Basic Concepts

Removable Storage can be described in terms of five basic concepts: media, physical locations, media pools, work queue items, and operator requests. The last four of these are the top level nodes in the Removable Storage snap-in. The first, media, is the most fundamental concept and affects all the others.

Media

Units of media store information. Each unit of media (or medium, also referred to as a "cartridge") is of a certain type, such as 8mm tape, magnetic disk, optical disk, or CD-ROM.

Most types of media have a single side. For example, a tape must always be oriented in a certain way. When the tape is placed in a drive, all of its data is accessible. Some types of media, such as magneto-optic (MO) disks, have two sides. An MO disk has an "A" side and a "B" side. When an MO disk is placed in a drive with the "A" side up, then the "A" side is accessible and the "B" side is not. To access the "B" side, the disk must be inserted with the "B" side up.

Removable Storage represents media physically and logically. Physical media are the tangible media that are inserted and removed from libraries and mounted in drives.

When an application wants to access data on a medium, Removable Storage generates a logical identifier (ID) that allows the application to request the data on that medium. Since access to the data occurs only through that ID, Removable Storage can manage the physical location of the data. For example, if the original medium begins to fail, Removable Storage can move the data to a new medium without having to notify the application.

Physical media and the sides they contain are tracked in Removable Storage and can take on various states as they are entered into a library and used. For more information about media and side states, see "Media Handling and Usage" later in this chapter.

Removable Storage provides the infrastructure necessary to share media among various applications. Removable Storage ensures that all media are used in such a way as to preserve the data they contain. Removable Storage accomplishes this by identifying and verifying the identity of each medium. For more information about how Removable Storage verifies the identity of media, see "Media Identification and Naming" later in this chapter.

Physical Locations

Removable Storage manages two classes of physical locations: libraries and the offline media physical location. Libraries include both media and the means to read and write them. The offline media physical location is a special holder for media that is cataloged by Removable Storage, but does not reside in a library.

The set of all physical locations includes the libraries and the offline media physical location.

Libraries

In its simplest form, a library is composed of data storage media and a means to read and write the media. A CD-ROM drive with a disc inserted is a simple library with one drive, no slots, and no transport. A more complex example of a library is a robotic-based tape library, which holds several (up to several thousand) tapes, has one or more tape drives, and has a mechanical means to move tapes into and out of the drives.

Specifically, libraries are composed of the following:

Slots   Slots are storage locations in the library. For example, a tape library has one slot for each tape that the library can hold. A stand-alone drive library has no slots. However, most libraries have at least four slots. Sometimes slots are organized into collections of slots called magazines. Magazines are usually removable.

Drives   A drive is a device that can read or write to a medium. An Iomega Jaz drive, for example, can read and write to a Jaz cartridge which is inserted into it. A library has at least one drive.

Transports   A transport is the robotic device that moves a medium from its slot to a drive and back again. Robotic libraries usually have only one transport.

Bar Code Readers   A label with a bar code is attached to the outside of a cartridge that is both human readable and computer readable. Libraries that hold media with bar codes attached might have a bar code reader. There is only one reader, which is usually mounted on the transport.

Doors   A door is a means to gain unconstrained access to the media in a library. On larger libraries a door might resemble an actual door. When the door is open, an administrator can add and remove media from the library. Doors on smaller libraries might not always look exactly like doors, but they still provide the same functionality. For example, some small changers have all their slots in a magazine. When an administrator instructs Removable Storage to open the door, the changer pushes the magazine out through the front. The administrator can add, replace, or remove media and then reinsert the magazine into the changer. When an administrator adds media to a library, the media are placed directly into a slot. Some libraries have no doors, while others have several.

Insert/Eject Ports   While doors provide unconstrained access to the media in a library, an insert/eject (IE) port provides very controlled access. When an administrator adds media to a library through an IE port, the media are not placed directly into a slot. Instead, the media are placed in the IE port and then the library uses the transport to move the media from the IE port to a slot. Some libraries have no IE ports, while others have several. Also called "mailslots," some IE ports handle only one cartridge at a time, while others may handle several.

Types of Libraries   A robotic library can have any of the components described above. At a minimum, a robotic library has some media, slots to hold the media, one or more drives, a transport, and either a door or an IE port. Robotic libraries are sometimes referred to as changers or jukeboxes. No human intervention is required to place a medium in a library in one of its drives.

A stand-alone drive library or stand-alone drive is a special kind of library. A medium must be manually placed in a drive. The CD-ROM drive found on most desktop computers is a stand-alone drive library. Removable Storage treats stand-alone drives as libraries with one drive and an IE port. Removable Storage models these drives as libraries to make its program interfaces simpler. For this reason, an application need not know whether a medium is mounted by a transport or a human.

Offline Media Physical Location

The offline media physical location is where Removable Storage lists media that are not in a library. The physical location of media in an online library is the library in which it resides. Media that are not in any of these libraries, such as archived backup tapes on a shelf, are offline media and reside in the offline media physical location. When a user or administrator moves an offline medium into a library, Removable Storage changes its physical location to be the library into which it was placed. When a medium is taken out of an online library, Removable Storage notes that it now resides in the offline media physical location.

Media Pools

A media pool is a logical collection of media that share some common attributes. A media pool contains media of only one type, but media in the media pool can be in more than one library. Each medium is in a media pool. There are two classes of media pools: system and application. System media pools are created by Removable Storage for its own use and include the free, import and unrecognized pools. Application media pools are created by applications to group media. Grouping media is especially important if several applications are sharing the libraries attached to a system and the media they contain.

Each media pool has access permissions that control access to the media that belong to it. While these permissions do not control access to the data contained on the media, they do control the manipulation of the media, including an application's ability to move media from the pool or to allocate it for its own use.

Media pools can be used hierarchically. A media pool can be used to hold other media pools, or it can be used to hold media. An application that needs to group media of several types into one collection can create an application media pool for the whole collection and additional media pools within it, one for each media type. Removable Storage actually uses this technique for its system pools. Within the free pool, for example, is a media pool for each media type. Both sides of a two-sided medium are always in the same pool.

System Pools

The system pools are used to hold media that are not currently being used by an application. The free pool holds unused media that are available to applications, and the unrecognized and import pools are temporary holding places for media that have been newly placed in a library. For more information on how Removable Storage uses these pools, especially the unrecognized and import pools, see "Media Handling and Usage" later in this chapter.

Free Pools   The free pools support sharing media among applications. They contain media that are freely available to any application, and they hold no useful data. An application can draw media from the free pools when it needs additional media, and it can return media to the free pools when the media are no longer needed and should be made available to other applications.

Unrecognized Pools   When a medium is placed in a library, Removable Storage tries to identify it. If it has not seen this particular media before nor discerned its format or the application which last wrote data on it, Removable Storage places the medium in the unrecognized pool for its media type. Blank media are treated this way. Media in the unrecognized pools might have data on them, but they are not cataloged by Removable Storage nor is Removable Storage able to read any data on these media.

Import Pools   If Removable Storage can identify the format or application associated with a medium that has just been placed in a library but has never been seen before, it places the medium in the import pool. For example, if an administrator placed a tape written by Backup on one system into a library attached to a second system, the instance of Removable Storage on the second system recognizes that the tape was written using Microsoft Tape Format (MTF) and places it in the proper media type import pool.

Application Pools

Each application that uses media managed by Removable Storage uses one or more application pools. Applications can create these pools, or they can be created using the Removable Storage snap-in. Permissions on application pools can be set up to allow applications to share pools, or to assign each application its own set of pools.

As mentioned earlier, a pool can either be created to hold other pools or it can be created to hold media. A pool created to hold media has additional properties that specify the actions to be taken when an application allocates or deallocates media.

The following example shows one way in which media pools might be configured to support the needs of several client applications. The example system contains two libraries — one optical disk library and one tape library — and two data mover applications — a backup application and a document management application.

Table 2.1 shows how the two applications use the two media types.

Table 2.1 Media Type and Data Mover Applications

Work Queue

When applications make a library request , Removable Storage places these requests in a queue and processes them as resources become available. For example, a request to mount a tape in a library results in a mount work queue item, which might wait until a drive is available.

Table 2.2 describes the states that a work queue item can have while it is being handled by Removable Storage.

Table 2.2 Work Queue States

Operator Requests

Note

"Operator" as used here, is synonymous with "administrator."

Sometimes, even with robotic libraries, manual assistance is needed to complete a request or perform maintenance or support. If an application requests that a medium in the offline media physical location be mounted, the medium must be manually entered into an online library, generating a request to the operator to enter the cartridge.

Operator requests can be presented to an administrator through the Windows 2000 Messenger Service or the system tray. An administrator can refuse or complete the request. Definitions of the operator request states are shown in Table 2.3.

Table 2.3 Operator Request States

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