Confusion with the PREVENT field in the PREVENT ALLOW MEDIUM REMOVAL command
Ralph O. Weber
roweber at IEEE.org
Thu Feb 24 10:48:02 PST 2005
* From the T10 Reflector (t10 at t10.org), posted by:
* "Ralph O. Weber" <roweber at ieee.org>
Is this important enough to handle as a late letter ballot comment
against SPC-3? If the issue can get sorted out by the March CAP
meeting, then the SPC-3 letter ballot is still open.
Banther, Michael wrote:
> We're running into some problems correctly interpreting SMC-2 and
> SPC-2/3 when implementing a local SMC device server as specified in ADC.
> SMC2r07 includes PREVENT ALLOW MEDIUM REMOVAL as an optional command
> for independent media changers in Table 3 - Commands For Independent
> Media Changers. It references SPC-2 for the definition of the command.
> SSC2r09 includes PREVENT ALLOW MEDIUM REMOVAL as an optional command
> in Table 11 - Explicit Address Command Set For Sequential-Access
> Devices and in Table 18 - Implicit Address Command Set for
> Sequential-Access Devices. Both of these tables reference SPC-3 for
> the definition of the command.
> SPC3r21d, Table 119 - PREVENT Field discusses the effect of the values
> in terms of data transport elements and attached medium changer
> elements. It includes a table note that the field values 10b and 11b
> are only valid when the INQUIRY bits RMB and MCHNGR equal 1b (by the
> way, the note has become garbled). SPC2r20, Table 78 - PREVENT ALLOW
> MEDIUM REMOVAL PREVENT Field contains very similar text. However it
> includes the restriction on field values 10b and 11b in text just
> below the table.
> SMC2r07, 5.2 Attached Media Changer makes it pretty clear that an
> attached media changer sets MCHNGR to 1b. Although SMC-2 is silent,
> I've always read it to imply that an independent media changer sets
> MCHNGR to 0b. SPC2r20, 7.3.2 Standard INQUIRY Data and SPC3r21d,
> 6.4.2 Standard INQUIRY Data include text on the meaning of the MCHNGR
> bit that supports this interpretation.
> Hence I interpret the restriction on PREVENT field values as meaning
> that a logical unit that does not include an attached media changer
> shall respond to a PREVENT ALLOW MEDIUM REMOVAL CDB with PREVENT set
> to either 10b or 11b with CHECK CONDITION status and shall set the
> sense key to ILLEGAL REQUEST and the additional sense code to INVALID
> FIELD IN CDB. This behaviour holds for a local SMC logical unit, in a
> device that implements ADI Bridging, that supports the independent
> media changer model as much as for any other logical unit.
> The problem with this interpretation lies in SPC3r21d, Table 119 and
> SPC2r20, Table 78. These tables associate the lsb of the PREVENT
> field with a 'data transport element.' Neither SPC-2 nor SPC-3 define
> this term. SMC2r07 defines a 'data transfer element' as 'the address
> in media changer element space of a data transfer device.' It then
> goes on to give examples of data transfer devices as magnetic disk
> drives, cartridge tape drives, optical disk drives, and CD-ROM
> drives. Some people here interpret 'data transport element' and 'data
> transfer element' as synonyms.
> If a 'data transport element' is a 'data transfer element', then an
> independent media changer device server that receives a PREVENT ALLOW
> MEDIA REMOVAL command with PREVENT set to 01b is being told to prevent
> removal of the medium from some removable media drive, not from the
> media changer itself. Indeed no method exists for an application
> client to tell a media changer to prevent the removal of a piece of
> media from the media changer itself.
> I believe that the usage of 'data transport element' in Table 119 of
> SPC3r21d and Table 78 of SPC2r20 is incorrect. The lsb of the PREVENT
> field is associated with preventing/allowing media removal from the
> physical device associated with the addressed logical unit. If
> sufficient consensus exists within the removable media community, I'll
> prepare a proposal for SPC-4 that addresses this problem.
> Comments welcome.
> Michael Banther
> Hewlett-Packard Ltd.
> Telephone +44 (117) 312-9503
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