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SAS Features And Basics

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

SAS Fundamentals

Unlike SATA, SAS works on a full-duplex basis and provides full bandwidth in both directions. As mentioned earlier, SAS connections are always established through physical links using unique device addresses. In contrast, SATA can only address port numbers.

Each SAS address may consist of multiple PHYs to create wide links on InfiniBand (SFF-8470) or mini-SAS cables (SFF-8087 and -8088). Typically, four SAS links using one PHY each are routed through a wide SAS link to connect to SAS devices. Links can also flow through expanders, which act more like switches than SAS devices.

Features such as zoning now allow admins to assign specific SAS devices to initiators. This is where the increased SAS 6 6 Gb/s speed kicks in, as the four-way wide connection now offer twice the bandwidth. Finally, SAS devices may even have multiple SAS addresses. Since SAS drives can be dual-ported, with one PHY on each port, one drive can support two SAS address.

Links, Connectors, and Connections

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

SAS connections are addressed to SAS ports using SSP (Serial SCSI Protocol), but established from PHY to PHY using one or multiple links for bandwidth reasons. SAS uses 8/10-bit encoding to convert 8-bit of data into 10-bit characters for clock recovery, DC balance, and error detection. This results in an effective gross bandwidth of 300 MB/s for the 3 Gb/s transfer mode and 600 MB/s for 6 Gb/s connections. Fibre Channel, Gigabit Ethernet, FireWire, and other storage device technologies work on the same encoding scheme.

SAS and SATA power and data connectors are largely the same. While SAS merges power and data into one physical connector (SFF-8482 on the device side), SATA requires two separate cables. The notch between the data and the power pins (see image above) is filled in the case of SAS, preventing a SAS device from being connected to a SATA controller.

On the other hand, SATA devices can operate very well on SAS hardware through STP or natively, if no expander hardware is used. STP introduces extra latency, as dwords flow through expanders. The expander has to establish the connection, which is a slower process than SATA’s direct communication. Still, the latency is acceptably low.

Domains, Expanders

SAS domains can be compared to tree-like structures similar to what you may know from Ethernet networks. SAS expanders can manage a large number of SAS devices, but they work based on link switching rather than the more common packet switching. Some expanders include SAS devices while others don’t.

SAS 1.1 recognizes edge expanders, which allow a SAS initiator to communicate with up to 128 additional SAS addresses. Only two edge expanders can be used in a SAS 1.1 domain. However, one fanout expander can connect up to 128 edge expanders, which dramatically increases the possible structure of your SAS solution.

Source: SCSI-TA and HP.Source: SCSI-TA and HP.

Compared to SATA, SAS may seem complex, with different initiators accessing target devices through expanders, which take care of the appropriate routing. SAS 2.0 simplifies and improves how this routing is accomplished.

It is important to know that SAS prohibits loops or multiple paths. All connections have to be point-to-point and exclusive, but the link architecture itself is very scaleable.

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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 (,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 ( it shows 3 Gb/sec. However on the list of device speeds Wiki site ( 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?

  • 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.