Enterprise SSD testing spec released

SNIA, which designed the spec to level the benchmark playing field, is also working on application-specific tests for solid-state drives

The Storage Networking Industry Association today announced the release of a specification that can be used to test the performance of solid-state drives regardless of the vendor.

SNIA, an industry trade group of vendors and universities that develops and promotes standards for storage systems, said its Solid State Storage Initiative (SSSI) came up with the SSD Performance Test Specification to level the playing field in benchmark testing.

The SSSI is releasing two versions of the test specification: one this week for enterprise SSDs, and another for server or client-side SSDs, which will be released in the third quarter of this year.

The Enterprise Performance Test Specification defines a set of device-level tests and methodologies intended to enable comparative testing of SSD devices in enterprise systems, such as storage arrays.

Previously, there has been no widely accepted test methodology for measuring SSD device performance. Each SSD manufacturer used different measurement methodologies to derive performance specs for their products.

"You couldn't compare one data sheet to another data sheet and expect to understand if one drive was faster than another, because the manufactures used different metrics," said Paul Wassenberg, chairman of the SSSI Governing Board.

"Today, the SSD market is where the HDD market was in the 1970s. There are a lot of different suppliers offering products with a lot of different abilities, and there's a lot of variability," he added.

More than 40 companies spent two years developing the Performance Test Specification (PTS), Wassenberg said. Among those companies were all of the major SSD and storage system manufacturers, including Samsung, Intel, Marvell, Toshiba, IBM, Seagate, Dell, EMC, Hitachi Data Systems, and Western Digital.

Jim Handy, an analyst at market research firm Objective Analysis who was in the specification's technical working group, said, "The SNIA test specification is not an end-all, but it is certainly a big step ahead of the specifications that are commonly used by SSD makers."

Handy said one of the most important aspects of the specification is that it takes care to ensure that SSDs are first "pre-conditioned" prior to testing, meaning data is first written to them and then erased to break the drives in.

All SSDs slow down after initial use because once a sufficient amount of data has been written to them, the processor in the drive begins to move data around in a process known as the read-modify-erase-write cycle.

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.

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