Learn more on Network backup ,Server components(Job scheduler, Error handling, Metadata database, Media manager,)

Learn more on Network backup ,Server components(Job scheduler, Error handling, Metadata database, Media manager,)

 SERVER COMPONENTS

Back-up servers consist of a whole range of component parts. In the following we will discuss the main components:

ü  Job scheduler

ü  Error handler metadata database

ü  Media manager

 Job scheduler

The job scheduler determines what data will be backed up when. It must be carefully configured; the actual back-up then takes place automatically. With the aid of job schedulers and tape libraries many computers can be backed up overnight without the need for a system administrator to change tapes on site. Small tape libraries have a tape drive, a magazine with space for around ten tapes and a media changer that can automatically move the various tapes back and forth between magazine and tape drive. Large tape libraries have several dozen tape drives, space for several thousands of tapes and a media changer or two to insert the tapes in the drives.

 Error handling

If a regular automatic back-up of several systems has to be performed, it becomes difficult to monitor whether all automated back-ups have run without errors. The error handler helps to prioritize and filter error messages and generate reports. This avoids the situation in which problems in the back-up are not noticed until a back-up needs to be restored.

 Metadata database

The metadata database and the media manager represent two components that tend to be hidden. The metadata database is the brain of a network back-up system. It contains the following entries for every back-up up object: name, computer of origin, date of last change, date of last back-up, name of the back-up medium, etc. For example, an entry is made in the metadata database for every file to be backed up. The cost of the metadata database is worthwhile: in contrast to back-up tools provided by operating systems, network back-up systems permit the implementation of the incremental-forever strategy in which a file system is only fully backed up in the first back- up. In subsequent back-ups, only those files that have changed since the previous back-up are backed up. The current state of the file system can then be calculated on the back-up server from database operations from the original full back-up and from all subsequent incremental back-ups, so that no further full back-ups are necessary. The calculations in the metadata database are generally performed faster than a new full back-up. Even more is possible: if several versions of the files are backed up on the back-up server, a whole file system or a subdirectory dated three days ago, for example, can be restored (point-in-time restore) – the metadata database makes it possible.

 Media manager

Use of the incremental-forever strategy can considerably reduce the time taken by the back-up in comparison to the full back-up. The disadvantage of this is that over time the backed up files can become distributed over numerous tapes. This is critical for the

restoring of large file systems because tape mounts cost time. This is where the media  manager comes into play. It can ensure that only files from a single computer are located on one tape. This reduces the number of tape mounts involved in a restore process, which means that the data can be restored more quickly. A further important function of the media manager is so-called tape reclamation. As a result of the incremental-forever strategy, more and more data that is no longer needed is located on the back-up tapes. If, for example, a file is deleted or changed very frequently over time, earlier versions of the file can be deleted from the back-up medium. The gaps on the tapes that thus become free cannot be directly overwritten using current techniques. In tape reclamation, the media manager copies the remaining data that is still required from several tapes, of which only a certain percentage is used, onto a common new tape. The tapes that have thus become free are then added to the pool of unused tapes. There is one further technical limitation in the handling of tapes: current tape drives can

only write data to the tapes at a certain speed. If the data is transferred to the tape drive too slowly this interrupts the write process, the tape rewinds a little and restarts the write process. The repeated rewinding of the tapes costs performance and causes unnecessary wear to the tapes so they have to be discarded more quickly. It is therefore better to send the data to the tape drive quickly enough so that it can write the data onto the tape in one go (streaming). The problem with this is that in network back-up the back-up clients send the data to be

backed up via the LAN to the back-up server, which forwards the data to the tape drive. On the way from back-up client via the LAN to the back-up server there are repeated fluctuations in the transmission rate, which means that the streaming of tape drives is repeatedly interrupted. Although it is possible for individual clients to achieve streaming by additional measures (such as the installation of a separate LAN between back-up client and back-up server) (Section 7.7), these measures are expensive and technically not scalable at will, so they cannot be realized economically for all clients. The solution: the media manager manages a storage hierarchy within the back-up server. To achieve this, the back-up server must be equipped with hard disks and tape libraries. If a client cannot send the data fast enough for streaming, the media manager first of all stores the data to be backed up to hard disk. When writing to a hard disk it makes no

difference what speed the data is supplied at. When enough of the data to be backed up has been temporarily saved to the hard disk of the back-up server, the media manager automatically moves large quantities of data from the hard disk of the back-up server to its tapes. This process only involves recopying the data within the back-up server, so that streaming is guaranteed when writing the tapes. This storage hierarchy is used, for example, for the back-up of user PCs Many user PCs are switched off overnight, which means that back-up cannot be guaranteed  overnight. Therefore, network back-up systems often use the midday period to back up user PCs. Use of the incremental-forever strategy means that the amount of data to be backed up every day is so low that such a back-up strategy is generally feasible. All user PCs are first of all backed up to the hard disk of the back-up server in the time window from 11 : 15 to 13 : 45. The media manager in the back-up server then has a good twenty hours to move the data from the hard disks to tapes. Then the hard disks are once again free so that the user PCs can once again be backed up to hard disk in the next  midday break. In all operations described here the media manager checks whether the correct tape has been placed in the drive. To this end, the media manager writes an unambiguous signature to every tape, which it records in the metadata database. Every time a tape is inserted the media manager compares the signature on the tape with the signature in the metadata database. This ensures that no tapes are accidentally overwritten and that the correct data is written back during a restore operation. Furthermore, the media manager monitors how often a tape has been used and how old it is, so that old tapes are discarded in good time. If necessary, it first copies data that is still required to a new tape. Older tape media formats also have to be wound back and forwards now and then so that they last longer; the media manager can also automate the winding of tapes that have not been used for a long time. A further important function of the media manager is the management of data in a so-called off-site store. To this end, the media manager keeps two copies of all data to be backed up. The first copy is always stored on the back-up server, so that data can be quickly restored if it is required. However, in the event of a large-scale disaster (fire in the  data centre) the copies on the back-up server could be destroyed. For such cases the media manager keeps a second copy in an off-site store that can be several kilometres away. The media manager supports the system administrator in moving the correct tapes back and forwards between back-up server and off-site store. It even supports tape reclamation for tapes that are currently in the off-site store and it.