Read Part 1
Both SATA and SAS devices come in 2.5” disk form factors. Until recently, PCIe devices were only available in the Half-Height, Half-Length (HH-HL) card form factor, meaning that the buyer would have to open up the server to install the SSD. This has changed in recent months. Almost all server vendors now offer machines where PCIe Flash can be accessed in the front of the server just like a traditional hard drive. Adoption of this server and storage combination is growing rapidly as it allows simple maintenance (like hot-swap) and gives customers the choice of easily adding or changing SSDs as needed.
SSD endurance is usually described in terms of Drive Writes per Day (DW/D). Specifically, this is how much data that can be written to the device for a specified time period (typically three years or five years). For many vendors, this time period is the same as the SSD’s warranty period. But this is not always the case, so understanding the definition of DW/D is important. For example, if a 1TB SSD is specified for 1DW/D, it should handle 1TB of data written to it every day for the warranty period.
It is important to pay close attention to how DW/D is presented. Some vendors show DW/D in a best case scenario using Total Flash Writes. This is very different from measurements that use Application Writes. The latter takes into consideration worst-case, small block (4K) random I/O patterns with all device activities including writes, reads, wear leveling and garbage collection. It is common to hear about “Write Amplification” which is a reference to the realistic view of what happens over time when writing to an SSD. Other considerations like random or sequential writes will have an impact on endurance. The above reference is for random writes, which will yield lower endurance than sequential writes.
Another metric that is used for SSD write endurance is Terabytes Written (TBW), which describes how much data can be written to the SSD over the life of the drive. Again, the higher the TBW value, the better the endurance of the SSD.
Depending on the supplier, endurance may be reported as either DW/D or TBW. To convert between the two metrics, the drive capacity and the supplier measurement period must be known. To convert TBW to DW/D, the following formula can be used.
TBW = DWD * Warranty * 365 * Capacity/1024
Note: 1024 is simply the conversion for gigabytes to terabytes.
A few years ago, endurance was the top criteria for purchasing an SSD. What the industry has found over time is that SSD technology has improved and generally use-cases tend to be more read intensive. As such, there is now a broad mix of SSD endurance, capacities and DW/D annotations for High Endurance (HE), Medium Endurance (ME), Read Intensive (RI) and Very Read Intensive (VRI) along with associated DW/D warranties.
There are several good ways to choose the right DW/D for a specific environment’s needs. Options include vendor-supplied profiling tools or historical storage information with Self-Monitoring, Analysis and Reporting Technology (S.M.A.R.T.).
Endurance, footprint, cost and performance are all directly impacted by the underlying NAND technology used by the SSD maker. Early on, Single-Level Cell (SLC) NAND Flash, which uses a single cell to store one bit of data, was the primary choice as it provided high endurance for write intensive applications. The downside, however, was that SLC was extremely expensive. To allow the cost of SSDs to reach the mainstream, the industry moved to Multi- Level Cell (MLC) architectures. While less expensive, MLC also has lower endurance. Pioneering SSD vendors addressed MLC endurance challenges with specialized controller architectures for error handling and data protection, yielding Unrecoverable Bit Error Rates (UBER) of 1 error in 100,000 trillion bits read over the full write endurance of the device.
With broad adoption of MLC NAND today, the industry continues to seek new ways to reduce cost and expand the use cases for SSDs. To address both capacity and cost, a new technology is emerging called 3D NAND, where the NAND cells are arranged vertically in the NAND die to gain more density in the same footprint.
NAND manufacturers have chosen different paths for the construction of NAND cells. Some fabrications use traditional floating gate MOSFET technology with doped polycrystalline silicon. Others use Charge Trap Flash (CTF) where silicon nitride film is used to store electrons.
Floating gate is more mature based on its long history, but CTF may have advantages in certain areas. Enterprises should look to vendors with a strong track record of delivering high- quality and high-reliability to successfully manage the 3D NAND transition.