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Next-Generation SAS: 6 Gb/s Storage Hits The Enterprise

Next-Generation SAS: 6 Gb/s Storage Hits The Enterprise
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For years, Serial Attached SCSI (SAS) has been gradually replacing the aging Small Computer System Interface (SCSI) in the business and enterprise storage segments. While SCSI was a parallel bus technology with many limitations, SAS is characterized as a serial interconnect architecture, allowing businesses to manage complex storage solutions in a very scalable and flexible fashion. Second-generation SAS now makes the architecture even better, starting by doubling the link speed from 3 Gb/s to 6 Gb/s.

Source: SCSI-TA.Source: SCSI-TA.

Why SAS?

Serial Attached SCSI is not just a serial implementation of the SCSI protocol. There’s more to it than simply carrying forward SCSI features, such as TCQ (Tagged Command Queuing), across a new connector. If simplicity were the name of the game in storage, we’d all be using Serial ATA (SATA), which employs a basic point-to-point connection between a host and an end device, such as a hard drive.

However, SAS is based on an object model that defines a “SAS domain,” a delivery subsystem that includes optional expanders and SAS end devices, such as hard drives and host bus adapters (HBAs). Unlike SATA, SAS devices may have multiple ports, each of which can utilize multiple physical connections to enable faster (wider) SAS links. In addition, multiple initiators may access any given target, and cable length can be up to eight meters (for the first generation of SAS) instead of SATA’s one meter. Obviously, this enables a plethora of options for creating redundant or high-performance storage solutions. In addition, SAS implements SATA Tunneling Protocol (STP), allowing you to run SATA drives from a SAS controller.

SAS 2.0

Second-gen SAS is supposed to speed up connections from 3 to 6 Gb/s. This speed boost is very important in complex environments, where applications require high-bandwidth links to high-performance storage appliances. The new SAS version also aims at reducing the complexity of cabling and the number of connections per Gb/s of bandwidth by increasing available cable length and improving expander zoning/self-discovery. We’ll talk about these changes in detail.

Ramping Up 6 Gb/s SAS

In an effort to promote the benefits of SAS to a broad audience, the SCSI Trade Association (SCSI TA) presented a turorial on SAS technology at the Storage Networking World Conference earlier this year in Orlando, FL. A so-called SAS Plugfest, which demonstrated SAS 6 Gb/s interoperability, compatibility, and functionality, had preceded in November of 2008. LSI and Seagate are the first to market with SAS 6 Gb/s-compatible hardware, but others will follow soon. This article provides an overview on the state of the SAS union and introduces some of the first devices. For more on what we've seen SAS 6 Gb/s do already performance-wise, check out our efforts to break a storage throughput record with the technology.

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  • 0 Hide
    bunz_of_steel , August 31, 2009 1:17 PM
    Now I like this, can't wait to see it implemented on EMC & IBM platforms. And on the servers side I'd like to see some numbers on spec's and performance. Good article Patrick Schmid and Achim Roos! Next.... benchmark 2.5 vs 3.5 on enterprise systems and low budget small business ....yes???
  • 0 Hide
    Shadow703793 , August 31, 2009 3:32 PM
    SCSI is still not dead?!?!? :lol: 

    Joking aside, I wonder if any motherboard manufacture like ASUS will get this on their high end WS models (see P6T WS). And I wonder if they will make any SSDs using this for the server sector.
  • 0 Hide
    kittle , August 31, 2009 8:45 PM
    Shadow703793SCSI is still not dead?!?!? citation]
    no its not -- its a long death cause most scsi drives run forever. I have a bunch of scsi drives running on my systems at home and as much as Id like the performance boost of SAS or even Sata, the things just wont die.

    but 6g/b SAS looks to be the next upgrade step
  • -5 Hide
    falchard , August 31, 2009 10:07 PM
    6Gb for SAS is kind of pointless considering SAS drives only peak 100MB
  • 0 Hide
    rhodenator , September 1, 2009 7:11 PM
    On this page (http://www.tomshardware.com/reviews/sas-6gb-hdd,2392-8.html), I see that you stated "SAS 1.1 at 3 Gb/s (300 MB/s)"

    Well this has always confused me. If SAS 1.1 is 3 Gb/s, that would be 375 MB/sec (3 x 1000 to convert to 3000 Mb/sec and then 3000 \ 8 to convert to 375 MB/sec.)

    On the SAS Wiki site (http://en.wikipedia.org/wiki/Serial_Attached_SCSI) it shows 3 Gb/sec. However on the list of device speeds Wiki site (http://en.wikipedia.org/wiki/List_of_device_bandwidths) it shows 2.4 Gb/sec with a 300MB/sec. The only maximum that appears to stay the same is MB, which is 300. So is it really 2.4 Gb/sec or is it 3.0 Gb/sec really for SAS 1.1?

    Thanks!
  • 0 Hide
    jrst , September 2, 2009 4:12 AM
    @kittle -- Not to mention that SCSI is a protocol, not just a physical/electrical interface. SAS simply changes the physical/electrical interface, but the good old SCSI protocol is still in there (just like SATA still has ATA underneath).

    @falchard -- No, it's not pointless. A dedicated controller channel for every drive would be prohibitively expensive for large drive arrays (not to mention the cabling nightmare), which is why SAS (unlike SATA) allows for more complex topologies, not just point-to-point. When you put multiple drives/expanders on a channel, you can quickly hit SAS bus speed limits.

    @rhodenator -- SAS (like SATA) uses 8b/10b encoding. That is, 8 data bits end up as 10 bits on the wire (typical of high speed serial buses), so: 3.0Gbs wire * 8/10 encoding = 2.4Gbs data = 300MBs data.
  • 0 Hide
    JohnnyLucky , September 2, 2009 4:35 PM
    I bet the hardcore gamers are drooling at the prospect of SAS 6Gb/sec. Too bad they'll have to wait.
  • 0 Hide
    Anonymous , September 3, 2009 1:26 PM
    Actually, LSI was not 1st to market w/ 6g controllers. ATTO Tech was shipping their H6xx 6G HBAs months ago based on the PMC Sierra chip. Also HP has been shipping their 6G RAID controllers - P212 & P410 (also PMC Sierra chip) for a couple of months also. Granted LSI is the industry leader w/ SAS but has stumbled in execution of releasing 6G product.
  • 0 Hide
    s_a_r99 , September 3, 2009 1:59 PM
    Actually, LSI was not 1st to market w/ 6G SAS controllers. That distinction goes to ATTO Tech with their H6xx 6G HBA line up and HP for their 6G RAID controllers - P212 & P410. All of these are based on the PMC Sierra chipset. LSI has been stumbling to get 6G products out the door.
  • 0 Hide
    tygrus , September 7, 2009 4:28 AM
    re: rhodenator. As mentioned in the article; 8b/10b encoding means that 8bits data is encoded using 10bits accross the link. 3000Mb/10b = 300MB. Command and control overhead and processing delay further limits real world performance to about 270MB/s. I think someone managed over 580MB/s using the new 6Gb/s signalling and more data per transfer to reduce overhead (was that using SATA or SAS ?).


    re: falchard "..kind of pointless.." NO.
    "Build it and they will come". You have to start building the infrastructure and plan for the future otherwise it's pointless making the drives go faster. I want a server that in a few years time can take additional drives or replacements where performance boost is not nullified by slow IO bandwidth.

    It will take 12months after the standard's release to be more common as HBA, motherboard and drive designs are refreshed.

    SATA is no longer a 1-to-1 link and burst bandwidth is close to current limit.
    600MB/s means enough bandwidth to aggregate 4 of today's SAS HD per SAS port or maybe 2 SSD's. Within 2 years a single HD will exceed current limit and within 1 year SSD's will reach the new limit.