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    SubjectRE: The limitation of scsi target and lun supported under Linux?
    Date
    Thanks for the response.

    The scsi support depends on two sources. One from OS target driver and
    another from spec.
    Even wide scsi support 16 targets, most Unix OSes support 32 and more. For
    example, Solaris
    support 128 target and 32 lun each. Unixware has 8 bit for target under
    UW7. From scsi
    III, massive lun is supported. It all depends on how OS target driver
    implemented. Since
    I don't see any document regarding this isuue for Linux, I need to know the
    limitation.

    I am doing Fibre Channel device support which using SCSI III protocal.
    Currently many
    vendor requires large number device support. For example, EMC asks at least
    256 LUN is
    supported.

    In the HBA driver, the limitation is defined in the structure of Scsi_Host
    with max_id and
    max_lun. Both value could be vary from host side but the limitation from
    target driver
    side is the key.

    From code "scsi_.c" routine "scan_scsi()", it looks like there is no
    limitation for max
    target supported. It all depends on the max_id passed by host driver.
    However, there is
    a limitation for max_lun which is 8 defined by "max_scsi_luns = 8".

    As you mentioned here the max LUN could be 127, could you tell me where it
    is defined and
    how it was passed during the scsi device scanning phase?

    Even by looking into the source code and got the conclusion of no limitatin
    for scsi target support and max. 8 LUN is supported, still I have question
    of the limitation of target and lun current Linux can support? This is due
    to my feeling that there should be a limitation for target support and 8 lun
    support is too few. Also Benjamin mentioned 127 lun supoort although I
    don't know where the number comes from.

    If anyone knows for sure, please confirm!

    Many thanks!

    -eddie

    -----Original Message-----
    From: Benjamin Scott [mailto:dragonhawk@iname.com]
    Sent: Monday, September 27, 1999 9:30 PM
    To: WANG,YIDING (HP-SanJose,ex1)
    Subject: Re: The limitation of scsi target and lun supported under
    Linux?


    On Mon, 27 Sep 1999, WANG,YIDING (HP-SanJose,ex1) wrote:
    > Linux scsi support has max_id and max_lun defined in Scsi_Host structure.
    > Both of them are defined as "unsigned int" which is a 4 byte value.

    They are defined that way because an int is (per the C standard) supposed
    to
    be the most "comfortable" (read: fast) data type a particular architecture
    can
    work with.

    > Does Linux really be able to support defined bit number of device?
    > max_id could be 2^31 = 2G, and max_lun could be the same.

    The SCSI specification limits you to 127 LUNs per SCSI target.

    The number of SCSI targets depends on the width of your SCSI bus. Narrow
    SCSI has 8 pins, thus 8 targets (including the host adapter). 16-bit wide
    SCSI supports a maximum of 16 targets. And so on...

    --
    Benjamin Scott
    dragonhawk@iname.com

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