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# 20.8 UFS Journaling Through GEOM

Beginning with FreeBSD 7.0, support for UFS journals is available. The implementation is provided through the GEOM subsystem and is configured using gjournal(8).

Journaling stores a log of file system transactions, such as changes that make up a complete disk write operation, before meta-data and file writes are committed to the disk. This transaction log can later be replayed to redo file system transactions, preventing file system inconsistencies.

This method provides another mechanism to protect against data loss and inconsistencies of the file system. Unlike Soft Updates, which tracks and enforces meta-data updates, and snapshots, which create an image of the file system, a log is stored in disk space specifically for this task, and in some cases, may be stored on another disk entirely.

Unlike other file system journaling implementations, the `gjournal` method is block based and not implemented as part of the file system. It is a GEOM extension.

To enable support for `gjournal`, the FreeBSD kernel must have the following option which is the default on FreeBSD 7.0 and later:

    options	UFS_GJOURNAL

If journaled volumes need to be mounted during startup, the `geom_journal.ko` kernel module needs to be loaded, by adding the following line to `/boot/loader.conf`:

    geom_journal_load="YES"

Alternatively, this function can be built into a custom kernel, by adding the following line in the kernel configuration file:

    options	GEOM_JOURNAL

Creating a journal on a free file system may now be done using the following steps. In this example, `da4` is a new SCSI disk:

    # gjournal load
    # gjournal label /dev/da4

At this point, there should be a `/dev/da4` device node and a `/dev/da4.journal` device node. A file system may now be created on this device:

    # newfs -O 2 -J /dev/da4.journal

This command will creates a UFS2 file system on the journaled device.

`mount` the device at the desired point with:

    # mount /dev/da4.journal /mnt

> Note: In the case of several slices, a journal will be created for each individual slice. For instance, if `ad4s1` and `ad4s2` are both slices, then `gjournal` will create `ad4s1.journal` and `ad4s2.journal`.

For better performance, the journal may be kept on another disk. In this configuration, the journal provider or storage device should be listed after the device to enable journaling on. Journaling may also be enabled on current file systems by using `tunefs`. However, always make a backup before attempting to alter a file system. In most cases, `gjournal` will fail if it is unable to create the journal, but this does not protect against data loss incurred as a result of misusing `tunefs`.

It is also possible to journal the boot disk of a FreeBSD system. Refer to the article Implementing UFS Journaling on a Desktop PC for detailed instructions.

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