ENDURANCE RATING: TBW & WORKLOAD EXPLAINED

Most of Industrial applications used SSD as OS boot up drive and data storage with 24x7 operation. Applications may vary from POS terminal to mission critical Transportation/Defense equipment. Highest Data reliability, availability and integrity of SSD are important to Industrial customers. SP Industrial SSDs evaluated endurance with Enterprise application case to simulate customers' application behaviors.

The SSD manufacturer shall establish an endurance rating for an SSD that represents the maximum number of terabytes that may be written by a host to the SSD, using the workload specified for the application class, such that the following conditions are satisfied:

1. the SSD maintains its capacity;
2. the SSD maintains the required UBER for its application class;
3. the SSD meets the required functional failure requirement (FFR) for its application class; and
4. the SSD retains data with power off for the required time for its application class.

KEY FACTORS OF ENDURANCE 

Program/erase (PIE) cycles

• The writing of data to one or more pages in an erase block and the erasure of that block, in either order.
• NANO has a limit on how many P/E cycles it can withstand until data retention is not reliable. UBER and data retention are adversely affected by P/E cycles.

Write Amplification Factor (WAF)

• The amount of data written to the NANO flash divided by the amount of data written by the host to the SSD
• WAF is directly associated with P/E cycles. Higher WAF = More P/E cycles happen
• An SSD usually writes more data to the memory than it is asked by the host to write
• The nature of the workload has a significant impact on WAF
  

Figure 1  Workloads & SSD Applications

Figure 2   SSD Endurance Classes And Requirements

In the more general case, WAF may not be a constant. More extensive characterization would be needed to determine f(TBW) in equation before estimating the endurance rating.
The P/E cyclyes of NANO flash is obtained from component qualification data. The WAF may be obtained from SSO data using the specified workload for endurance testing.

Measurement of WAF requires access to information about NANO program/erase cycles which is generally not available to third parties. Under the assumption in this example where WAF is constant, WAF may be measured after operating the SSO long enough to reach a steady state, without needing to operate the drive to its full endurance rating. The guard band for wear leveling effects (2) may be measured from similar SSO data or estimated from the design of the wear leveling scheme.

Figure 3  Endurance Test Workload

Endurance TBW definition issue 

• Not a real world workload
• TBW is based on WAF, WAF is based on WORKLOAD

Solution : Use JEDEC Workload for Endurance Tests

WORKLOAD FACTORS THAT IMPACT WAF

• Sequential versus random
• Large transfers versus small transfers
• Boundary alignment
• Transfer size vs program page size/alignment
• Transfers crossing erase blocks
• Data content/patterns (especially for SSDs using data compression)

WORKLOAD AND INDUSTRIAL APPLICATIONS

Workload of Industrial applications may vary with different segments. However 24x7 operation and high availability/reliability/integrity of SSD are the key designs for Industrial SSD.
SP implemented industrial standard Vdbench utility to generate disk 1/0 workloads to be used for validating storage performance and storage data integrity. Each Industrial SSD is evaluated with two types of workload.

• Sequential Write workload with sector size alignment for most common application with sequential write behavior
• Enterprise workload defined by JESD219A (Figure 4) which consists of random data distributed across an SSD in a manner like some enterprise workload traces that are publicly available for review.
  

SP has capability to develop workload script to generate different disk 1/0 workload to simulate customer's real read/write behavior such as 75% Write/25% read intensive write behavior to estimate endurance of SSDs with workload behavior.

Figure 4  The enterprise endurance workload shall be comprised of random data with the following payload size distribution:

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