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Talos C Series - from OCZ

Serial Attached SCSI - is it worth the wait?

by S R Prasana Kumhar - Head of Products at Xserve India Pvt Ltd - March 2006
Editor's comments:- Here at we've had a magazine page dedicated to SAS market news, articles and vendors since 2001. SAS products have been shipping to users since 2005 - but the market has been stuck in a prolonged "innovator" phase. As the SAS market is poised to advance to the "early adopter" phase - I thought it would be good to have an article (which is less technical than those we've previously published) to introduce the benefits of this technology to more readers. This article is published here with the kind permission of InfoStore - Asia's leading storage magazine. (...Later note - like many other storage publications - InfoStore no longer exists.)

see also:- Serial Attached SCSI - Delivering Flexibility to the Data Center, and the Benefits of SAS (Serial Attached SCSI) for External Subsystems

Serial Attached SCSI
Serial Attached SCSI on
Megabyte found it difficult
adapting to the newer thinner
Serial SCSI connections.
Serial Attached SCSI - is it worth the wait?
Factors such as larger capacity, density, security, scalability and accessibility, brought forward demand for storage devices that were more affordable than Fibre-channel and more robust and reliable than SATA. The answer comes in the form of Serial Attached SCSI, which will have the capability of fulfilling these requirements as well as providing the necessary performance and scalability to move data at gigabit speeds.

Nature has played its role in pointing the right direction for data flow. It has taken decades for our technology innovators to realize that serial is the way to go, when it comes to accessing storage devices. Data movement within the system or outside the system has been the target of various parallel technologies only to succumb to the serial phenomenon. The most recent one to show how this makes a difference is the death of printers and associated devices that worked on parallel technology. They simply could not keep up with the demands of data movement.

The current trends

Most of the industry watchers and IT users have been witness to the phenomenon of Serial Attached ATA popularly known as SATA drives. These devices have, in perhaps the shortest span of time, captured a significant market share, eating into various other technology products namely PATA, SCSI and for that matter even FC.
Let us briefly understand why this happened.

Was it because the technology based on this technology performed well? Was it the demand for faster data movement that made them the success they are today? Delving deeper into these questions, will perhaps answer why this phenomenon has made the industry look for more such offerings. Was it because the technology was good? The answer is an emphatic yes!

Let us first understand why it was good and it will answer why the end result was good.

Data that is transferred serially moves one bit at a time in linear succession through a fast single path. While, data transferred in parallel mode consists of many data bits moving together simultaneously on a shared pathway. The primary differentiator between serial and parallel architectures is the timing and throughput.

In serial technology the clock is embedded in the data. This results in data constantly moving through the system from one point to another using the bandwidth to the fullest extent.

Meanwhile in parallel architectures the clock moves data at a slower pace to take into account data skew. This skew is due to the fact that the data bits which are theoretically transmitted all at the same time do not actually start or arrive together due to variations in the signal path length, variations in gating and buffer delays and variations in signal quality. The only way to guarantee data integrity is to wait for the slowest signals to settle. This results in intermittent periods of inactivity and much slower frequency throughputs on parallel than serial lines.

Once the first answer was an emphatic "yes" the rest followed suit. The serial drives were commercially cost effective (serial interfaces take up less board space, use less cable cores and result in lower cost logic, cables and connectors than parallel interfaces). The new Serial ATA drives also performed well raising the bar on data transfers and reliability to the level of SCSI and FC technologies. SATA II drives that deliver data transfer rates of up to 3 Gbps have become mainstream products, with drives of capacities 400GB plus available; something that was unimaginable a couple of years ago. All these factors lead to the innovations that incorporated this technology at the drive interface, with conversion to host interfaces of U320 and FC to ensure the end users benefit from the technology, which in turn has lead to the invasion of serial technology based ATA drives into the realms of datacenters, inconceivable couple of years ago.
Does serial attached SCSI make sense?

Any layman after seeing the serial phenomenon would agree it makes common sense!

Earlier in the article I mentioned nature, why would one think of nature, when we talk technology here? Well what ever we are, we have a lot to learn from nature. Nature has simply demonstrated that serial is the way to go, be it waves in the ocean, sound waves, electrons in electricity or perhaps even the way we eat, we eat serially-has anybody observed this? Well coming back to whether Serial Attached SCSI makes sense? Why would anybody look at SCSI as one of the stalwarts of data communication technologies to change itself? Well serial technology has ensured that SCSI too changed to meet the evolutionary need of data movement and storage.

Information has become recognized as an important asset of companies, governments and even individuals. The ubiquity of the Internet and the proliferation of connectivity technologies have increased demands to harvest information for every need. This in turn has placed huge and uncompromising demands on how enterprise data is moved, stored and retrieved. This unprecedented demand on storage and retrieval has also meant that implementations of data storage subsystems at datacenters should allow for constant growth and expansion and be maintainable while in operation.
SAS disk duplicators for  server oems from ICS
Serial Attached SCSI - disk duplicators
from Intelligent Computer Solutions
These factors brought forward demand for storage devices that were more affordable than Fiber Channel and more robust and reliable than SATA. The answer was in the form of Serial Attached SCSI (SAS) which will have the capability of fulfilling these requirements as well as providing the necessary performance and scalability to move data at gigabit speeds-speeds that meet or exceed current storage I/O performance found in ATA, SATA, SCSI or Fiber Channel systems.

What about business sense?

Until now we have explored what the SAS (Serial Attached SCSI) hype is all about and what has driven vendors, users and storage administrators, the CIOs and definitely the CFOs to look forward towards a technology that promises to address the demands posed by data on move, in storage and on demand. The most important question that one should ask is "does SAS make Business Sense?" Let us see whether the technology and what it promises to do, makes business sense and would perhaps tell whether it was worth waiting for it to get into mainstream.

With the amalgamation of SCSI and select features of SATA and Fiber Channel technologies, SAS is evolving into a storage solution superior and more flexible than any other existing technology. These factors ensure that SAS will be deployed in storage environment and is likely to be deployed in other associated computing environments like servers, workstations, etc, too. SAS's ability to scale beyond the known boundaries of SCSI of 15 devices to envisaged 4,032 devices per port promises unprecedented opportunities of deploying in commercially and technically viable implementations.
One of the key deciding factors for success of any technology is the backward compatibility to legacy systems, technologies that have not been able to do so without enough marketing support and hype have died natural deaths.

The advantage SAS brings to the table is the support for legacy SCSI software support with the interface ensuring that a cost effective migration path is possible. The other key factor is the development at the initial stages of standards evolution of various associated protocols to enhance SAS's chances of deployment:-
  • the (Serial SCSI Protocol) SSP used to leverage the existing SCSI software
  • the (Serial Management Protocol) SMP to help manage the new point-to-point connectivity topology; and
  • the (SATA Tunneling Protocol) STP to allow for seamless interconnectivity and support for SATA devices.
This multi-protocol support, associated with SATA compatible signaling and a physical mating scheme capable of directly connecting SATA drives, offers a powerful reason for deploying storage systems that earlier were impossible. This unique advantage provided by design to ensure a compatible connection scheme, enclosure and infrastructure with SATA

Serial Attached SCSI provides exceptional flexibility to customers when it comes to choosing the right class of storage to meet their requirements. Since both the technologies have similar electrical interfaces, users have the choice of deploying cost-effective SATA drives for low-cost bulk storage or higher performance SCSI drives for mission-critical applications. A Serial Attached SCSI backplane will ensure that the end user as well as the solution provider has the choice of either drives, viz., SATA or SAS. The final application determines what type of drives needs to be used in a particular deployment to bring down the cost of the deployment.
SAS is likely to have a deep impact on how data is moved, stored and retrieved similar to the impact its predecessor (parallel SCSI) had on the storage industry in the late 1980s and early 1990s. SAS's serial point-to point connection provides the high performance that today's storage systems (and those of tomorrow) are likely to demand. The compatibility with SATA drives will ensure system integrators and end users have unparalleled choice of configuring their storage in ways that are most advantageous to a given application.

SAS will benefit the end user in terms of easy storage resource management and deployment. It also ensures that constant upgrades or replacements due to changes in storage technology are done away with. Further, it ensures that the storage systems deployed are more user-friendly in terms of interoperability, flexibility and scalability. Last but not the least, with its roots in the rugged SCSI technology with easy migration path, commonality in terms of connectivity and deployment with other storage technologies like ATA, SAS has the right pedigree to make business sense and well worth the wait.

The author - S R Prasana Kumhar is a member of SNIA India Technical Committee and Head of Products at Xserve India Pvt Ltd

Article republished with the kind permission of InfoStore - Asia's leading storage magazine.

See also:- Serial Attached SCSI, Storage portals & specialised information sources
the Problem with Write IOPS

the "play it again Sam" syndrome
Editor:- Flash SSD "random write IOPS" are now similar to "read IOPS" in many of the fastest SSDs.

So why are they such a poor predictor of application performance?

And why are users still buying RAM SSDs which cost 9x more than SLC? - even when the IOPS specs look similar.
the problem with flash SSD  write IOPS This article tells you why the specs got faster - but the applications didn't. And why competing SSDs with apparently identical benchmark results can perform completely differently. the article

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