Simply put, when an SSD is new, data can be written to it without interference from management software. However, once the drive has had a certain amount of data written to it, the NAND flash memory used to make SSDs requires that old data first be marked for deletion before new data is written to memory. Then, once the new data is written, the old blocks marked for deletion are actually deleted in a process known as "garbage collection."
SNIA has created a set of nomenclatures used to describe the life cycle of an SSD. A new SSD is called FOB, for "fresh out of the box."
After an SSD's initial use, it settles into a stage that SNIA terms the "steady state," which is when performance levels out and can be accurately measured. "In terms of performance, reads are fastest, writes [are] slower, and erases are the slowest yet," Wassenberg said.
Handy and Tom Coughlin, founder of consultancy Coughlin Associates, teamed up with Calypso to compile a study on SSD performance that involved 18 different drives.
"We found that there was no performance consistency between any two SSDs. They vary all over the map," Handy said, adding that some single-level cell (SLC) SSDs perform worse than less expensive multilevel cell (MLC) SSDs.
"And some MLC-based SSDs are slower than an enterprise hard-disk drives once they have entered their steady state," Handy said.
How long it takes an SSD to move into a steady state that's reliable for testing varies from product to product, but the new spec requires that an SSD go through five separate performance tests prior to its being benchmarked.
"The key thing with the PTS spec is it tells you what to do and how to prepare the drive. Is this the only way to test performance? No. But over time, we found it to be very efficient and the most dependable way. You can run this test multiple times and get the same result," Wassenberg said.
The PTS Test Sequence is as follows:
- Purge: Put SSD into a near-FOB condition, by erasing data.
- Workload Independent Precondition: Write prescribed data to entire SSD to facilitate reaching steady state.
- Workload Based Precondition: Run the Test loop itself until SSD is in steady state.
- Test: Take measurements when SSD is in steady state.
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