Solid-state disks (SSDs) are hardly new, but their growing usage represents a significant shift in the primary storage landscape. SSDs have been increasing in capacity and decreasing in cost at an accelerating rate, so the chances that you're going to bump into them in the wild are climbing as a result. However, SSDs are not perfect. A solid understanding of their history and differentiating factors will help you debunk some of the hype and leverage them more effectively in your environment.
The idea of a solid-state disk has been around for a very long time. Essentially, an SSD is persistent storage media constructed using transistors rather than an electromechanical disk or tape. SSDs have been holding the firmware for our switches, routers, cell phones, calculators, and just about any other kind of non disk persistent memory for easily 30 or more years.
What is different today is that we're well down the path of using these SSDs in our enterprise primary storage environments -- either augmenting traditional disks or replacing them completely. This type of application for SSD hasn't been possible until recently due to the tremendous difficulty involved in constructing very large SSD memory modules that are cheap, reliable, and fast and that have a long lifetime. We're still working to overcome some of these challenges, and being aware of them is key to implementing them successfully.
Volatile versus non-volatile SSDs
The biggest distinction to make right off the bat when talking about SSDs is whether they are volatile DRAM-based devices (RAM storage) or non-volatile NAND memory (flash storage) devices. They both often fall under the SSD moniker, so it's easy to get them confused.
DRAM-based devices essentially use the same type of memory that makes up the primary system memory of your server; they are both extremely fast and susceptible to total data loss if power is interrupted for some reason. To combat this, most DRAM-based SSD devices require a battery backup to power the memory and ensure data integrity until power is restored.
In some cases, this battery backup is a super capacitor that can power the device for a few days; this is common in very high-performance DRAM SSDs that ship in a PCIe card factor. However, in the event that power isn't ever restored, your data probably won't be, either. In other cases, the DRAM is paired with an equal-capacity array of hard disks or slower NAND flash memory in a rack-mount chassis that is used to stage and de-stage the DRAM memory during power up and power down (with an internal bank of batteries or capacitors providing enough power to perform the de-stage operation in the event of an unexpected power outage).
NAND-based (flash) devices use the same general breed of memory found in cell phones and USB sticks. These memory devices do not require power to hold their state. Thus, they don't require a battery backup of any kind to ensure data integrity. On the other hand, they're several times slower than DRAM-based devices, though their speed is improving as the devices and their controllers mature.
MLC versus SLC SSDs
NAND devices come in two major flavours: MLC (multilevel cell) and SLC (single-level cell). MLC devices are so named because they can store a few bits of data in the same cell, whereas SLC devices can store only a single bit of data per cell. SLC devices are much more expensive to make because they require more transistors to store the same quantity of data, but they're significantly faster and have a longer lifespan than MLC devices.