SCSI Trends and Future Features

    If you want to know what innovations await the SCSI device market in the not too distant future, and you have not heard anything about SAS-3, SCSI Express (SCSI over PCIe) or Multilink SAS, then you are the very person who reads this article to the end !

    SAS 12 Gb / s

    We start with the relatively old, but nonetheless relevant “roadmap” for developing the SAS protocol from the SCSI Trade Association.

    First of all, she tells us that this year there will be products using SAS-3 technology (12 Gb / s) that will be able to use the capabilities of the PCI Express 3.0 interface. The new generation of devices is backward compatible with SAS-2 (6 Gb / s) and SAS (3 Gb / s). For SAS-3, the SFF 8680 connector is used. It looks no different from SFF 8482, which was used in SAS-2. He uses all the same 29 pins, which are located in the same way (3 sets of contacts: 7 + 7 + 15).

    However, this year it is not worth waiting for manufacturers of SSD and HDD to produce SAS-3 disks, the new interface will first appear on RAID controllers and expanders. In particular, LSI said that its devices will support the multiplexing of two streams of 6 Gbit / s to one 12 Gbit / s and called it the sonorous word DataBolt . Adaptec by PMC already has SAS expanders (24 to 68 ports) and SAS-3 controllers in its product line. At least all major manufacturers to one degree or another are already available for sale or SAS-3 devices are about to start selling.

    Multilink SAS

    Also in the very near future we will see a new type of slot - Multilink SAS. This standard expands the SAS slot to four ports (and thereby doubles the bandwidth). Over the usual SAS-3, it has the following advantages:
    • Greater bandwidth. For a Multilink SAS device inserted into the Multilink slot, we can achieve a throughput of 96 Gb / s (4 ports * 12 Gb / s SAS in full duplex mode).
    • Devices connected to such a slot will receive more power - up to 25 watts. Together with the huge bandwidth, this will allow you to create SSD disks consisting of a very large number of memory modules. In such a slot, they will be able to reveal their full potential.
    • Backward compatible with all generations of SATA and SAS.

    Multilink SAS uses the new SFF 8630 (43 pin) standard connector. It looks like a SAS-2 and SAS-3 connector, but has a group of 14 additional contacts (shown in gray in the figure).

    SCSI Express

    In the first half of 2014, the first devices using SCSI Express technology may appear. The concept is very simple - the SCSI device will connect directly to the PCI Express bus bypassing the SCSI controller. To do this, both (initiator and target) must support SOP (SCSI over PCIe). In the first version of the standard, the device will use up to 4 PCIe bus channels. It makes sense to use this technology, for example, to connect devices that will be used to cache "hot data". SCSI Express will have to provide an extremely low latency when accessing a storage device. SCSI Express will receive its own slot (SFF-8639). The slot will support hot add / remove devices and will also be compatible with all SATA and SAS devices.

    SCSI innovations

    The SCSI protocol itself is also actively expanding. Of the interesting ones, one can note, for example, a set of instructions for implementing atomic writes (atomic writes), which will allow combining a group of SCSI instructions into transactions at the SCSI protocol level. In this case, the storage device must either execute all the commands combined in one transaction, or if it is impossible to execute all the commands, return to the original state. This feature should increase performance when using journaling file systems and databases.

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