Section 9.10. Building a Simple File System

9.10. Building a Simple File System

It is straightforward to build a simple file system image. Here we demonstrate the use of the Linux kernel's loopback device. The loopback device enables the use of a regular file as a block device. In short, we build a file system image in a regular file and use the Linux loopback device to mount that file in the same way any other block device is mounted.

To build a simple root file system, start with a fixed-sized file containing all zeros:

# dd if=/dev/zero of=./my-new-fs-image bs=1k count=512

This command creates a file of 512KB containing nothing but zeros. We fill the file with zeros to aid in compression later and to have a consistent data pattern for uninitialized data blocks within the file system. Use caution with the dd command. Executing dd with no boundary (count=) or with an improper boundary can fill up your hard drive and possibly crash your system. dd is a powerful tool; use it with the respect it deserves. Simple typos in commands such as dd, executed as root, have destroyed countless file systems.

When we have the new image file, we actually format the file to contain the data structures defined by a given file system. In this example, we build an ext2 file system. Listing 9-20 details the procedure.

Listing 9-20. Creating an ext2 File System Image

# /sbin/mke2fs ./my-new-fs-image mke2fs 1.35 (28-Feb-2004) ./my-new-fs-image is not a block special device. Proceed anyway? (y,n) y Filesystem label= OS type: Linux Block size=1024 (log=0) Fragment size=1024 (log=0) 64 inodes, 512 blocks 25 blocks (4.88%) reserved for the super user First data block=1 1 block group 8192 blocks per group, 8192 fragments per group 64 inodes per group Writing inode tables: done Writing superblocks and filesystem accounting information: done This filesystem will be automatically checked every 24 mounts or 180 days, whichever comes first.  Use tune2fs -c or -i to override. #

As with dd, the mke2fs command can destroy your system, so use it with care. In this example, we asked mke2fs to format a file rather than a hard drive partition (block device) for which it was intended. As such, mke2fs detected that fact and asked us to confirm the operation. After confirming, mke2fs proceeded to write an ext2 superblock and file system data structures into the file. We then can mount this file like any block device, using the Linux loopback device:

# mount -o loop ./my-new-fs-image /mnt/flash

This command mounts the file my-new-fs-image as a file system on the mount point named /mnt/flash. The mount point name is not important; you can mount it wherever you want, as long as the mount point exists. Use mkdir to create your mount point.

After the newly created image file is mounted as a file system, we are free to make changes to it. We can add and delete directories, make device nodes, and so on. We can use tar to copy files into or out of it. When the changes are complete, they are saved in the file, assuming that you didn't exceed the size of the device. Remember, using this method, the size is fixed at creation time and cannot be changed.