UNIX Filesystems: Evolution, Design, and Implementation

Filesystem Switchout Commands

Many of the commands that apply to filesystems may require filesystem specific processing. For example, when creating a new filesystem, each different filesystem may support a wide range of options. Although some of these options will be common to most filesystems, many may not be.

To support a variety of command options, many of the filesystem-related commands are divided into generic and filesystem dependent components. For example, the generic mkfs command that will be described in the next section, is invoked as follows:

# mkfs -F vxfs -o …

The -F option (-t on Linux) is used to specify the filesystem type. The -o option is used to specify filesystem-specific options. The first task to be performed by mkfs is to do a preliminary sanity check on the arguments passed. After this has been done, the next job is to locate and call the filesystem specific mkfs function.

Take for example the call to mkfs as follows:

# mkfs -F nofs /dev/vx/rdsk/myvol
mkfs: FSType nofs not installed in the kernel

Because there is no filesystem type of nofs, the generic mkfs command is unable to locate the nofs version of mkfs. To see how the search is made for the filesystem specific mkfs command, consider the following:

# truss -o /tmp/truss.out mkfs -F nofs /dev/vx/rdsk/myvol
mkfs: FSType nofs not installed in the kernel
# grep nofs /tmp/truss.out execve(“/usr/lib/fs/nofs/mkfs”, 0x000225C0, 0xFFBEFDA8) Err#2 ENOENT execve(“/etc/fs/nofs/mkfs”, 0x000225C0, 0xFFBEFDA8) ...