Removable Storage Management and Windows

Updated: January 7, 2002

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  Removable Storage Management
  RSM Benefits
  Software Support
  RSM Implementation
  Driver Model for Changer Drivers

Removable Storage Management

Removable Storage Management (RSM) is a standard interface in the Microsoft Windows XP and Windows 2000 operating systems for accessing removable media, especially removable media in automated devices such as changers, jukeboxes and libraries. It was designed to simplify access to libraries of media in both standalone drives and in these automated devices.

RSM is available on all client and server versions of Windows. RSM uses functionality that is new to NTFS 5, so there are no plans to port RSM to earlier Windows versions.

RSM is installed by default to control most types of removable media, including CD-ROM, DVD-ROM, magneto-optical, Jaz, and Zip in both standalone and library configurations. RSM can be used to manage anything except the A and B drives.

RSM models all changers as subsets of an "ideal" changer. A given minidriver tells RSM what functionality the actual changer implements so that RSM can treat it appropriately. The implementation of the features varies from changer to changer, but client applications can access them in the same way, using the ideal model. This library model supports changer-based libraries as well as stand-alone drives, by simply modeling them as changers with one drive, no slots, and no robot.

Client programs such as backup applications and Hierarchical Storage Management (HSM) services use RSM to gain access to their media. Essentially, RSM is an API that is called by these programs to make media accessible. Once media is accessible, these client programs use other Microsoft Win32 APIs to read and write the media. In addition, there is a Microsoft Management Console (MMC) snap-in client (called "Removable Storage") that provides a human interface to most functionality such as inject/eject, cleaning, and so on.

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RSM Benefits

RSM offers these benefits:

• Common driver model. A single driver will allow a tape library to be used with any RSM-compatible application. Conversely, an RSM-aware application will work with all changers on the market that have RSM drivers.

• Library sharing. Multiple applications can now share a single library. Previously, a user who wanted to use backup and HSM solutions from two different vendors needed two changers. With RSM, both applications can share the same changer.

• Abstraction of offline media. A backup application does not need to worry about whether a tape is in a changer or on a shelf. It simply requests the media, and RSM loads it or asks an operator to fetch it, as appropriate.

• Media tracking. RSM tracks in an internal database all media that it recognizes. Applications can register with RSM so that it can identify their media. They can also use the API to search the database and load particular media.

• Identical interface for libraries and standalone drives. A backup application written to RSM will work equally well with a changer and with a standalone drive.

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Software Support

RSM will interfere with legacy applications. By default, RSM binds exclusively to all media changers on the system. This will break applications that expect to access these changers through other methods. All legacy applications that are changer-aware must be reauthored to access changers through RSM. Disabling RSM control of the device might allow the application to function, but this has not been widely tested and will not be supported by Microsoft.

The Windows Backup Utility uses RSM for tape media, but not for media with file systems (such as Zip or Jaz). The HSM tool called RSS that is included with Windows Server uses RSM to interface with tapes. Many third-party data management application vendors are currently working on RSM-aware versions of their applications. If you have software partners, make sure that they are working on RSM versions of their applications.

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RSM Implementation

RSM keeps a database of all media, organized by both pool and media type. Each application creates its own pool, so (for example) backup media and HSM media can be identified separately, even if they are present in the same library. The database is system-wide, so a tape removed from a library and inserted into a standalone drive on the same system will be properly identified.

RSM fully supports tape, magneto-optical, all existing CD, and all existing DVD media types in changers. However, although Windows can read universal disk format (UDF) 1.5, it does not have UDF-write capabilities. RSM will function with third-party UDF write solutions, but no write-once media is supported, because RSM cannot write a free label to it.

To make a changer work with RSM, the developer must write an RSM changer miniclass driver. The Windows DDK contains a chapter under "Storage Drivers" that describes interfaces that must be supported. The DDK also contains the source for several changer drivers distributed with Windows as examples.

After the driver is complete, Microsoft Windows Hardware Quality Labs (WHQL) will test and validate changer drivers. If a manufacturer follows the WHQL testing process and the device and driver pass testing, Microsoft will distribute the driver on the Windows Update site at http://windowsupdate.microsoft.com. For information, see http://www.microsoft.com/whdc/whql/default.mspx.

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Driver Model for Changer Drivers

Changer drivers are constructed using the standard class driver/miniclass driver architecture that most Windows drivers use. A changer class driver is provided as part of Windows to handle all device-independent driver services required of all changer drivers. The class driver uses the miniclass driver that the vendor provides to handle the details of communicating with specific changers.

The standard changer driver interface has a variety of "feature bits" that allow a minidriver to describe a changer's unique functionality. Most changers' features can be fully described by some combination of these feature bits. If this is true for a particular changer, then any application can seamlessly use the features provided by that changer.

If the drives in any changer are supported in Windows as stand-alone drives, then they will automatically be supported inside changers. For drives that are not currently supported in Windows, the vendor must supply drivers. Information is provided in the Windows DDK.

RSM has an auto-configuration component that, if given appropriate information by the changer driver, matches drives with drive bays inside a changer.

Call to action for Removable Storage Media:

• Write a driver for your changer using information about changer drivers in the current Windows DDK. Documentation is provided under the following topic: Storage Drivers > Design Guide > Changer Drivers. Also, several sample changer minidrivers are provided in the DDK in the %ntddk%\src\storage\changers directory.

• Make sure that your software partners are using RSM to communicate with the changer.

• For information about the RSM API for the user-mode service that opens changer drivers, see the Microsoft Platform SDK, available through MSDN Professional Subscription. In the MSDN Library on the web, this information can be found at Windows Development > Windows Base Services > Hardware > Removable Storage Manager.

• Write a driver for your changer using the information provided in the Windows DDK. If you find that you cannot describe your changer's functionality with the provided feature bits, you will need to contact Microsoft.

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