Some operating systems support filesystems other than UFS (or ext2fs). Journaling filesystems are a common alternative. The primary difference between UFS and journaling filesystems is in the way that they handle updates. With UFS, updates are made in-place. For example, when you change a file and save it to disk, the new data replaces the old data. When you remove a file, UFS updates the directory directly.
A journaling filesystem, on the other hand, writes updates to a separate journal, or log file. You can typically select whether to journal file changes, metadata changes, or both. A background process reads the journal during idle moments and applies the actual changes. Journaling filesystems typically recover much faster from crashes than UFS. After a crash, the filesystem simply reads the journal and commits all the outstanding changes.
The primary drawback of journaling filesystems is that they require additional disk writes. Changes are first written to the log and later to the actual files and/or directories. This is particularly relevant for web caches because they tend to have more disk writes than reads in the first place.
Journaling filesystems are available for a number of operating systems. On Linux, you can choose from ext3fs, reiserfs, XFS, and others. XFS is also available for SGI/IRIX, where it was originally developed. Solaris users can use the Veritas filesystem product. The TRU64 (formerly Digital Unix) Advanced Filesystem (advfs) supports journaling.
You can use a journaling filesystem without making any changes to Squid's configuration. Simply create and mount the filesystem as described in your operating system documentation. You don't need to change the cache_dir line in squid.conf. Use a command like this to make a reiserfs filesystem on Linux:
# /sbin/mkreiserfs /dev/sda2
For XFS, use:
# mkfs -t xfs -f /dev/sda2
Note that ext3fs is simply ext2fs with journaling enabled. Use the -j option to mke2fs when creating the filesystem:
# /sbin/mke2fs -j /dev/sda2
Refer to your documentation (e.g., manpages) for other operating systems.