Subject: Storage Network Management and Retrieval

Difficulty: Easy.

Marks: 05M

Journaling is a technique that guarantees consistency of the file system even after a system crash. Before creating any changes within the file system, it initially writes each and every modification to log file that is invisible to applications or end users. If a system crash occurs, the file system solely has to run through the log file entry in order to recreate the filing system consistency..

In file systems without journaling – basically older file systems such as Microsoft’s FAT32 file system or the UNIX file system (UFS) that is widespread in UNIX systems – the consistency of the entire file system should be checked after a system crash. On the other hand, in large file systems this may consume several hours. In file systems without journaling, it will take several hours after a system crash which depends on the dimensions of the file system before the data and applications are back in operations.

Snapshots

Snapshots freeze the state of a file system at a given point of time. Applications end users access the frozen copy via a special path. As is the case for fast copies, the creation of the copy solely takes some seconds.

Similarly, while taking snapshots, care should to be taken to make sure that the state of the frozen knowledge/data is consistent. Table I compares the instant copies and snapshots. The most important advantage of snapshots is that they are compatible to any hardware. On the other hand instant copies within disk subsystems place less load on PC machine and also the buses of the server, thus using more system resources for the actual applications.

Table 1 snapshot are hardware independent but they load the server’s central processing unit.

Snapshots Example

Figure 3 shows three example snapshots of the ‘/data1’ file system within the ‘datavg’ volume cluster.

As seen in the figure, the file system snapshots are often a powerful resolution to form stable backups from a running system, that is, the system keeps modifying the information. Another choice to use snapshots is that it allows access to information from earlier time.