19.8. UFS Journaling Through GEOM

Beginning with FreeBSD 7.0, support for journals on UFS file systems is available. The implementation is provided through the GEOM subsystem and is configured using gjournal. 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.

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. For better performance, the journal may be stored on another disk. In this configuration, the journal provider or storage device should be listed after the device to enable journaling on.

The GENERIC kernel provides support for gjournal. To automatically load the geom_journal.ko kernel module at boot time, add the following line to /boot/loader.conf:

geom_journal_load="YES"

If a custom kernel is used, ensure the following line is in the kernel configuration file:

options	GEOM_JOURNAL

Once the module is loaded, a journal can be created on a new file system using the following steps. In this example, da4 is a new SCSI disk:

# gjournal load
# gjournal label /dev/da4

This will load the module and create a /dev/da4.journal device node on /dev/da4.

A UFS file system may now be created on the journaled device, then mounted on an existing mount point:

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

Journaling may also be enabled on current file systems by using tunefs. However, always make a backup before attempting to alter an existing 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. Refer to gjournal(8) and tunefs(8) for more information about these commands.

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

All FreeBSD documents are available for download at http://ftp.FreeBSD.org/pub/FreeBSD/doc/

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