Free Tutors on Management features of removable media Tape libraries and drives

Free Tutors on  Management features of removable media Tape  libraries and drives

In what follows we give an overview of the most important features and terms, plus a brief explanation:

Cartridge

 A cartridge is a physical medium upon which storage space is available. A cartridge can be moved and has one or more sides. External cartridge label A label that is applied to the outside of a cartridge and serves to identify the cartridge, for example, a mechanically readable barcode. Internal cartridge label A dataset in a certain format at a certain position on the data carrier that serves to identify the cartridge. Side A physical part of a cartridge that provides storage space. A side contains one or more partitions. Tapes normally have only one side. DVDs and magneto-optical media

are also available in double-sided variants. Partition Part of a side that provides storage space as a physical unit of the cartridge.

Volume A volume is a logical data container. It serves to reserve storage space for applications on data carriers. A partition can hold as many volumes as desired. Please note that the term volume may have different meanings depending on the context it is used: in terms of the SNIA Shared Storage Model (Chapter 10) a tape volume is called a tape extent and may span multiple physical tape cartridges. In terms of back-up

software and mainframes a volume is often used synonymously with cartridge. Scratch tape A new tape without any content or a tape, the content of which is no longer of interest, and the entire storage capacity of which can be used for new purposes. Access handle An identifier that an application can use to access the data of a volume. Under UNIX operating systems an access handle is equivalent to the name of a device special file (for example: /dev/rmt0). Mount request The command to place a certain cartridge in a drive. Audit trail Audit trails consist of a series of data sets, which describe the processes that have been performed by a computer system. Audit trails are used primarily in security-critical fields in order to record and check access to data. As already mentioned, a system for the management of removable media should also be able to represent the logical and physical properties and features of a cartridge. Ideally a cartridge can consist of as many sides as desired (tapes generally have only one side, optical media often have two, holographic media could, at least theoretically, provide even more sides). Each side can hold one or more partitions, and any desired number of volumes can be allocated to each partition.

LIBRARIES AND DRIVES

Operating systems and applications that use removable data carriers must be able to deal with a large amount of different library hardware. In general, libraries possess a media changer, slots to accept cartridges, and drives with which the cartridges can be read and written. The media changer takes cartridges from the slots and transports them to the drives. In automatic libraries, the media changer can be controlled via an interface. This interface can be realized in the form of an in-band interface (e.g. SCSI) or an out- band interface depending upon the device (cf. Section 9.3.3). The bandwidth of these automatic libraries ranges from individual small autoloaders with 1–2 drives and a few slots through to large automatic tape libraries, in which one or more media changers can transport thousands of cartridges in dozens or possibly even hundreds of drives.

In addition to automatic libraries, a removable media management system should also consider manually operated libraries. In these, an operator takes on the function of the media changer and inserts the cartridges in the drives accordingly. It is thus possible to include even individual (standalone) drives in the system as a whole. Depending upon the level of abstraction, a shelf or a safe filled with cartridges and without any drives can also be viewed as a library. These libraries are also called vaults or vaulting locations. Particularly if both automatic libraries and vaults are used, it is wise to choose the level of abstraction for the management of the media so that vaults can also be handled like manual libraries without drives. This means that, for all libraries of whatever type, the same procedures can be applied for auditing (the requesting of all components, particularly the cartridges of a library), export (the removal of a cartridge from a less library) and import (the insertion of a cartridge into a library). In what follows we will consider the individual components of libraries.

 Drives

Like hard drives, drives for removable media are currently equipped with a SCSI or Fibre Channel interface in the Open Systems environment and are connected to the storage network via these. In the mainframe field, ESCON and FICON are dominant. As already mentioned, tape drives in particular can only work at full speed if they read and write many blocks one after the other (streaming). Although it is possible, and in the mainframe environment totally normal, to write individual files consisting of just one logical block to tape, or to read them from tape, different drives are necessary for this than those used in the Open Systems back-up operation. These enterprise drives have larger motors and can position the read-write heads significantly more quickly and precisely over a certain logical block.

Media changers

Media changers have the job of transporting cartridges within a library. The start and end of a transport operation can either be a slot or a drive. To this end, a library has an inventory in which all elements of the library and their attributes are noted. The media changer has access to this inventory. Like drives, media changers have an interface for the control and checking of their functions. It is normal to use this interface for requesting data from the inventory, as well as for controlling the transport operations. The following information can therefore be requested via the media changer interface:

• the number of drives and their properties (addresses, type, etc.);

• the number of slots and their properties;

• the number of cartridges and their properties (slot, label, etc.);

• the number of further media changers and their properties.