Free tutors on Media tracking Grouping, pooling Drive pools

Free tutors on Media tracking Grouping, pooling Drive pools

As an integral part of a disaster recovery solution, a management system must ensure that all removable media plus the appropriate metadata remain in the system until they are deleted by an appropriately authorized user. Thus, under no circumstances may tapes be 'lost' or removed from the system without authorization. Furthermore, it must be possible to determine the storage location of each medium at all times. If access is available to the media online, for example, in automatic tape libraries in which tapes can be automatically identified by the reading of a barcode label by a scanner, a suitable audit can be performed at any time. In such an audit, the content of the inventory is compared with the real existing tapes. Such libraries also automatically report the opening of a door. After the door has been closed an audit should once again be automatically performed in order to ensure that no media has been removed from the system without authorization. If a part of the media is withdrawn from direct access, either to a well-protected safe or to another manually-operated library, this storage place must be managed with appropriate care by the responsible administrators. Ideally, the management software provides an interface for this vaulting. In order to increase the reliability of a disaster recovery concept and to fulfil statutory provisions, a two-stage or multi-stage strategy made up of online and offline storage is often performed. Storage media are first written in automatic libraries, then stored offline for a certain period of time and subsequently either taken out of circulation or reused (Figure 9.5). Media that are in transit from an online library to an offline storage place must be identified. A management system for removable media should serve both as a central repository for all resources and as a universal interface for applications. It should not be possible for any application to withdraw itself from the control of the system and access or move media in an uncontrolled manner. Only thus is it actually guaranteed that all media in the system can be located at any time. Such a central interface is always in danger of becoming a single point of failure. It is therefore very wise to use appropriate measures to guarantee a high level of availability for the entire solution. Consequently, care must be taken to ensure that all components of the entire system, from the hardware through the operating systems used with the media management to the back-up software, are designed to have a high level of availability. The hardware often offers suitable options. Drives and media changers are available with a redundant power supply and redundant access paths. Modern operating systems such as AIX or Solaris can automatically use such redundantly designed access paths in the  event of a fault.

Grouping, pooling

Systems for the management of removable media must be able to deal with a great many media and facilitate access to many applications. In order to plan and execute access control in a sensible manner and to guarantee its effective use, it is necessary to combine cartridges and drives into groups. Grouping also allows budgets for storage capacities or drives to be created, which are made available to the applications. Scratch pools

A scratch pool contains unused cartridges that are available to authorized applications so that they can place volumes upon them. As soon as an application has placed a volume upon a cartridge from such a scratch pool, this cartridge is no longer available to all other applications and is thus removed from the scratch pool. If several scratch pools are available, they cannot only be grouped for different media, it is also possible to define groups for certain application purposes. For example, an administrator should be able to make a separate pool of cartridges available for important

back-up jobs, whilst cartridges from a different scratch pool are used for 'normal' back-up jobs. The back-up application can choose which pool it would like to have a cartridge from. If it is possible to assign priorities to scratch pools on the basis of which the management system can decide from which pool a new cartridge will be provided, then such a request can be automated to a certain degree without the back-up application

having to know all available pools. To this end, the request for a new tape must be given an appropriate priority, whereupon the management system searches for a cartridge from a scratch pool with a suitably high priority. In addition to the requirement that all cartridges can be located at all times, a management system for removable media should be capable of offering free storage space to an application at any time. Only thus can back-up windows actually be adhered to. As is the case for media tracking, in order to fulfil this requirement a high-availability solution

covering all levels, from the hardware to the application software, should be pursued. In addition, scratch pools can help to contain the cartridges from two or more libraries (Figure 9.6). They also offer the guarantee that, even in the event of the failure of indi- vidual libraries, cartridges in the other libraries will remain usable. It is precisely in this case that the advantages of a storage network, together with an intelligent management

system for removable media, fully come to bear in the optimal utilization of resources that are distributed throughout the entire system.

In order to be able to react flexibly to changes, scratch pools should be dynamically expandable. To this end, an administrator must make additional storage space available to the system dynamically, whether by the connection of a new library or by the addition of previously unused cartridges. Ideally, this can be achieved without making changes to the applications that have previously accessed the scratch pool. An adjustable minimum size (low water mark) makes the management of a scratch pool easier. If this threshold is reached, measures must be taken to increase the size of the

pool, as otherwise there is the danger that the system will cease to be able to provide free storage space in the foreseeable future. The management system can help here by flexibly offering more options. Many actions are possible here, from the automatic enlargement of the scratch pools – as long as free media are available in the libraries – to the 'call home' function, in which an administrator is notified. If a cartridge has several partitions, then it would occasionally be desirable to collect just the free partitions – rather than the complete cartridges – into a scratch pool. Then

it would be possible to manage free storage capacity with a finer granularity and thus achieve an optimal utilization of the total amount of available storage capacity. Since, owever, the individual partitions of a medium cannot be accessed at the same time, a cartridge is currently generally managed and allocated to an application as the smallest unit of a scratch pool. As capacity increases, however, the additional use of partitions for

this may also be required.

Drive pools

The mere fact that cartridges are available does not actually mean that the storage space can be used. In addition, drives must be available that can mount the cartridges for reading or writing. Similarly to cartridges, it is also possible to combine drives into pools. A pool of high- priority drives can, for example, always be kept to fulfil mount requests if all other drives are fully utilized. In order to save the applications from having to know and request all drive pools, it is a good idea to have a priority attribute that is used by the management system to automatically locate a drive with an appropriate priority. If several libraries are available, drive pools should include drives from several libraries (Figure 9.7). This ensures that drives are still available even if one library has failed. At the very least, this helps when writing new data if free cartridges are still available.