2. Understanding Journaling in FreeBSD

The journaling provided by GEOM in FreeBSD 7.X is not file system specific (unlike for example the ext3 file system in Linux®) but is functioning at the block level. Though this means it can be applied to different file systems, for FreeBSD 7.0-RELEASE, it can only be used on UFS2.

This functionality is provided by loading the geom_journal.ko module into the kernel (or building it into a custom kernel) and using the gjournal command to configure the file systems. In general, you would like to journal large file systems, like /usr. You will need however (see the following section) to reserve some free disk space.

When a file system is journaled, some disk space is needed to keep the journal itself. The disk space that holds the actual data is referred to as the data provider, while the one that holds the journal is referred to as the journal provider. The data and journal providers need to be on different partitions when journaling an existing (non-empty) partition. When journaling a new partition, you have the option to use a single provider for both data and journal. In any case, the gjournal command combines both providers to create the final journaled file system. For example:

The amount of disk space you need to reserve for the journal provider depends on the usage load of the file system and not on the size of the data provider. For example on a typical office desktop, a 1 GB journal provider for the /usr file system will suffice, while a machine that deals with heavy disk I/O (i.e. video editing) may need more. A kernel panic will occur if the journal space is exhausted before it has a chance to be committed.

Note:

The journal sizes suggested here, are highly unlikely to cause problems in typical desktop use (such as web browsing, word processing and playback of media files). If your workload includes intense disk activity, use the following rule for maximum reliability: Your RAM size should fit in 30% of the journal provider's space. For example, if your system has 1 GB RAM, create an approximately 3.3 GB journal provider. (Multiply your RAM size with 3.3 to obtain the size of the journal).

For more information about journaling, please read the manual page of gjournal(8).

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