As we have discussed the differences between Enterprise SSDs and Consumer SSDs in regards to performance and reliability in the first part of this article last week, we now continue in showing their differences in endurance. Following will be a short summary of the findings of this study. Let's start with:
Endurance
For all NAND flash memories in flash memory devices, the reliability of storing data bits decreases with each program or erase cycle (P / E) of a NAND flash memory cell until the NAND flash blocks can no longer reliably store data. At this point, a degraded or weak block is moved from the user-addressable storage pool and the logical block address (or LBA) to a new physical address in the NAND flash memory array. A new memory block replaces the bad using the spare block pool that is part of the Over Provisioned (OP) memory on the SSD.
As the cell is constantly being programmed or erased, the BER also increases linearly, and therefore, a complex set of management techniques must be implemented on the enterprise SSD controller to manage cell capacity, thus reliably predicting the expected life of the SSD can be stored. [4]
The P / E lifetime of a particular NAND flash memory can be significantly different, depending on the current lithographic manufacturing process and the type of NAND flash produced.
| NAND- Flash Memory Type |
TLC |
MLC |
SLC |
| Architecture |
3 Bits per Cell |
2 Bits per Cell |
1 Bit pro Cell |
| Capacity |
Highest capacity |
Highest capacity |
Lowest capacity |
| Lifespan (P/E) |
Lowest lifespan |
Medium lifespan |
Highest lifespan |
| Price |
$ |
$$ |
$$$$ |
| Approximate NAND bit error rate (BER) |
10^4 |
10^7 |
10^9 |
Table 2 - NAND flash memory types [5] [6]
Enterprise SSDs also differ from client SSDs in terms of their duty cycle. An enterprise-class SSD must be able to handle high-level read or write activity typical of data center server scenarios that require access to data throughout the 24 hours, every day of the week, as opposed to an SSD the client class, which is typically fully utilized for only 8 hours a day per week. Enterprise SSDs have a 24x7 work cycle, unlike client SSDs that have a 20/80 work cycle (20% of the time active, 80% in standby or sleep mode during computer use).
Understanding the write-resistance of applications or SSDs can be very complex. Therefore, the JEDEC Committee has proposed a lifetime measurement metric that uses the TeraBytes Written (TBW) value to display the amount of raw data that can be written to an SSD before the NAND flash in the SSD starts to store unreliably and should be removed.
The proposed JESD218A test methods and JESD219 enterprise-class workloads by the JEDEC simplify the task of interpreting SSD manufacturer life cycle calculations using TBW and extrapolating a more understandable lifetime measurement that can be applied to data centers.
As noted in documents JESD218 and JESD219, different workloads in the application class may also suffer from a Write Amplification Factor (WAF), which is larger in magnitude than the actual host-supplied writes. This can easily lead to uncontrollable NAND flash wear, over time due to over-description, to higher NAND flash BER and to slower performance due to invalid pages scattered throughout the SSD.
While TBW is an important topic for discussion between SSDs of the enterprise class and the client class, TBW is just a predictive model for the NAND flash lifetime and the Mean Time Between Failure (MTBF) is considered to be the component of the predictive life model and the Reliability based on the reliability of the components used in the device. Expectations of enterprise-grade SSD components include ongoing and tougher work to manage the tensions across all NAND flash memories over the life expectancy of the SSD. All Enterprise SSDs should be rated at least one million hours MTBF, which is more than 114 years! Kingston gives the specifications of his SSDs very conservative, and it is not uncommon to see higher MTBF specifications on SSDs. It's important to note that 1 million hours is more than a good starting point for enterprise SSDs.
With S.M.A.R.T. monitoring and reporting of enterprise-class SSDs, the device can easily query its life expectancy based on the current Write Amplification Factor (WAF) and wear status before failure. Predictive warnings of failures such as a power failure, bit errors occurring on the physical interface, or uneven wear are also commonly supported. The Kingston SSD Manager utility can be downloaded from the Kingston website and used to indicate the status of a drive.
For client-class SSDs, only the minimum SSM.A.R.T. services may be available to monitor the SSD during default use or after a failure.
Depending on the application class and capacity of the SSD, an increased reserve capacity of the NAND flash memory may also be allocated as oversized (OP) reserve capacity. Op Capacity is hidden in user and operating system access and can be used temporarily as a write buffer for higher, sustained performance and as a replacement for defective flash memory cells throughout the life expectancy of the SSD to increase the reliability and longevity of the SSD (with a larger number of spare blocks) to improve.
Summary
The differences between enterprise-class and client-class SSDs are significant, ranging from the lifecycle of their NAND flash memory program and erase cycles to their complex management techniques to accommodate workloads across different application classes.
Understanding these differences in application classes can be an effective tool in minimizing and managing disruptive downtime in the demanding and often mission-critical business environment because it is about performance, reliability, and longevity. For further questions, please contact your Kingston representative or use the „Ask An Expert“ or the Tech Support Chat feature on Kingston.com.
[4] JEDEC Committee JESD219: JESD219: Solid State Drive (SSD) Endurance Workloads JEDEC Committee
[5] The Bleak Future of NAND Flash Memory, University of California
[6] 10. Characterization and Error-Correcting Codes for TLC Flash Memories, University of California
[7] NAND Flash Qualification Guideline, California Institute of Technology.
Text Copyright: Kingston Technology
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