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Tech blog  February 23, 2016

Can You Truly Enjoy the Reported SSD Performance?

Most SSDs, especially consumer models, suffer from a phenomenon where performance deteriorates over time. Although users can enjoy the peak performance of these SSDs initially, once the drives have been used for a certain amount of time, performance may be 3x-10x slower than that of the fresh-out-of-box condition.

Storage Networking Industry Association (SNIA) Solid State Storage Initiative defines three states of SSD’s time variant performance:

  • FOB(Fresh-Out-of-Box)State: In this state the number of P/E (program/erase) cycles of each NAND flash memory cell is very low.
  • Transition State: This state refers to the duration from FOB to Steady State.
  • Steady State: The performance of an SSD is consistent under a specific workload.

Most SSD vendors provide benchmark results of the peak performance in FOB State. These benchmark results do not always reflect the expected performance in real applications. The performance deterioration of SSDs comes from the characteristics of NAND flash memory and SSD firmware.

NAND flash memory is structured as Figure 1 illustrates. A NAND cell can retain a data size of 1 bit, whereas users cannot write just 1 bit to a NAND cell. Due to the structure of NAND flash memory circuits, page sized writes are required (writing data to NAND cells is called “programming”). Erasing data must be done by block, each block comprising of several pages. Both erasing and programming operations are required to overwrite data with NAND flash. Such operations are time-consuming, which takes the form of the performance deterioration in SSDs, especially, for Steady State SSDs.

Figure 1: Structure of NAND Flash Memory

To transition an SSD from FOB to Steady State, the workload shown in Table 1 should be performed. Such a workload causes data overwriting where both erasing and programming operations are performed inside the SSD. After the workload finishes, the SSD will be in Steady State.

Table 1: Preconditioning Workload

Step Sub-workload Data Pattern Duration
1 Disk Pre-Fill Entropy 100% Write 2x capacity
2 4K Random Write Entropy 25% 10 minutes
3 4K Random Write Entropy 50% 10 minutes
4 4K Random Write Entropy 100% 10 minutes

We evaluated Fixstars SSD-3000M, Samsung SSD 850PRO, Samsung SSD PM863, Crucial MX200, Sandisk Extreme Pro SSD, Sandisk Ultra II SSD, and Intel SSD 520 to compare their steady state performance by using an OakGate SAS/SATA Desktop Systems (http://oakgatetech.com/products-top/sas-sata-top). Figure 2 depicts the sequential write throughput after performing Table 1’s preconditioning workload.

You must ask if you’re still enjoying the expected performance from your SSD.

The Fixstars SSD sustains more than 390 MB/s while many popular drives aren’t able to sustain even half of that. As you might notice, some of them have performance even low er than spinning HDDs!

Figure 2: Steady State (Dirty State) Performance Comparison

Article by: Masana Murase

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