Learn more about Removable Media Management THE SIGNIFICANCE OF REMOVABLE MEDIA LIKE TAPE CD, DVD and magneto-optical media

Learn more about Removable Media Management THE SIGNIFICANCE OF REMOVABLE MEDIA LIKE TAPE  CD, DVD and magneto-optical media

Removable media is a central component of the storage architectures of large data centers The use of storage networks means that several servers – and thus various different applications – can now use media and libraries jointly. The management of removable media in storage networks is therefore becoming increasingly important. Hence this chapter describes the network based virtualization of tape libraries and other removable media resources. In the following section we first of all explain why, in spite of the ever-increasing capacity of hard disks and intelligent disk subsystems, removable media is indispensable (Section 9.1). Then we consider various types of removable media (Section 9.2) and libraries (Section 9.3), giving special consideration to media management. We then discuss the problems and requirements related to the management of removable media (Section 9.4). Finally, we introduce the IEEE 1244 Standard for Removable Media Management – an approach that describes both the architecture of a system for the management of removable media and also its communication with applications.

The significance of removable media

Articles with such titles as 'Tapes Have No Future' or 'Is Tape Dead?' keep appearing in the press. Some storage manufacturers proclaimed the end of tapes as early as twenty years ago. Then it was to have been all over in the last few years ... After all, they said, hard disks (e.g. serial ATA disks) have now become so cheap that it is unnecessary to move data to other data carriers. In our opinion (in 2003), removable media is, and will remain, an important building block in the storage architectures of data center In addition to their high capacity and low price, for many companies the fact that

removable media can be stored separately from read and write devices and thus withdrawn from direct access is particularly relevant. Viruses, worms and other 'animals' are thus denied the possibility of propagating themselves uncontrollably, as they could on storage that is continuously available online. Furthermore, with removable media a very large quantity of data can be stored in a very small and possibly well-protected area at low storage costs. WORM (Write Once Read Multiple) properties, which are now available not only for optical media but also for magnetic tapes, additionally increase security. Furthermore, the requirement for storage capacity is increasing continuously. Progress in storage density and the capacity of cartridges can scarcely keep up with the ever-growing requirement, which means that the number of cartridges is also growing continuously. For the film The Lord of the Rings alone, 160 computer animators generated and edited a data volume of one terabyte every day, which was stored to tape. At the end of the three-year production period, the digital material for the final version of the film – 150 terabytes in size – was stored on tape. In the scientific field, and also in the field of medicine and bioinformation, data volumes in the petabyte range have been handled for a long time. This immense requirement for storage space cannot be provided exclusively in the form of storage that is available online, such as hard disks. Power consumption, heat and space requirements would drive the costs of this so high that this type of storage could not currently be justified by the shorter access time. For example, the power consumption of an average 120 GB S-ATA drive is currently (mid-2003) approximately 13 Watts. An installation with 400 terabytes of storage thus has a power consumption of more than 42 kW! This corresponds with approximately the average power consumption for 120 german single family homes. A further important advantage of removable media in comparison to hard disks is their robustness. They are less sensitive to impact and a service life of up to 30 years is possible for media in the field of high-end tapes stored in the correct manner.

 REMOVABLE MEDIA

Various types of removable media are currently in use. These are primarily magnetic tapes (Section 9.2.1), optical media such as CDs and DVDs and magneto-optical media (Section 9.2.2). In these sections we are primarily interested in how the special properties of the various media types should be taken into consideration in the management of removable media.

 Tapes

Tapes have firmly established themselves as a back-up and archiving medium for large data quantities due to their very low costs per megabyte storage space in comparison to other media. However, tapes can only be accessed sequentially. The position of the head of a tape drive cannot, therefore, be chosen at will, but must be determined by the  appropriate fast-forwarding and rewinding of the tape. This movement of the tape costs

significantly more time than the movement of the head of a hard disk drive and an optimal speed can, therefore, only be achieved if as many associated data blocks as possible are read and written one after the other, i.e. sequentially. Access to back-up and archive data at will is often unnecessary. The speed at which the large quantities of data can be backed up and restored is likely to be a significantly more important factor than the random access to individual files. Back-up and archiving applications available today utilize this special property of tapes by aggregating the data to be backed up into a stream of blocks and then writing these blocks onto tapes sequentially (cf. Section 9.3.1). Such programs use an internal management system to ensure that they are capable of identifying at any time both the tape on which a file or database is saved and also the position of the start of the file or database on the tape (cf. Section 7.3.4) Today (end of 2003), the term 'removable media' is primarily used to refer to tapes and tape libraries. In almost all large data centres, tape libraries with several drives and a great many tape cartridges are used. They are currently the most commonly used medium for back-up and archiving purposes. Therefore, systems for the management of removable media are currently used primarily where tapes have to be managed and tape libraries have to be controlled. Nevertheless, in view of future developments of new storage technologies, current management systems should also support media that possess several sides and several partitions.

 CD, DVD and magneto-optical media

When writing to CDs, DVDs and magneto-optical media, a file system (e.g. ISO-9660) is generally applied. When writing to these media, the same limitations apply as for tapes, since only one application can write to the data carrier at any one time. Normally, this application also writes a large portion – if not the whole – of the available storage space. However, once these data carriers have been written, applications can access them like

hard disk drives. As a result, the applications have available to them the full support of the operating system for the read access to optical media, which is why in this case they behave like write-protected hard disks and can be shared accordingly. Magneto-optical media are generally readable and writeable on both sides. Depending upon the drive, the cartridge may have to be turned over in order to access the second side. This property makes it necessary for management systems to be able to manage a second side of a cartridge and control the changing mechanism so that

the cartridge can be turned over. Furthermore, the WORM properties must be suitably represented for these data carriers.