Learn more on How to BACK-UP clients , (PERFORMANCE GAINS AS A RESULT OF NETWORK BACK-UP Application-specific performance bottlenecks)

Learn more on How to BACK-UP clients , (PERFORMANCE GAINS AS A RESULT OF NETWORK BACK-UP Application-specific performance bottlenecks)

BACK-UP CLIENTS

A platform-specific client (back-up agent) is necessary for each platform to be backed up. The base client can back up and archive files and restores them if required. The term platform is used here to mean the various operating systems and the file systems that they support. Furthermore, some base clients offer HSM for selected file systems. The back-up of file systems takes place at file level as standard. This means that each changed file is completely retransferred to the server and entered there in the metadata database. By using back-up at volume level and at block level it is possible to change the granularity of the objects to be backed up. When back-up is performed at volume level, a whole volume is backed up as an

individual object on the back-up server. We can visualize this as the output of the Unix command 'dd' being sent to the back-up server. Although this has the disadvantage that free areas, on which no data at all has been saved, are also backed up, only very few metadata database operations are necessary on the back-up server and on the client side it is not necessary to spend a long time comparing which files have changed since the last

back-up. As a result, back-up and restore operations can sometimes be performed more quickly at volume level than they can at file level. This is particularly true when restoring large file systems with a large number of small files. Back-up on block level optimizes back-up for members of the external sales force, who only connect up to the company network now and then by means of a laptop via a dial-up line. In this situation the performance bottleneck is the low transmission capacity of modem or ISDN connections. If only one bit of a large file is changed, the whole file

must once again be forced down the dial-up connection. When backing up on block level the back-up client additionally keeps a local copy of every file backed up. If a file has changed, it can establish which parts of the file have changed. The back-up client sends only the changed data fragments (blocks) to the back-up server. This can then reconstruct the complete file. As is the case for back-up on file level, each file backed up is entered

in the metadata database. Thus, when backing up on block level the quantity of data to be transmitted is reduced at the cost of storage space on the local hard disk. In addition to the standard client for file systems, most network back-up systems pro- vide special clients for various applications. For example, there are special clients for MS Exchange or Lotus Domino that make it possible to back up and restore individual documents. We will discuss the back-up of file systems and NAS servers  and databases (Section 7.10) in more detail later on.

PERFORMANCE GAINS AS A RESULT OF NETWORK BACK-UP

The underlying hardware components determine the maximum throughput of network back-up systems. The software components determine how efficiently the available hard- ware is actually used. At various points of this chapter we have already discussed how network back-up systems can help to better utilize the existing infrastructure:

• Performance increase by the archiving of data: deleting data that has already been archived from hard disks can accelerate the daily back-up because there is less data to back up. For the same reason, file systems can be restored more quickly.

• Performance increase by hierarchical storage management (HSM): by moving file contents to the HSM server, file systems can be restored more quickly. The directory entries of files that have been moved can be restored comparatively quickly; the majority of the data, namely the file contents, do not need to be fetched back from the HSM server.

• Performance increase by the incremental-forever strategy: after the first back-up, only the data that has changed since the last back-up is backed up. On the back-up server the metadata database is used to calculate the latest state of the data from the first back-up and all subsequent incremental back-ups, so that no further full back-ups are necessary. The back-up window can thus be significantly reduced.

• Performance increase by reducing tape mounts: the media manager can ensure that data that belongs together is only distributed amongst a few tapes. The number of time-consuming tape changes for the restoring of data can thus be reduced.

• Performance increase by streaming: the efficient writing of tapes requires that the data is transferred quickly enough to the tape drive. If this is not guaranteed the back-up server can first temporarily store the data on a hard drive and then send the data to the tape drive in one go.

• Performance increase by back-up on volume level or on block level: as standard, file systems are backed up on file level. Large file systems with several hundreds of thousands of files can sometimes be backed up more quickly if they are backed up at volume level. Laptops can be backed up more quickly if only the blocks that have changed are transmitted over the modem to the back-up server.