Know basics of Back-up of file systems

Know basics of  Back-up of file systems

For the classical network back-up of file systems, back-up on different levels (block level, file level, file system image) has been discussed in addition to the incremental-forever strategy. The introduction of storage networks makes new methods available for the backup of file systems such as server-free back-up, application server-free back-up, LAN-free back-up, shared disk file systems and instant copies. The importance of the back-up of file systems is demonstrated by the fact that manufacturers of file systems are providing new functions specifically targeted at the acceleration of back-ups. In the following we introduce two of these new functions – the so-called archive bit and block level incremental back-up. The archive bit supports incremental back-ups at file level such as, for example, the incremental-forever strategy. One difficulty associated with incremental back-ups is finding out quickly which files have changed since the previous back-up. To accelerate this decision, the file system adds an archive bit to the metadata of each file: the network back-up system sets this archive bit immediately after it has backed a file up on the back-up server. Thus the archive bits of all files are set after a full back-up. If a file is altered, the file system automatically clears its archive bit. Newly generated files are thus not given an archive bit. In the next incremental back-up the network back-up system knows that it only has to back up those files for which the archive bits have been

cleared. The principle of the archive bit can also be applied to the individual blocks of a file system in order to reduce the cost of back-up on block level. In Section 7.4 a comparatively expensive procedure for back-up on block level was introduced: the cost of the copying and comparing of files by the back-up client is greatly reduced if the file system manages the quantity of altered blocks itself with the aid of the archive bit for blocks and the network back-up system can call this up via an interface. Unfortunately, the principle of archive bits cannot simply be combined with the principle of instant copies: if the file system copies uses instant copy to copy within the disk subsystem for back-up (Figure 7.11), the network back-up system sets the archive bit only on the copy of the file system. In the original data the archive bit thus remains cleared even though the data has been backed up. Consequently, the network back-up system backs this data up at the next incremental back-up because the setting of the archive bit has not penetrated through to the original data.