Media Identification and Naming

As a manager of shared resources, Removable Storage ensures that client applications do not corrupt each other's data. Placing permissions on media pools prevents an application from modifying the data of another application. Another way to accomplish this is to assign each medium a unique name which applications can use to refer to it, and to track the identities of all media in a system, ensuring that the mapping between the unique name and the physical media is always accurate.

A library management system could simply use a media's home slot as its identity. A request to mount a medium in a drive could be as simple as "Mount the medium in slot 6 of drive 2." There are problems with this minimalist approach, however. An administrator could exchange the medium in slot 6 for that in slot 7 during a door access. An application executing the request might get a medium it did not expect and not know it. To prevent this, Removable Storage assigns each medium a unique name, which is used to refer to the medium programmatically. In keeping with Windows 2000 naming conventions, Removable Storage uses globally unique identifiers (GUIDs) as the form for these names.

Removable Storage identifies each medium side by reading its label. At various times, Removable Storage reads the label to ensure that the name it has for a medium maps correctly. Consider a refinement of the example given earlier. If the name of a medium simply referred to the medium in slot 7, an administrator could change the contents of slot 7 during a door access and the next time an application used the name it would get a different medium. Removable Storage prevents this by associating information stored in the medium's label with its name. Each time a medium is mounted, for example, Removable Storage reads the label to ensure that the medium it mounted is correct. An inventory is an operation that an administrator can request or that Removable Storage can initiate, which verifies the labels associated with each named medium in a library.

Since administrators also need to identify media uniquely, and since GUIDs are difficult to read in printed form, Removable Storage also assigns each medium a display name. These names are changeable so if an administrator has a naming scheme for media, a display name can be assigned that is different from the one Removable Storage initially assigned.

On-media Identifiers

On-media identifiers (OMIDs) are electronically recorded labels on each medium side in a Removable Storage system. Each label is composed of at least two parts: a type and an ID. A label type identifies the format used to record information on the medium. A label ID is an identifier that uniquely identifies each medium.

Different media types have different types of labels. Media sides with file systems (optical disk and Jaz cartridges, for example) have file system labels. An NTFS-formatted Jaz cartridge, for example, has an NTFS label type, and the ID is the volume serial number. Tapes using MTF have a label type of MTF and the ID is derived from various fields in the MTF header.

On-media identifiers are handled somewhat differently for CD-ROM and DVD-ROM media. Serial numbers for instances of these media might not be unique — a library can hold multiple copies of the same CD-ROM title, for example, with each having the same serial number — so Removable Storage allows non-unique label IDs in this case. This doesn't degrade the media identification objectives of Removable Storage; any of the CD-ROMs with identical IDs can be used interchangeably since their contents are identical and cannot be changed.

Duplicate Copies of Media

Although Removable Storage tries to uniquely identify each cartridge that it manages, it also handles media duplicates. It treats identical media as separate media that are identical in every way and can be used interchangeably. For example, if a user has two copies of the Windows 2000 Server Setup CD and places both in a changer, Removable Storage shows both media and each can be mounted, ejected, and so on, separately. Since each has its own record in the database, it is possible to attach different attributes to each media. You can give each a different display name, for example, although this is discouraged because there is no guarantee that these differing attributes will always be associated with the same cartridge. If both are ejected, the attributes may be reversed when both are inserted again.

Bar Codes

Bar code labels on physical media, in libraries with bar code readers, are used as a shortcut to determine the identity of physical media and its sides. Reading an on-media label can be time consuming since it requires reading data on the media, which requires that the medium be placed in a drive if it's not already there. With unique bar codes, a bar code scan is a much faster way of identifying a physical medium and the sides it contains.

Media Names

When new physical media are placed into a library, Removable Storage assigns an initial display name to the media that an administrator can use to visually identify the media. This name is derived from several sources, including the on-media label and the bar code. The rules include the following:

1. If the media has a bar code label, its alphanumeric value is used as the display name.
2. If the media is single-sided and has a recognized label type on its single side, the name is taken from the label. If the format is MTF, the label is taken from information in the MTF header. If the format is a file system, the name is taken from the volume name.
3. The sequence number is used.

Media Change Detection

Many removable media devices, including all those connected to the system by means of SCSI and IDE, cannot themselves report when media are entered or removed. Instead the host system must notice such changes either by querying the device specifically for this information or when performing some other operation on the drive.

Since Removable Storage controls changers and knows when media enter and leave drives, this is not a problem for drives in changers. It is an issue, however, for stand-alone drives. The information that Removable Storage has about the contents of a stand-alone drive might be incorrect if the state of the drive has changed. For example, the snap-in might show that a drive is empty for a period of time after a tape is inserted into a tape drive. An inventory operation on the stand-alone drive library brings the state up-to-date.

Some applications need to have current information. For these applications Removable Storage provides a media change detection mechanism that can be turned on and controlled through Removable Storage API calls. With media detection turned on for a stand-alone drive library, the state of the drive that Removable Storage presents to applications is updated almost immediately after the physical state of the drive changes.

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