SES-2 Enclosure Management

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The introduction of Serial Attached SCSI (SAS) as the most recent evolution of SCSI required redefining the related standard for enclosure management, called SCSI Enclosure Services . SES-2, or SCSI Enclosure Management 2 first revision, was introduced in 2002 and is now at revision 20. SES-2 SCSI Enclosure Services (SES) permit the management and sense the state of power supplies, cooling devices, LED displays, indicators, individual drives, and other non-SCSI elements installed in an enclosure. SES2 alerts users about drive, temperature and fan failures with an audible alarm and a fan failure LED.

Serial Attached SCSI point-to-point serial protocol

In computing, Serial Attached SCSI (SAS) is a point-to-point serial protocol that moves data to and from computer-storage devices such as hard drives and tape drives. SAS replaces the older Parallel SCSI bus technology that first appeared in the mid-1980s. SAS, like its predecessor, uses the standard SCSI command set. SAS offers optional compatibility with Serial ATA (SATA), versions 2 and later. This allows the connection of SATA drives to most SAS backplanes or controllers. The reverse, connecting SAS drives to SATA backplanes, is not possible.

SCSI set of standards for physically connecting and transferring data between computers and peripheral devices

Small Computer System Interface is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, electrical, optical and logical interfaces. SCSI is most commonly used for hard disk drives and tape drives, but it can connect a wide range of other devices, including scanners and CD drives, although not all controllers can handle all devices. The SCSI standard defines command sets for specific peripheral device types; the presence of "unknown" as one of these types means that in theory it can be used as an interface to almost any device, but the standard is highly pragmatic and addressed toward commercial requirements.

Most recent SCSI enclosure products support a protocol called SCSI Enclosure Services (SES). The initiator can communicate with the enclosure using a specialized set of SCSI commands to access power, cooling, and other non-data characteristics.

Contents

SES-2 commands

The SES-2 command set uses the SCSI SEND DIAGNOSTIC and RECEIVE DIAGNOSTIC RESULTS commands to obtain configuration information for the enclosure and to set and sense standard bits for each element installed in the enclosure. The SEND DIAGNOSTIC command is used to send control information to internal or external LED indicators or to instruct one enclosure element to change its state or perform an operation.

The application client has two mechanisms for accessing the enclosure service process:

a) Directly to a standalone enclosure services process, for example an enclosure controller chip. SCSI conditions communicated directly include hard reset, logical unit Reset and I_T nexus loss.
b) Indirectly through a LUN of another peripheral device – for example a drive within the enclosure. The drive will communicate with the Enclosure through the Enclosure Services Interface. In this case the only SCSI device condition communicated through the LUN is hard reset.

Subenclosures

The SES-2 process handles a single primary subenclosure or multiple subenclosures. In the second case, one primary subenclosure will manage all the other secondary subenclosures.

Thresholds

Like SES, SES-2 establishes two types of thresholds for elements with limited sensing capability, like voltage, temperature, current etcetera: critical and warning. So for example in the case of temperature we may have:

When managed values fall within the warning range, the SES-2 processor will communicate a warning signal to the application client, typically a Host Bus Adapter (HBA). When values fall outside acceptable ranges, depending from the commands supported by the device server, the sense code shall be HARDWARE FAILURE or ENCLOSURE FAILURE.

Reporting methods

SES-2 lists four types of reporting methods:

  1. Polling
  2. Polling based on the limited completion function
  3. CHECK CONDITION status
  4. Asynchronous event notification

The standard

If you are a member of the T10 working group, the Standard, controlled by the T10 technical committee, can be found at:

http://www.t10.org/cgi-bin/ac.pl?t=f&f=ses2r19a.pdf

Due to INCITS policy changes the SCSI T10 drafts for released standards are no longer available online for non-T10 members and must be purchased from INCITS at http://www.incits.org . See the official INCITS policy at http://www.incits.org/rd1/INCITS_RD1.pdf .

Alternative technologies

SES-2 over I²C is still used for storage backplanes enclosure management, although a competing method for enclosure management communication is now becoming prominent. Serial GPIO (SGPIO), provides a simpler, less expensive solution and is now more widespread than SES-2.


Serial General Purpose Input/Output (SGPIO) is a four-signal bus used between a host bus adapter (HBA) and a backplane. Of the four signals, three are driven by the HBA and one by the backplane. Typically, the HBA is a storage controller located inside a server, desktop, rack or workstation computer that interfaces with hard disk drives (HDDs) to store and retrieve data. It is considered an extension of the general-purpose input/output (GPIO) concept.

Existing products using SES-2

American Megatrends’ Backplane controllers, the MG9071 and MG9072 can used either SES-2 or SGPIO for enclosure management with a simple configuration selection.

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References

    Old link : [broken] http://www.t10.org/ftp/t10/drafts/ses2/ses2r19a.pdf

    New link : [restricted] http://www.t10.org/cgi-bin/ac.pl?t=f&f=ses2r19a.pdf

    American Megatrends, Inc. Company Website AMI

    Backplane Controllers from AMI

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