instead most employ NAND-based flash memory. NAND-based flash technology is also used in flash drives, or removable USB storage devices. Using flash technology makes SSDs silent as opposed to the noisy HDD, and also less susceptible to shock damage.

They also have lower access times and latency, but of course are more expensive – though the price continues to lower as the technology becomes more popular.

SSDs also share interface technology developed for HDDs, which means that replacing them is simple.

How do they work? 

An SSD is comprised of the controller, and the memory to store the data. The memory component used to be a DRAM volatile memory, but since 2009 the NAND flash non-volatile memory is more commonly used because of the reduced cost. NAND is also able to retain data even without a constant power supply, giving it another advantage over DRAM. When the technology was first used, NAND was slower than DRAM but this problem was resolved in 2009 with the use of new controllers.

The controller is the part that contains electronics to transfer data between the host computer and the memory components. It also performs other functions, including error correction, bad block mapping and encryption amongst others.

The memory is made up of made NAND chips operating in parallel – a single NAND chip is very slow by itself, and has a high latency. Microsoft and Intel first employed data striping to improve SSD speed, and this was improved further still by SandForce in 2011.

NAND uses floating gate transistors in a similar way to NOR flash, but it uses groups connected in series rather than a single transistor.

Some SSDs are still based on DRAM volatile memory, but must come with either an internal battery or an AC/DC adaptor and backup storage in the event of a power failure.

One major drawback of flash memory is that the program-erase cycles are finite; this means that they have a limited life span. Most products on the market can withstand up to 100,000 P/E cycles before the integrity of the storage begins to deteriorate, however.

Most NAND devices are shipped with some bad blocks; this enables the manufacturers to produce more units at a lower cost than if all the blocks were required to be perfect, and only reduces the storage capacity by a small, acceptable amount.

Comparison with HDDs

The startup time for a SSD is almost instantaneous, whereas a HDD will often need several seconds for the disk to speed up. The random access time is also much faster with SSD at about 0.1 ms compared with anywhere from 2.9 to 12 ms for the HDD.

Fragmentation is irrelevant for SSD as there is no need to read data sequentially. This means that the defragmentation needed for HDDs is not necessary for the SSD.

HDDs are vulnerable to magnetic fields – although in practice, the magnetic platters are usually shielded well enough to avoid damage. No shielding is necessary for SSDs, which are not susceptible to magnetic fields.

The maximum storage capacity of the SSD is lower than HDD – SSD devices are available that can store up to 2TB and is very costly. HDD can store up to 4TB at a fraction of the price.

The lowest-power HDDs require as little as 0.35 watts to operate; SSD of a high performance only need half to one third of the power of a HDD.

Solid State Drives are not only finding themselves in discreet devices such as smart phones, USB memory sticks and laptops but they are also being seen as the next big hardware shift for data heavy processors such as data servers. As such cloud server companies are already offering SSDs as configuration options to enable their users to take advantage of the processing speeds of SSDs with many companies encouraging database heavy servers, who have high I/O requirements, to move to SSD cloud servers.

Solid State Drives are faster, quieter and less susceptible to damage from shock and magnetic interference than Hard Disk Drives, but are more expensive and have a limited life span.

References:

http://en.wikipedia.org/wiki/Solid-state_drive#Comparison_of_SSD_with_hard_disk_drives
http://en.wikipedia.org/wiki/NAND_flash#NAND_flash
http://en.wikipedia.org/wiki/Persistence_%28computer_science%29

ElasticHosts, a cloud server provider, offers fully flexible cloud server configurations – from OS to hard drive technology.