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Implementing UFS Journaling on a Desktop PC
Abstract
A journaling file system uses a log to record all transactions that take place in the file system, and preserves its integrity in the event of a system crash or power failure. Although it is still possible to lose unsaved changes to files, journaling almost completely eliminates the possibility of file system corruption caused by an unclean shutdown. It also shortens to a minimum the time required for after-failure file system checking. Although the UFS file system employed by FreeBSD does not implement journaling itself, the new journal class of the GEOM framework in FreeBSD 7._X_ can be used to provide file system independent journaling. This article explains how to implement UFS journaling on a typical desktop PC scenario.
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While professional servers are usually well protected from unforeseen shutdowns, the typical desktop is at the mercy of power failures, accidental resets, and other user related incidents that can lead to unclean shutdowns. Soft Updates usually protect the file system efficiently in such cases, although most of the times a lengthy background check is required. On rare occasions, file system corruption reaches a point where user intervention is required and data may be lost.
The new journaling capability provided by GEOM can greatly assist in such scenarios, by virtually eliminating the time required for file system checking, and ensuring that the file system is quickly restored to a consistent state.
This article describes a procedure for implementing UFS journaling on a typical desktop PC scenario (one hard disk used for both operating system and data). It should be followed during a fresh installation of FreeBSD. The steps are simple enough and do not require overly complex interaction with the command line.
After reading this article, you will know:
How to reserve space for journaling during a new installation of FreeBSD.
How to load and enable the `geom_journal` module (or build support for it in your custom kernel).
How to convert your existing file systems to utilize journaling, and what options to use in [.filename]#/etc/fstab# to mount them.
How to implement journaling in new (empty) partitions.
How to troubleshoot common problems associated with journaling.
Before reading this article, you should be able to:
Understand basic UNIX(R) and FreeBSD concepts.
Be familiar with the installation procedure of FreeBSD and the sysinstall utility.
The procedure described here is intended for preparing a new installation where no actual user data is stored on the disk yet. While it is possible to modify and extend this procedure for systems already in production, you should _backup_ all important data before doing so. Messing around with disks and partitions at a low level can lead to fatal mistakes and data loss.
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(R)) 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 [.filename]#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 [.filename]#/usr#. You will need however (see the following section) to reserve some free disk space.