Know more about extension of the snia shared storage model to tape functions logical and physical structure of tapes and Extension of the model

Know more about extension of the snia shared storage model to tape functions logical and physical structure of tapes and Extension of the model

The SNIA Shared Storage Model described previously concentrates upon the modelling of disk-based storage architectures. In a supplement to the original model, the SNIA Technical Council defines the necessary extensions for the description of tape functions and back-up architectures.

The SNIA restricts itself to the description of tape functions in the Open Systems environment, since the use of tapes in the mainframe environment is very difficult to model and differs fundamentally from the Open Systems environment. In the Open Systems field, tapes are used almost exclusively for back-up purposes, whereas in the field of mainframes tapes are used much more diversely. Therefore, the extension of the SNIA model concerns

itself solely with the use of tape in back-up architectures. Only the general use of tapes in shared storage environments is described in the model.

The SNIA does not go into more depth regarding the back-up applications themselves. We have already discussed network back-up in Chapter 7. More detailed information on tapes can be found in Section 9.2.1. First of all, we want to look at the logical and physical structure of tapes from the

point of view of the SNIA Shared Storage Model (10.3.1). Then we will consider the differences between disk and tape storage (10.3.2) and how the model is extended for the description of the tape functions (10.3.3).

 Logical and physical structure of tapes

Information is stored on tapes in so-called tape images, which are made up of the following

logical components (Figure 10.20):

• Tape extent A tape extent is a sequence of blocks upon the tape. A tape extent is comparable with a volume in disk storage. The IEEE Standard 1244 (Section 9.5) also uses the term volume but it only allows volumes to reside exactly on one tape and not span multiple tapes.

• Tape extent separator The tape extent separator is a mark for the division of individual tape extents.

• Tape header The tape header is an optional component that marks the start of a tape.

• Tape trailer The tape trailer is similar to the tape header and marks the end of a tape. This, too, is an optional component. In the same way as logical volumes of a volume manager extend over several physical disks, tape images can also be distributed over several physical tapes. Thus, there may be precisely one logical tape image on a physical tape, several logical tape images on a physical tape, or a logical tape image can be distributed over several physical tapes. So- called tape image separators are used for the subdivision of the tape images (Figure 10.21). 10.3.2 Differences between disk and tape At first glance, disks and tapes are both made up of blocks, which are put together to form long sequences. In the case of disks these are called volumes, whilst in tapes they are called extents. The difference lies in the way in which they are accessed, with disks being designed for random access, whereas tapes can only be accessed sequentially. Consequently, disks and tapes are also used for different purposes. In the Open Sys- tems environment, tapes are used primarily for back-up or archiving purposes. This is completely in contrast to their use in the mainframe environment, where file structures – so-called tape files – are found that are comparable to a file on a disk. There is no definition of a tape file in the Open systems environment, since several files are generally bundled to form a package, and processed in this form, during back-up and archiving.

This concept is, therefore, not required here.

 Extension of the model

The SNIA Shared Storage Model must take into account the differences in structure and application between disk and tape and also the different purposes for which they are used. To this end, the file/record layer is expanded horizontally. The block layer, which produces the random access to the storage devices in the disk model, is exchanged for a sequential access block layer for the sequential access to tapes. The model is further supplemented by the following components (Figure 10.22):

• Tape media and tape devices Tape media are the storage media upon which tape images are stored. A tape devices is a special physical storage resource, which can process removable tape media. This differentiation between media and devices is particularly important in the context

of removable media management (Chapter 9). The applicable standard, IEEE 1244, denotes tape media as cartridge and tape device as drive.

• Tape applications The SNIA model concentrates upon the use of tapes for back-up and archiving. Special tape applications, for example, back-up software, are used for back-up. This software can deal with the special properties of tapes.

• Tape format system In the tape format system, files or records are compressed into tape extents and tape images. Specifically in the Open Systems environment, the host generally takes over this task. However, access to physical tape devices does not always have to go through the tape format system. It can also run directly via the extent aggregation layer described below or directly on the device.

• Extent aggregation layer The extent aggregation layer works in the same way as the block aggregation layer (Section 10.1.7), but with extents instead of blocks. However, in contrast to the random access of the block aggregation layer, access to the physical devices takes place

sequentially. Like the access paths, the data flows between the individual components are shown as arrows.