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SSD Interfaces and Performance Effects


By Steve Leibson for Denali Software

IDC’s Research Director John Rydning and Micron’s Director of SSD Marketing Justin Sykes tackled the merging abilities of fast enterprise-class SSDs and evolving disk interface standards, particularly SATA 6G (also called SATA 6.0) and USB 3.0, while speaking on a panel about the technology of storage during the Storage Visions 2010 conference held early this year in Las Vegas. Rydning spoke first and he compared and contrasted two new external disk-interface standards, namely USB 3.0 and eSATA 6.0. These standard disk interfaces improve on their predecessors. USB 3.0 maximum data rates are 3.2 to 4.8 Gbps versus USB 2.0’s 480 Mbps--a 6.7x to 10x boost in theoretical I/O performance. SATA 6.0 and eSATA 6.0 essentially double the theoretical maximum data rate of SATA 3.0 and eSATA 3.0 from 3 Gbps to 6 GBps. Consequently the new SATA 6.0 and eSATA 6.0 interfaces are theoretically faster than the new USB 3.0 interface just as SATA 3.0 and eSATA 3.0 are faster than USB 2.0.

The SATA and USB standards seem to be in lock step with respect to adoption rates according to Rydning. He showed comparison graphs that forecast increasing adoption rates for both SATA/eSATA 6.0 and USB 3.0, with some minor amount of adoption in 2010 and about 50% market penetration for each interface by the year 2012.

To aid this transition, laptop makers have started to build eSATA interface ports into laptops. This is not a particularly difficult feat because most motherboard chipsets include several SATA ports so implementing an eSATA port for such a machine is a matter of adding an eSATA connector to the laptop motherboard. For desktop and enterprise-class server systems, adding an eSATA port requires little more than a SATA extension cable that connects the motherboard SATA connector to an eSATA connector mounted on a metal expansion-card bracket or a case bulkhead because SATA ports are plentiful on most desktop and server motherboards. Rydning also pointed out that officially, eSATA connectors supply no power to the external SATA drive but connector manufacturers have developed an “unofficial” hybrid eSATA/USB 2.0 connector that allows a properly designed cable to tap into the co-located USB port’s 5V power while simultaneously coupling the eSATA disk-interface signals to the external drive.

Sykes’ panel presentation corroborated Rydning’s and provided some important test data to reinforce some of Rydning’s points and to make new ones. First, Sykes presented a historical chart showing the uneven throughput progress for SCSI and ATA disk interfaces as they evolved into the SAS (serial attached SCSI) and SATA (serial ATA) interfaces.

SATA disk interface data rates over time

SCSI/ATA/SAS/SATA disk interface data rates over time (Micron Technology)

The graph shows that the SCSI disk interface led in throughput until both SAS and SATA interface standards hit 3 Gbps around 2005. With the development of a 6 Gbps standard in 2008, the SAS interface pulled ahead of the SATA interface and will remain in the lead even with the development of the new SATA 6.0 specification.

Sykes then showed a different sort of performance graph for an existing MLC (multi-level cell) SSD using SATA 3.0 and SATA 6.0 interfaces:

MLC SSD performance with SATA 3.0 and SATA 6.0 interfaces

MLC SSD performance with SATA 3.0 and SATA 6.0 interfaces

The graph shows that sequential reads for this particular SSD benefit greatly from the faster interface although the read speed does not double with a doubling of the interface transfer rate. This result indicates that the SATA 3.0 interface definitely limits this SSD’s read performance. Although the SSD’s random read performance benefits some from the faster disk interface, the SSD’s sequential and random write performance essentially gains nothing from SATA 6.0.

These figures could lead you to the wrong conclusion, so take care in your interpretation. What the above figures do show is that the drive being tested was designed and optimized for the SATA 3.0 interface. In other words, the number of NAND Flash channels implemented in the tested drive is sufficient to support the SATA 3.0 data rate. Slapping a faster interface on this existing SSD architecture doesn’t produce a substantally faster SSD. To fully exploit the faster performance abilities of the SATA 6.0 interface, SSDs need more internal NAND Flash channels to boost internal read/write parallelism. That’s what Sykes’ next graph depicted:

Boosting NAND Flash channels increases SSD performance to SATA 6.0 rates

Boosting NAND Flash channels increases SSD performance to SATA 6.0 rates
(Micron Technology)

Increasing the number of NAND Flash channels implemented in an SSD substantially increases the SSD’s read and write speeds (using either multi-level-cell or single-level-cell NAND Flash devices). In fact, the theoretical performance of SSDs that support 16 or 32 active NAND Flash channels greatly exceeds the bandwidth of 6-Gbps disk-interface standards, which means that the SAS and SATA disk-interface standards will need to evolve even further to keep pace with future SSD developments.


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