High Availability SCSI Device Profile draft

Doug, dtn 237-2145 Flames to NL: 12-Sep-1995 1433 hagerman at starch.enet.dec.com
Tue Sep 12 11:36:57 PDT 1995


        From:    Doug Hagerman
        To:      X3T10
        Date:    8 September 1995
        Subject: SCSI-3 High Availability Device Profile

        Abstract: Proposal for a profile that describes requirements
        that apply to highly available systems based on the SCSI-3

        1  PURPOSE

        This profile describes the baseline SCSI requirements for
        building Highly Available storage subsystems based on SCSI bus

        Both parallel and serial versions of SCSI-3 are covered.
        These requirements are addressed from the electrical, SCSI
        adapter/system, software and SCSI device perspectives.  In
        particular this profile focuses on the requirements inherent
        in building SCSI multihost configurations.  Multi-host SCSI is
        a requirement for building High Availability subsystems
        because it allows recovery from a host failure.

        This profile is organized into two major sections.  The first
        deals with general multi-host and high availability issues,
        ignoring any requirements that are specific to a given SCSI
        bus implementation.  The second section deals with the
        specific requirements of parallel SCSI, SSA, and FC-AL
        implementations of the SCSI bus.


        In order to support highly available storage subsystems, the
        use of more than one host initiator on the SCSI bus is a
        requirement.  By allowing more than one host on the bus it
        becomes possible to move disk traffic from one host to
        another.  To allow this "host failover" to occur, the
        requirements outlined below must be implemented in the SCSI
        systems, adapters and target devices.

        Note that host failover is different from "controller
        failover", which describes the way that a controller (usually
        a RAID controller) failure is handled by a system.  Controller

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        failover is usually implemented inside a controller cabinet,
        although it may extend out onto the SCSI bus.

        There are a number of system issues that must be addressed to
        achieve a true high availbility system, including the failover
        of network traffic, shared access control, and management of
        the many issues related to system security, device naming, and
        access control.  However, multi-host SCSI is a basic component
        that is needed because the system must have multiple paths to
        the same device on the bus.

        Other configurations of storage interconnect topology may be
        used to achieve high availability.  One method is to use dual
        ported devices with the ports dedicated to different hosts,
        while several network-based approaches are possible.  However,
        multi-host access to the storage devices over a single bus is
        the configuration under discussion here.

        The basic requirement is that the SCSI devices must be capable
        of coexisting on a SCSI bus with multiple hosts present.

        This profile recognizes that the requirements for a host
        device are slightly different from those of a target device,
        and that diagnostic software has a slightly different set of
        requirements from the boot software or the run-time driver,
        but for the most part the requirements are common.  Separate
        sections are reserved to describe any specific requirements
        for special cases.

        3  DEFINITIONS

        The following terms are defined for use in this profile.

        3.1  High Availability SCSI Device

        A SCSI device which meets the requirements of this profile.

        3.2  High Availability SCSI Target

        A SCSI device which meets the requirements of this profile,
        particularly in regards to those requirements are specifically
        applicable to SCSI targets.

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        3.3  High Availability SCSI Initiator

        A SCSI device which meets the requirements of this profile,
        particularly in regards to those requirements are specifically
        applicable to SCSI initiators.


        This section describes the generic requirements for multi-host
        high availability SCSI systems.  These requirements apply
        equally to Fast-20, SSA, and FC-AL implementations of SCSI-3.

        4.1  Generic SCSI-3 Physical Requirements

        The basic physical requirement is that the SCSI bus should be
        capable of supporting multiple hosts.  This means that it
        should be possible to construct a bus configuration that
        includes multiple hosts, devices, subsystems or other
        components without the need to perform "objectionable" levels
        of hardware manipulation.  An example of an objectionable
        manipulation would be if a host required that the enclosure be
        opened to remove bus terminators before it could be used in
        the multi-host configuration.

        In most installations the SCSI bus for a multi-host
        configuration should be expected to be at least several meters
        in length.  (Clearly special enclosures with multiple hosts
        would require shorter minimum lengths.) In those situations
        where it is desired to maintain the hosts on separate power
        grids, the SCSI bus might need to be several kilometers in

        The installation should protect against SCSI bus
        configurations that are possible to construct with available
        hardware but that do not operate satisfactorily because of
        electrical or other reasons.  It is desireable that the system
        have a means of performing an automatic verification of the
        configuration.  This means that one should not construct a
        Fast-20 single ended system with a 20 meter bus length, and
        that the system should detect it if it happens.

        The SCSI bus should support the "plugging" (removal or
        insertion of a device on the SCSI bus) of devices or hosts
        without disturbing the other devices on the bus.  This means
        that the cabling should regain full continuity after the
        plugging operation is complete.  This may require the use of
        "Y" cables for Fast-20 SCSI-3 or "Loop Resiliancy Circuits"

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        for FC-AL or "blank device enclosures" for.

        4.2  Hot Vs Warm Vs Cold Plug

        Depending on the level of plugging support desired, the SCSI
        bus should maintain continuity for all or most of the period
        during which devices are plugged.  The levels are defined and
        discussed below.

        There is no standard terminology to describe device plugging.
        "To plug" means either to remove or to insert a device,
        including the removal or connection of any mechanical
        interlocks or retainers, and the disconnection or connection
        of the electrical bus interface to the device.  In 1990 IBM
        proposed the term "live plugging" for the case that is now
        commonly called "hot plugging".  Other possibilities
        identified at that time are listed here for reference.

        0  No power applied anywhere during the
            device plugging operation.
        1  Concurrent maintenance: device power off,
            others on, but bus idle.
        2  Hot plugging: device power on, others on,
            bus idle.
        3  Live plugging: device power on, bus in use
            by other devices.
        4  Yanking: bus in use by device being plugged.

        In the case of highly available systems, either case 3 or case
        4 makes sense.  It is undesireable to require that the bus be
        idle during the plug operation, so cases 0, 1, and 2 are not
        considered further in this profile.

        For the purposes of this profile, "hot plugging" refers to
        case 3, above.

        4.3  Generic SCSI-3 Electrical Requirements

        The SCSI bus and associated electronics should support hot
        plugging.  There may be a need for longer pins in some
        connectors in order to properly sequence the connection and
        disconnection of the power, ground, and data circuits.

        The driver and receiver electronics should be able to
        withstand the hot plugging operation.  The signal lines should
        be well-behaved during the hot plugging operation.  Other
        devices and in-progress transfers should not be disturbed by

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        the hot plugging operation.

        The driver and receiver electronics should be well-behaved
        during power cycles.  No glitches or other irregular signals
        may be caused on the bus as a result of the application or
        removal of power to the device during or in preparation for
        the plug operation.

        After power is initially applied to a device after it is
        plugged, the unit attention flag should be set.  The SCSI unit
        attention flag should be maintained on a per-initiator basis
        in the device.

        4.4  Generic SCSI-3 Logical Requirements

        The device ID mechanism used by the bus should be fully
        supported by the multi-host environment.  If the host software
        requires fixed bus IDs, then the devices on the bus should
        implement fixed bus IDs.  If the host software supports
        dynamic device addressing, the devices on the bus may
        implement dynamic addressing.

        In order to be able to uniquely identify a device regardless
        of where it is in a configuration, or after swapping, or in
        the case of multiple access paths, all devices should support
        the vital product data unit serial number page.

        Since general bus resets can be extremely costly in terms of
        performance across the entire system, they should be issued
        only as a last resort.  Only hosts should issue general

        Since specific device resets (BUS DEVICE RESET) may interfere
        with device activity that was started by another host, device
        resets should be sent only by hosts.

        All devices should handle all possible incoming messages at
        all times.  In particular, console microcode used during the
        host initialization process should implement the complete
        message protocol because other traffic will be active on the
        bus during host initialization.

        All initiators should renegotiate any bus options (e.g. wide
        SCSI) with any device that may have been replaced or power
        cycled since it was last used.  This should not be done on
        every command, but after a host determines that a bus event
        has occurred, as detected by the use of the Unit Attention

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        All initiators should renegotiate any bus options such as wide
        or synchronous before issuing the INQUIRY command.

        4.5  Generic SCSI-3 Command Requirements

        Devices should implement all the mandatory SCSI commands for
        the device type they report.  Optional commands that are not
        implemented should be handled properly according to the SCSI

        4.6  Generic SCSI-3 Target Device Requirements

        Devices should properly handle bus resets that may occur at
        any time.  Setup information should not be carried across a
        bus reset (except for saved mode parameters as described in
        SCSI) because a newly added host cannot predict the earlier

        Devices should maintain mode pages on a per-LUN basis.  This
        is needed because hosts view each LUN as a separate device.

        Devices should properly support simultaneous hosts.  Devices
        should be able to accept and process commands from multiple
        initiators at any bus IDs without hanging the bus, violating
        the SCSI standard, or crashing or hanging themselves.

        Devices should properly handle device reservation (using
        either RESERVE or PERSISTENT RESERVE) in a multi-host

        Devices should support tagged commands in a timely manner.

        4.7  Generic SCSI-3 Initiator Device Requirements

        Hosts should minimize the number of bus reset operations that
        they initiate.  This means that a host should attempt to never
        reset the bus during initialization, normal processing, and
        shutdown.  A bus reset should be sent only when it is
        determined that no other method of restarting the bus is
        possible.  Prior to resetting the bus the host should
        coordinate with other hosts on the bus.

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        4.8  Generic SCSI-3 System Level Requirements

        Device mode pages should be coordinated between all hosts and
        devices on the bus.  Devices are not required to maintain mode
        pages on a per-initiator basis, so all hosts should be able to
        operate with the same mode page setup on each device.  Each
        device may have different mode parameter values, but the
        values for a given device apply across all hosts.

        Device resource locking should be controlled using RESERVE or
        PERSISTENT RESERVE commands.  The locking should be
        coordinated between all hosts and devices on the bus.

        SCSI Device reservations should be coordinated between all the
        hosts in the system.  Since the status of a reservation may
        change upon removal of device power, the hosts should
        coordinate the reservations between themselves.  The
        persistent reservation option may be used to improve this

        Bus ID assignments should be coordinated between all hosts in
        the system.  This applies both in the case of fixed bus IDs
        and dynamic IDs.

        The use of BUS DEVICE RESETs should be coordinated between all
        the hosts in the system.

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        The following sections define the high availability
        requirements for each of the three implementations of SCSI-3.

        5.1  Parallel SCSI-3 Requirements

        The SCSI system, including all devices, components, hardware
        and software, shall conform with all requirements of the
        latest revision of the SCSI-3 Parallel Interface standard
        (SPI).  Revision 15a is current as of the date of this

        Specific additional exclusions and expansions to the SPI
        standard are described in this profile.

        5.1.1  Parallel SCSI-3 Physical Requirements -

        In order to allow devices to be removed from the SCSI bus
        without interrupting bus activity, the cable plant shall
        provide electrical continuity and bus termination when a
        device is removed from the bus.

        SCSI devices and adapters shall conform to one of the
        following two cabling options.  Refer to Note 3, SPI (page 8).

        The two options may be mixed on a single SCSI bus as long as
        the continuity requirement is met.

             1.  The preferred option is the one connector option.
                 The enclosure shall be implemented with a single
                 external SCSI connector.  In order to remove such a
                 device from the SCSI bus without interrupting bus
                 activity, the cable plant shall be equipped with "Y"
                 SCSI cables.  SCSI bus terminators shall not be
                 installed inside the enclosure, but shall be
                 connected directly to the SCSI bus itself.  The stub
                 length of the connector and cable inside the
                 enclosure shall meet the requirements of SPI sections
                 6.4 and 6.5.

             2.  A less desireable option is the two connector option.
                 The enclosure shall be implemented with two external
                 SCSI connectors.  In this case the SCSI bus enters
                 and exits the device using the two connectors, and
                 the internal wiring is arranged to minimize the stub
                 length caused by the device connection.  Such an

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                 enclosure shall meet the requirements of SPI section

        It should be noted that enclosures that use the two connector
        option cannot be removed from the SCSI bus without disrupting
        bus activity.  However, an enclosure suitable for high
        availability systems may be wired this way if it allows the
        devices it contains to be removed without disruption of the
        SCSI bus, and if it provides internal bus continuity when
        devices are removed from the enclosure.

        The SCSI cable plant shall meet the recommendations of SPI
        Annexes D and F.  Cable lengths for a complete system shall
        not exceed the following values.

        Single Ended:
        Up to 5 Megatransfers per second     6 meters
        5 to 10 Megatransfers per second     3 meters
        10 to 20 Megatransfers per second  1.5 meters

        All speeds                          25 meters

        Since the short cable lengths associated with higher clock
        speeds may be impractical for high availability systems, it is
        expected that most such systems will use the differential
        signalling alternative.

        5.1.2  Parallel SCSI-3 Electrical Requirements -

        A device shall not be terminated internally or terminated in
        such a way that precludes it from occupying any position on
        the SCSI bus.

        Switchable terminators may be used if there is a mechanism for
        them to be disabled, such as by software or a jumper.

        Since bus extenders and converters (eg single-ended to
        differential) terminate an electrical bus segment, they shall
        provide bus termination.  This termination shall be external
        to the extender or converter.

        Terminator power shall be supplied as described in SPI section
        7.3, except that "optional internal terminators" shall not be

        Signals on the SCSI bus shall not be disrupted during power
        transitions.  SCSI targets and initiators shall retain their
        high impedance state during power cycles.  Refer to SPI

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        sections 7.1.2 and 7.2.2.

        A high availability system requires that devices be inserted
        or removed from the bus during bus activity to other devices.
        This is needed to allow system maintenance, repair, and
        reconfiguration while normal operation continues.  In order to
        insure glitch free insertion and removal of devices onto to,
        or off of the SCSI bus, SCSI devices shall conform to SPI
        paragraph A.4, "Current I/O Process Allowed During Insertion
        or Removal".

        The system software shall arrange to prevent bus activity to
        the device that is to be plugged (the device becomes inactive
        on the SCSI bus), and the system hardware shall guarantee that
        the device power and ground connections are made before the
        SCSI signal lines, in conformance with the requirements of SPI
        paragraph A.4.

        To meet the requirements of SPI paragraph A.4, one of the
        following two schemes shall be implemented:

             1.  The required signal sequencing may be done by using
                 staggered pins in the connector.

             2.  The enclosure hardware or software may cooperate with
                 the host software to achieve bus activity to the
                 device.  The required signal sequencing may be done
                 by using electrical or mechanical switching devices
                 that isolate the device being plugged from the SCSI

        5.1.3  Parallel SCSI-3 Message And Command Requirements -

        High availability SCSI devices, including targets and
        initiators, shall return a status of CHECK CONDITION with
        sense key of ILLEGAL REQUEST for any unsupported command.

        High availability SCSI devices, including targets and
        initiators, shall return a status of CHECK CONDITION with
        sense key of ABORTED COMMAND/MESSAGE ERROR for any unsupported

        Upon host bootup, the console code of high availability SCSI
        initiators shall negotiate synchronous speed and wide data
        transfer setup before attempting to execute any SCSI command
        that enters a data phase (including the INQUIRY command) to
        any target.  Initiators shall assume that any device on the
        bus may have been powered on, reset, or have changed to a fast

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        or wide mode since the last time it was used by the initiator.

        This DOES NOT mean that these negotiations should be done
        before each command, rather the console (NOT runtime drivers)
        should do this before the first I/O during the boot or
        shutdown sequence.

        High availability SCSI devices shall support the vital product
        data unit serial number page.

        5.1.4  Parallel SCSI-3 Target Device Requirements -

        High availability SCSI target devices shall not issue SCSI bus

        Targets in Multi-host systems shall be able to accept and
        process commands from multiple initiators.

        Targets shall accept and process commands from initiators
        located at any bus ID.

        When a target that is holding data in a cache before writing
        it to non-volatile storage receives a bus reset, it shall
        write the cache contents to the media before processing the

        Targets shall maintain the following on a per-initiator basis.

        Synchronous negotiated state.
        Width negotiated state.
        Contingent Allegiance state.
        Unit attention flag.

        The Unit Attention Condition shall indicate whether if the
        mode parameters in effect for this initiator have been changed
        by another initiator, or if the mode parameters in effect for
        the initiator have been restored from non-volatile memory, or
        if any of the normal SCSI-2 Unit Attention Condition
        conditions apply.

        If a target device supports LUNs, then mode pages shall be
        maintained on a per-LUN basis.

        Targets shall support the RESERVE and RELEASE SCSI commands.
        These commands allows hosts to allocate devices with exclusive
        access.  [question of PERSISTENT RESERVE...]

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        Targets shall support the following mechanisms to clear device

        RELEASE Command
        BUS DEVICE RESET Message
        SCSI Bus Reset
        Power Down/Remove

        Targets that are reserved by an initiator shall accept and
        process the following commands received from any initiator.
        All other commands shall be failed with a SCSI status of

        PREVENT ALLOW MEDIA REMOVAL (Bit set 0) (removable devices)

        If the RELEASE command is received from the initiator that has
        the outstanding reservation, the reservation is cancelled.  If
        the RELEASE command is received from another initiator, the
        command is failed with a SCSI status of RESERVATION CONFLICT.

        High Availability SCSI target devices shall support Tagged
        Command Queuing.

        High Availability SCSI target devices shall support drive
        based Bad Block Replacement (BBR) as described in SCSI-3.

        High Availability SCSI target devices shall implement a
        reselection retry algorithm that limits the amount of bus time
        spent attempting to reselect a non-responsive initiator.

        5.1.5  Parallel SCSI-3 Initiator Device Requirements -

        It is particularly important that host and host adapter
        designs comply with the requirements for external bus
        termination.  It is also important to consider the system
        implications of the choice between the single-connector and
        dual-connector options.

        If these requirements are not met, the SCSI system is limited
        to two hosts that cannot be hot-plugged.  This is not adequate
        for a high availability system.

        Because bus activity is expected to continue during the power
        sequencing, removal, replacement, and reboot procedure on a
        failed host in a high availability system, there is no
        distinction between the requirements placed on the console

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        microcode, the host adapter microcode, and the normal runtime
        driver software environment.  Every device on a high
        availability SCSI bus shall meet all the requirements at all
        times.  This is a notable difference from a single-user system
        where boot-time discrepancies from normal SCSI usage are

        If a host is halted by an operator command (such as a console
        halt command) the SCSI bus host adapter shall not stall in a
        state that prevents the other devices on the SCSI bus from
        continuing normal operation.

        The host shall support the processor type device mandatory
        requirements per ANSI SCSI-3.

        High availability SCSI initiators, including system consoles,
        and adapter microcode, and mainline driver code, shall not
        issue SCSI bus resets except when the SCSI bus is "hung".  The
        definition of "hung" is system dependent, but the following
        procedure is recommended.

        The initiator that suspects that a target is hung should first
        attempt to issue an INQUIRY command to the target.  If the
        command goes through the required bus phases then the bus
        itself is assumed not to be hung.  If this fails, the
        initiator should attempt to coax the target to MESSAGE OUT
        phase [describe how ???], and then send an ABORT message.  If
        that fails then the initiator should attempt to send the BUS
        DEVICE RESET MESSAGE, if that fails the initiator should
        communicate with the other cooperating hosts on the bus to
        determine whether it is ok to issue a bus reset.

        The SCSI bus reset signal is extremely disruptive to
        in-progress bus activity and can be require lengthy recovery
        activity, particularly in the case of systems with tape drives
        and highly cached storage subsystems.

        It may be better to remove a suspect device from the bus than
        to attempt on-line recovery.  The reset signal shall be used
        only as a last resort.

        A third party reset is a reset that an initiator detects that
        was generated by another device.  The initiator shall be able
        to recover from a single or repeated third party SCSI bus
        resets.  The initiator shall not take longer than 60 seconds
        to recover from a single SCSI bus reset or from the last of
        any series of resets.

        (Note:  The 60 second requirement is intended to define a
        guideline for the amount of time the SCSI I/O subsystem may
        take to recover from a bus reset.  The actual recovery time

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        for the entire system may be longer than 60 seconds, depending
        on a number of factors including the number of spindles on the
        bus, whether failover actions occur and whether or not the
        file system recovery is fast or slow.)

        Event during the period when a system is starting up or
        shutting down in a Multi-host environment, it is still
        possible for other initiators to select the system as a
        target.  This selection shall be treated as a normal event and
        handled in such a way that will allow the currently executing
        boot or shutdown activity to complete without error.

        Three optional solutions to this situation may be used.

             1.  Disable selection as a target in the console or
                 adapter card.  This option is most appropriate for
                 adapters that have little intelligence on them.

             2.  Enable selection as a target and support the INQUIRY,
                 TEST UNIT READY and REQUEST SENSE SCSI commands.  Use
                 of this option implies that until the host software
                 has completed its boot process, the console microcode
                 shall be able to respond to and process these
                 commands, and shall either completely implement all
                 of the SCSI message protocol or correctly REJECT any
                 unsupported SCSI bus messages received.

             3.  Option 3:  Enable SELECTIONs and return SCSI status
                 of BUSY, and then return the COMMAND COMPLETE

        Initiators shall not use all the tag queue depth in a device.
        The initiator shall reserve some number of tag queue elemenets
        so other initiators may still send commands (such as INQUIRY)
        to the device while it is in use by another initiator.

        5.1.6  Parallel SCSI-3 System Level Requirements -

        It is recommended that the use of "Y" cables and the
        single-connector option be chosen for all designs.  This
        approach maximizes configuration flexibility and provides the
        opportunity to maximize system availability.

        It is recommended that the mass storage devices in a high
        availability system be housed in a self-contained enclosure
        that is separate from the host enclosures.  By providing
        independent power supplies and cabinet services a storage
        subsystem can provide SCSI block data service to more than one

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        host system with minimal disruption if one host system

        It is recommended that dual redundant independent power
        supplies be provded in the storage enclosure since the
        maintenance of electrical power to the devices is critical in
        maintaining high availability.  If data redundancy is
        distributed between more than one storage subsystem this is
        not as important.

        If storage devices are housed within a host system enclosure,
        it is critical to insure that the external SCSI bus connectors
        to these internal devices do not violate the SCSI bus stub
        length requirements when connected to a "Y" cable.

        All high availability SCSI devices shall implement at least
        the "SCAM tolerant" level of SCAM as described in SPI Annex B.

        In order to have more than one initiator on the SCSI bus there
        shall be a cooperative method of handling the SCSI ID
        assignments on all the devices on the SCSI bus.  Either of the
        following may be used.

             1.  All devices on the bus have SCSI IDs assigned in
                 advance by the system administrator and set by
                 switches or jumpers.  Each initiator shall have a
                 unique SCSI bus ID.

             2.  The various levels of SCAM support shall be
                 implemented according to SPI Annex B.

        Console, host adapter, and device diagnostics, self tests, and
        boot sequences shall run correctly on an active SCSI bus.
        Console, host adapter, and device firmware shall not effect
        active I/O on the bus.

        High availability multi-host systems shall have a mechanism to
        coordinate the access to shared data.  SCSI provides such a
        mechanism in the RESERVE and RELEASE commands, but the details
        of data access coordination is vendor specific.

        High availability multi-host systems shall have a mechanism to
        coordinate the mode page settings on shared devices.  The
        details of mode page coordination is vendor specific.

        Particular examples of mode page values that shall be
        maintained on a system-wide basis include:

        Default block size
        Read/Write Error recovery page

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        Cache control page
        Disconnect/Reconnect page

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        5.2  FC-AL SCSI-3 Requirements

        [compare to list of proposed low cost FC-AL implementation]

        5.2.1  FC-AL SCSI-3 Physical Requirements -

        FC-AL requires active participation by each loop member to
        maintain loop continuity.  To avoid a single point of failure,
        FC-AL systems use a pair of independent redundant loops.

        5.2.2  FC-AL SCSI-3 Electrical Requirements -

        To minimize the disruption of the loop during device plugging,
        the subsystem shall use a multiplexor of some type to bypass a
        device during the plugging operation.  This may be located at
        the device or in a physically remote location that is
        topologically near the device.  The latter choice allows all
        the active redundancy hardware to be localized to a single
        physical location which may make it easier to provide
        redundant electrical power to the critical components.

        5.2.3  FC-AL SCSI-3 Message And Command Requirements -

        The dynamic addressing scheme used by FC-AL shall be supported
        by the devices and hosts used in a redundant system.  One
        option is to have the addresses set by hardware (switches or
        jumpers); another is to have full cooperative support for
        dynamic device addressing in the host systesm.

        5.2.4  FC-AL SCSI-3 Target Device Requirements -

        An FC-AL device shall meet the requirements of dual port SCSI.

        5.2.5  FC-AL SCSI-3 Initiator Device Requirements -


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        5.2.6  FC-AL SCSI-3 System Level Requirements -

        The FC-AL bus redundancy is based on redundant loops, The host
        shall support traffic in both directions from either port.

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        5.3  SSA SCSI-3 Requirements

        5.3.1  SSA SCSI-3 Physical Requirements -

        SSA requires active participation by each loop member to
        maintain loop continuity.  To avoid a single point of failure,
        SSA systems use a pair of interdependent redundant loops.

        5.3.2  SSA SCSI-3 Electrical Requirements -

        The SSA architecture results in the ability of SSA devices to
        be removed from a loop without disrupting the electrical
        status of the loop.

        The removal of more than one device from an SSA loop (or a
        partially-populated cabinet) may cause the isolation of some
        of the devices.  For a high-availability system, the isolation
        shall be prevented by the use of dummy plug-in modules or a
        multiplexor of some type to bypass a missing device.

        5.3.3  SSA SCSI-3 Message And Command Requirements -

        The dynamic addressing scheme used by SSA shall be supported
        by the devices and hosts used in a highly available system.

        5.3.4  SSA SCSI-3 Target Device Requirements -

        SSA devices shall meet the requirements of dual port SCSI.  In
        the case of a failure of a device in the loop that causes
        traffic coming to a device via one of its ports to be
        interrupted, the device shall support the failover mechanism
        that causes incoming traffic to a device to be reflected back
        using the outgoing connection on that port.

        5.3.5  SSA SCSI-3 Initiator Device Requirements -

        SSA hosts shall meet the requirements of dual port SCSI.  In
        the case of a failure of a device in the loop that causes
        traffic via one path to be interrupted, the host shall support
        the failover of that traffic to its other port.

                                                               Page 20

        5.3.6  SSA SCSI-3 System Level Requirements -

        Since SSA bus redundancy is based on dual-ported hosts that
        fail over to the alternate path, the SSA configuration shall
        be a loop.  The other SSA topologies shall not be used in high
        availability systems.

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