Proposal for Writable Device Identifiers

Tom Coughlan coughlan at
Thu Feb 5 06:14:20 PST 1998

* From the T10 (formerly SCSI) Reflector (t10 at, posted by:
* coughlan at (Tom Coughlan)
     A proposal for writable device identifiers has been posted for review
     as document 98-112R0 on
     Please review this in preparation for the the March WG.

     The first few sections of the document have been extracted and
     attached below to provide a summary of the proposal.

     Tom Coughlan
     OpenVMS SCSI Project Leader
     Digital Equipment Corporation
     Nashua, New Hampshire
     tom.coughlan at

     Subject:  Writable Device Identifiers


     There is a requirement in some environments for the user to be able to
     write an identifier to a SCSI device.  This identifier is subsequently
     returned in the Device identification page of the Inquiry data.  This
     functionality is provided by making two changes to the current
     standard, both in SPC-2:  1) define a new device identifier type
     value, to denote a user-written identifier, and 2) define a new mode
     page, to provide the application client with the means to set one or
     more device identifiers.  Implementation of the writable identifier is
     optional, and the mode page is optional for all device types.


     Many operating systems have traditionally assigned names to SCSI
     devices based on the path from the operating system (OS) to the
     device.  These naming schemes are convenient because they provide
     names that are unique, and they convey some information about the
     logical or physical location of the device.  Path-based names are
     problematic, however, when:

     1.  there are multiple paths from an OS instance to the device

     2.  there are multiple OS instances in the same naming domain with
         access to the same device (e.g.  a cluster)

     3.  the interconnect employs path identifiers that are not as
         persistent as is required for OS device identification.  Fibre
         Channel is one such interconnect.

     One solution to these problems is to devise a device naming scheme
     that is based on Word Wide Identifiers (WWIDs).  In such a scheme,
     each SCSI device is required to provide a persistent,
     path-independent, world-wide unique identifier in its Inquiry data.
     The operating system uses the WWID as the basis for device naming,
     thereby avoiding the problems with path-based naming listed above.

     The WWID formats that are typically being implemented for SCSI are
     either 64-bit or 128-bit binary values.  These identifiers are too
     long to be usable by humans as device identifiers.  To resolve this
     difficulty, the OS (or a group of OS instances in a cluster) will
     typically assign a short alias to each WWID, and will present this to
     the user as the device name.  The OS is responsible for maintaining
     the alias-to-WWID mapping in such a way as to provide the device
     naming consistency and persistence that is expected by the users.  For
     this discussion, we will assume that the OS implements a scheme for
     automatically assigning aliases to WWIDs, and that it implements a
     means for the user to modify aliases as desired to create meaningful
     device names.

     The Problem

     WWID-based naming does not work well in certain environments.  For

     1.  In many system installations, the individuals who replace failed
         storage devices do not have access to the operating system.  This
         presents a problem when WWID-based naming is in use, because the
         replacement storage device will, by default, have a different OS
         device name.  The default OS name can be overridden, or OS
         parameters can be changed to compensate for the new name, but
         these actions require the involvement of personnel with different
         expertise, adding to the cost of the repair.

     2.  Producers of turn-key systems desire to ship and maintain
         identical copies of the OS on identically-configured hardware
         systems.  This is not possible when WWID-based naming is in use,
         because the WWID-to-alias mapping on each system is necessarily

     3.  In some installations it is necessary to boot different instances
         of an operating system at different times.  It is desirable for
         the device names in these environments to match.  This is
         difficult to achieve in general with WWID-based naming, and may be
         impossible in an environment where there is a read-only (CD-ROM)
         system disk.

     4.  The default device name does not provide information about the
         location of the device.  New tools are needed to provide the user
         with a mapping from the default WWID-based device name to the
         device’s logical or physical location within the configuration.
         These tools may not be available in the same timeframe as when
         path-independent device names are required.

     The Proposal

     A variation on WWID-based naming is proposed to address the
     environments where the properties of WWID-based naming cause
     difficulties.  In the proposed scheme, each SCSI device optionally
     implements a writable, persistent, device identifier.  If the
     identifier is present, the SCSI device shall return it to the OS in
     the Device identification page of the Inquiry data.  A new identifier
     type shall be assigned to designate the user-supplied identifier.

     SCSI devices that have a management interface, such as RAID
     controllers, may provide a means for the user to specify a device
     identifier for each logical SCSI device provided by the controller.
     Once defined, the device shall include the identifier in the Device
     identification page.

     I addition, a standard method is required to allow an identifier to be
     written to the SCSI device, particularly for devices that do not have
     a separate management interface.  This is accomplished via a new mode
     page.  The new mode page shall be optional, and shall be defined for
     all device types.  For ease of implementation, the format of the new
     mode page resembles the format of the Device identification page.  In
     typical applications, there would be just one identifier in the page,
     and the identifier would be short, because to the requirement for a
     human to be able to disambiguate it from others.

     With these capabilities, it is possible to provide tools that can
     solve the problems described above.

     Detailed Proposal

     Please refer to 98-112R0 on
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