The Enclosure Services Interface (ESI) is a computer protocol used in

SCSI Small Computer System Interface (SCSI, ) 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 interface ...

enclosures. This is part of a chain of connections that allows a host computer to communicate with the enclosure to access its power, cooling, and other non-data characteristics. This overall approach is called SCSI attached enclosure services: The host computer communicates with the disks in the enclosure via a Serial SCSI interface (which may be either

FC-AL The arbitrated loop, also known as FC-AL, is a Fibre Channel topology in which devices are connected in a one-way loop fashion in a ring topology. Historically it was a lower-cost alternative to a fabric topology. It allowed connection of ma ...

or SAS). One of the disk devices located in the enclosure is set up to allow

SCSI Enclosure Services SCSI Enclosure Services (SES) is a protocol for more modern SCSI enclosure products. An initiator can communicate with the enclosure using a specialized set of SCSI commands to access power, cooling, and other non-data characteristics. SES device ...

(SES) communication through a LUN. The disk-drive then communicates with the SES processor in the enclosure via ESI. The data sent over the ESI interface is simply the contents of a

SCSI command In SCSI computer storage, computers and storage devices use a client-server model of communication. The computer is a client which requests the storage device to perform a service, e.g., to read or write data. The SCSI command architecture was or ...

and the response to that command. In fault-tolerant enclosures, more than one disk-drive slot has ESI enabled to allow SES communications to continue even after the failure of any of the disk-drives.

ESI electrical interface

The ESI interface was designed to make use of the seven existing "SEL_n" address signals which are used at power-on time for establishing the address (

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) of a disk-drive. An extra eighth signal called "-PARALLEL ESI" is used to switch the function of the SEL_n signals.

ESI command sequence

A SCSI Send Diagnostic command or Receive Diagnostic Results command is sent from the host computer to the disk-drive to initiate an SES transfer. The Disk-drive then asserts "-PARALLEL ESI" to begin this sequence of ESI bus phases: {, class="wikitable" , , , Phase, , Function , - , , , Discovery phase, , Disk-drive tests that the enclosure is SFF-8067 compliant , - , , , Command phase, , Disk-drive sends the SCSI CDB to the enclosure (similar to the write phase) , - , Either, , Read phase, , Disk-drive sends diagnostic page data to the enclosure , - , or, , Write phase, , Disk-drive receives diagnostic page data from the enclosure Finally, the disk-drive deasserts "-PARALLEL ESI". The above sequence is just a simple implementation of a 4-bit wide parallel interface which is used to execute a SCSI transaction. If the CDB is for a Send Diagnostic command then the data is sent to a SCSI diagnostic page in the enclosure. If the CDB is for a SCSI Receive Diagnostic Results command then the data is received from a SCSI diagnostic page. No other CDB types are allowed.

Alternatives to ESI

There are two common alternatives ESI: * Standalone Enclosure Services uses a direct connection which does not require ESI * SSA enclosures use an interface called DSI which is similar to ESI

Specifications

The definition of the ESI protocols is owned by an

ANSI The American National Standards Institute (ANSI ) is a private non-profit organization that oversees the development of voluntary consensus standards for products, services, processes, systems, and personnel in the United States. The organi ...

committee and defined in their specifications ANSI SFF-8067 and ANSI SFF-8045. SCSI