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TST

Serial Attached SCSI - New Interface, New Storage Rack?

by Sio Fu , Founder, TST - (September 2005)
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Editor's intro:- Users will need more than just host bus adapters and disk drives to deploy the new Serial Attached SCSI technology. But the traditional way of designing the backplanes in storage racks could lead to high cost and not use the expansion and high availability aspects of SAS to best advantage. In this article one of the world's leading suppliers of computer chassis describes their award winning new backplane concept which gets the best out of the new SAS technology while reducing costs.
Serial Attached SCSI - New Interface, New Storage Rack?

In today's economy, delivering high-density, scalable and reliable storage solutions to market quickly is a necessary for storage solution designer, OEMs, system integrators so they can keep the competitive edge they need for continued success. Serial Attached SCSI has defined a device called an expander, allowing thousand combinations of storage expansion to achieve the needs for IT professional on storage availability, flexibility, scalability, and performance. However, with the conventional method, this requires IT engineers to create many more complex storage backplanes that may be dedicated to only one single solution.

The limitation on conventional method backplane board

The predominant high availability physical interconnect technology between the hot swap hard disk drives and storage host bus adapters reply on transmission of data streams through a piece of physical PCB board (Backplane Board). In Serial Attached SCSI systems the SAS expander is located on the backplane as shown in fig 1 below.
Serial Attached SCSI
Serial Attached SCSI on
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Megabyte found it difficult
adapting to the newer thinner
Serial SCSI connections.
SAS backplane - fig 1
With the conventional method, all expanders are embedded on the backplane board. But because of the size of the expander foorprint and/or other IC components this can increase the form factor IT engineers have to carefully balance the trade-offs between reliability, scalability, performance and availability of the application requirements. This flexibility limitation on the conventional backplane design has forced designers to implement separate systems for each type of solution and bear the costs of additional backplane design, addition layers, prototypes, troubleshoot, manufacture and support. Failure of individual components in a backplane means a complete replacement of a new backplane causing single points of failure that can block access to the system, This results in high support cost and increased total cost of ownership.

The conventional method of backplane design (shown in fig 2 below) forces designers to use more than 20 PCB layers, generates unnecessary signal skew, crosstalk and DC interference, blocks airflow, restricts failed over and device addressability as well as configuration flexibility and stands as a barrier to throughput performance, storage scalability and system flexibility and availability.
Universal Solid State Disk USSD 200 from Solid Access Technologies with SAS, FC, SCSI or custom interfaces
Serial Attached SCSI solid state disks
from Solid Access Technologies
fig 2 - traditional storage backplane
Are MLC SSDs Safe in Enterprise Apps?
This is a follow up article to the popular SSD Myths and Legends which, a year earlier demolished the myth that flash memory wear-out (a comfort blanket beloved by many RAM SSD makers) precluded the use of flash in heavy duty datacenters.

This new article looks at the risks posed by MLC Nand Flash SSDs which have recently hatched from their breeeding ground as chip modules in cellphones and morphed into hard disk form factors.
which technology to choose? - read the article It starts down a familiar lane but an unexpected technology twist takes you to a startling new world of possibilities. ...read the article
The new backplane and daughter board

As the conventional backplane design method shown above has many shortcomings for implementing SAS systems IT engineers require new solutions that bring new levels of ease and simplicity.

The most important characteristic of the new method developed by TST (shown in fig 3 below) is its flexibility to support every storage solution available in the storage world. This includes two physical parts: the Drive Backplane Board and Expander Daughter Board. These two boards are connected with easy-swap high-speed connectors and enables highly flexible storage topologies. The backplane boards can support swappable daughter boards which are interchangeable within the same enclosure and can be used to build high-availability systems with no single points of failure. This method use the same backplane board for multiple solutions, just by changing the Daughter board with the expansion capability you require, will provide for a pay-as-you-grow platform so customers can migrate to their unique solutions as needed. Because every solution uses the same backplane, cost reduction then can achieved, In addition, it offers competitive advantage in the marketplace by meeting compliance deadlines, lowering the cost of building (as opposed conventional backplanes) and offering reliable, user-friendly products to their end-customers ahead of any other competitor in the marketplace.
Enlight's storage rack interconnect - fig 3
The new connection scheme provides many benefits:-
  • Backplane Manufacturer and Designer - Using the conventional method, backplane designers and manufacturers need to develop many backplanes for each solution, the emergence of SAS and the new backplane design enables manufacturers to develop only one backplane. Vendors can use the same Stock Keeping Unit (SKU) to support many types of storage application, resulting in a much less expensive board.
  • OEMs - The Emergence of SAS and this backplane solution means that OEMs can now sell standard backplanes and easily upgrade daughter boards. The use of this flexible solution shortens design time, simplifies inventory management, result in faster design, less validation efforts, reduces costs while maintaining margins and profitability
  • VARs and System Integrators - Serial Attached SCSI and the new backplane technology enables VARs and SIs to save time on integrating custom solutions, simply by changing the preferred daughter board. VARs no longer need to worry about stocking or integrating a wrong solution. Instead, they can simply populate the backplanes with the desired daughter board. Overall benefits include reduced inventory costs, easier product differentiation, simplified training, support, and reduced cost of ownership.
  • End users - benefit from the cost reductions from backplane manufacturers, OEMs and VARs, plus the ability to change storage solutions without purchasing new chassis systems simplifies the upgrade process and helps future-proof end-user investments.
Serial Attached SCSI will offer a new level of performance and data availability. The new backplane empowers customers with great flexibility. IT designers and users will be able to quickly and easily design storage systems where all these elements can be used without additional system and support costs. ...TST profile
Can You Trust Your Flash SSD's Specs?
Editor:- I've noticed is that the published specs of flash SSDs change a lot -from the time a product they are first announced, then when they're being sampled, and later again when they are in volume production.

Sometimes the headline numbers get better, sometimes they get worse. There are many good reasons for this.

The product which you carefully qualified may not be identical to the one that's going into your production line for a variety of reasons... ...read the article

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