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From: goykhman_a@apollo.HP.COM (Alex Goykhman)
Newsgroups: comp.periphs.scsi
Subject: Re: LUN on SCSI drives
Message-ID: <503975ca.20b6d@apollo.HP.COM>
Date: 7 Mar 91 16:35:00 GMT
Sender: root@apollo.HP.COM
Reply-To: goykhman_a@apollo.HP.COM (Alex Goykhman)
Organization: Hewlett-Packard Apollo Division - Chelmsford, MA
Lines: 31

In article <283400064@adaptx1> neese@adaptx1.UUCP writes:
>
>>/* ---------- "LUN on SCSI drives" ---------- */
>>I'm kind of confused about the "LUN" on SCSI drives...  For 
>>example, when I look at my drive configuration using scsicntl,
>>it shows both a SCSI id and a LUN for each unit.  I don't see
>>any documentation referring to LUN anyplace though.  Is that
>>something different from SCSI id?  How is it configured?  I don't
>>see any reference to it, at least in my CDC Wren manual (I'm still
>>waiting for Maxtor to send me the manual for the 8760S.)
>
>The LUN (Logical Unit Number) is a subset of the controller ID.  In SCSI
>you can have a total (currently) of 8 target ID's (0-7) on the SCSI bus.
>These target ID's are essentially the controllers/adapters on the bus.
>Attached to each target ID/controller you can have 8 LUN's (0-7).  The
>LUN is set in the actual SCSI command sent to the target/controller.
>All in all you could have 56 devices on the SCSI bus (7 targets with
>8 LUNS attached to each target = 56 (This assumes one adapter taking up
>one of the target ID slots)).

    SCSI allows up to 256 LUNs per target via an optional EXTENDED IDENTIFY
    message, though I am not aware of a single implementation that use it.
    SCSI-2 does not mention EXTENDED INTENTIFY at all, therefore is limited to
    8 LUNs per target.
>
>			Roy Neese
>			Adaptec Senior SCSI Applications Engineer
>			UUCP @  neese@adaptex


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