The Next Level of Performance in Storage Tiers

Sam Sawyer, Director of Product Marketing
Embedded Storage Products
Emulex Corporation

One of the hottest topics in storage is the adoption of solid state disks (SSDs) within external storage arrays. Over the last decade, advances in hard disk drive (HDD) capacity have far outpaced the random IOPS capability of HDDs, providing a catalyst for the increased adoption of SSDs. Inherent in the mechanical nature of HDDs is a latency that creates a significant bottleneck in the movement of data to and from the storage array. Common methods of improving the IOPS capability in storage systems include high-rotational speed HDDs and striping the data across additional disk drives, yet these approaches do not dramatically improve IOPS performance. Instead, they result in a dramatic increase in power consumption. SSD technology, by contrast, can vastly improve IOPS performance (by a factor of up to 1,000) while reducing total power consumption of a storage array.

SSDs and the External Storage Market
Adoption of SSD technology is a huge opportunity to the external storage market as SSDs are more than simply an extension of the HDD technology roadmap. They represent a whole new approach to storage technology by taking advantage of the commoditization in pricing for NAND Flash technology and moving the storage of data off spinning disks and onto Flash. This provides storage vendors with a much speedier alternative to traditional mechanical HDDs and alleviates the I/O bottleneck issue.

As with any technology, however, not all SSDs are created equal. Similar to creating tiered storage with traditional HDDs, SSDs have likewise been designed to be used for different applications, ranging from notebook applications to mission-critical transactional applications.

SSDs promise the ultra-high-performance capabilities required by today’s enterprise storage arrays, but as yet do not provide high capacity at low cost per Gigabyte (GB). Serial Advanced Technology Attachment (SATA) interface SSDs have recently emerged as lower cost alternatives to their Fibre Channel (FC) interface counterparts, but the SATA interface is not technically acceptable for use in the back-end of external storage systems, necessitating that all SATA SSDs be bridged to either the FC or Serial Attached SCSI (SAS) protocol.

Creating an Ultra-high-performance Storage Tier
As mentioned, the primary drivers for incorporating SSDs into external storage are alleviating the I/O bottlenecks and increasing energy efficiency. However, the variety of back-end storage protocol support needed (both FC and SAS) and the difficulty of meeting mission-critical storage requirements have proven to be very challenging for potential suppliers. One way of overcoming this challenge is to take advantage of the robust capabilities of an enterprise-class protocol bridge and incorporating it along with the SATA SSD devices. This can be accomplished with an external bridge interposer card, or by integrating the bridge directly into the SATA SSD. Figure 1 illustrates the former solution:


Figure 1. Converting SATA SSDs to SSP with SAS Bridge Interposer Card


There are challenges in doing this, however, not only because of the possible different protocols, but also because of differences in the physical connector between the types of SSDs. Table 1 provides a breakdown of these differences, along with typical applications in which each type would commonly be used:


Table 1. SSD Common Usage in External Storage Arrays


* While not prevalent in current data centers, SAS-based storage array back-ends are expected to start making inroads into data centers around the end of this year (2009), providing an alternative to mission-critical FC-based back-ends.

As shown in Table 1, there are differences in the type of drive connectors and availability for each of the three types of SSDs, which can be a challenge for storage vendors wanting to deploy SSDs in storage arrays. One is the addition of a drive interposer card. The protocol bridge on the interposer card provides several key benefits, including the intelligence to convert the SATA protocol into FC or SAS, the physical and operational conversion of the SSDs’ SATA drive connector to FC or SAS drive connector and dual-ports for path failover or multiple initiator purposes.

In adding a drive interposer card or integration of the bridge onto the drive itself, a SATA SSD is presented to the storage array’s controller as if it were an ultra-high-performance FC or SAS drive. Additionally, SATA SSDs from multiple vendors can be mixed and matched in the same storage array, because the protocol bridge provides an abstraction layer that can be customized to make any drive look and act the same to the storage array controller, regardless of the manufacturer.

SSDs represent a whole new era for storage arrays, but it is not necessary to start from scratch when creating a cost-effective and ultra-high-performance storage tier. As organizations struggle with finding a balance between increased performance and increased capacity, the solution is to use SATA SSDs with an enterprise-ready protocol bridge to move to the next level of performance in storage tiers. Through the simple addition of a drive interposer card or the integration of the protocol bridge into the SSD device, storage vendors can now mix and match ultra-high-performance SSDs with high-capacity HDDs, greatly increasing the value and functionality of their existing storage arrays.

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