Expanding and Shrinking

I recommend making a backup prior to expanding or shrinking. If problems arise unexpectedly, a backup reduces the risk of data loss. You can expand an Ext4 filesystem in an LVM environment, such as in this example, directly in a single step using the lvextend command.

The prerequisite is that the corresponding LVM volume group still has enough free disk space. The -r parameter means that, after expanding the LVM logical volume, the Ext4 resize2fs command is executed and thus the filesystem grows. Expanding is also possible on the fly, that is, the filesystem can be mounted.

You can also shrink an Ext4 filesystem in an LVM environment in a single step using the lvreduce command. Again, the -r switch causes a resize2fs to run before shrinking the LVM logical volume. Shrinking is only possible offline; the filesystem must not be mounted:

umount /mnt/ext4fs
lvreduce -L -10G -r /dev/vg00/ext4fs

Customizing Ext4 with tune2fs

With the Ext4 tune2fs tool, you can view and tweak all the adjustable parameters of an Ext4 filesystem. The -l switch shows you the parameters. One point of interest is the reserved block count, which indicates how many blocks of the filesystem are reserved for files belonging to the root user. This information helps you ensure that the filesystem has room for writing to log files and other essential operations. By default, the reserve is five percent of the filesystem size, which makes sense for the root filesystem. For other uses, a smaller reserve is fine; on a filesystem for backups, you can set the reserve to 0 percent:

tune2fs -m 0 /dev/mapper/vg00-ext4fs

FUSE: Filesystem in Userspace

"Filesystems in Userspace" (FUSE) filesystems are an interesting option. FUSE lets users create filesystems that do without root privileges and kernel code. For a long time, this simple approach to managing a filesystem in userspace was not possible. Until FUSE hit the scene, filesystems had to be implemented in the kernel, with all the complexity that comes with kernel-level operations. FUSE is based on the kernel module fuse.ko, which fields virtual filesystem requests, and the libfuse library, which passes the requests on to the filesystem in userspace.

If you have an innovative idea for a new filesystem, you will have an easier time programming if you use FUSE for it. FUSE supports countless API bindings: from C, through Perl and Python, to Ruby. Before you start a new FUSE project, however, take a look at existing FUSE implementations – you might find someone has already solved the problem.