CTS 2301 (Unix/Linux Administration I) Project #4
Working with Filesystems

 

Due: by the start of class on the date shown on the syllabus

Description:

This project assignment is designed to give you experience in creating, formatting, mounting, and unmounting Linux filesystems. 1  You will create a disk image file and mount it.  The commands you will use (and which you should lookup in the man pages) are dd, df, fsck, losetup (LOopback SETUP), mkfs, mount, umount, tune2fs, fuser, lsof, and the file /etc/fstab (also documented in the man pages).

Disk image files can be used to create bootable floppies and RAM disks. 2  An image of an entire CD (or of many CDs) can be stored on a hard disk.  (This is how CD duplicators or juke-boxes work.)

Make sure you keep an accurate system journal of any and all changes you make to your system!  You will need to turn this in, along with the answers to the questions asked below.

Requirements:

Answer the following questions and perform the following tasks:

  1. Create an empty image file, let's call it “myfs.img”.  Image files are regular files that hold filesystems.  They must be carefully created and formatted.  To create a 10 MB filesystem you must start by creating a 10 MB image file.  (Verify you have 10 MB of free disk space before starting this!  If not use a smaller count of say 1000.)  Use the dd command for this:

    dd if=/dev/zero of=/tmp/myfs.img bs=1k count=10000
  2. Attach the image file to a loopback device.  The various disk commands work with devices, not files.  So use /dev/loop0 (or another loopback device) which will be associated with the image file:

    losetup /dev/loop0 /tmp/myfs.img
  3. At this point the system will see /dev/loop0 as an unformatted disk.  Use the mkfs command to format it (and thus the image file) as an ext4 filesystem.  (The type for a CD would be iso for Linux and HSFS for Solaris.  Such image files can also be created and mounted, usually read-only though.)  This command is actually a common front-end for a family of related commands, one for each filesystem type.  What is the actual command ultimately invoked to create an ext4 filesystem?  (Use the man pages to determine this.)
        mkfs -t ext4 /dev/loop0 10000
    

    Examine the output.  Did the command succeed?  How many inodes were created?
  4. Now we are ready to mount our new filesystem.  The first step is to create a new directory to serve as its mount point.  Then the loop device previously associated with the image file (or any image file) can be mounted from the command line:
        mkdir /mnt/myfs
        mount -t ext4 /dev/loop0 /mnt/myfs
    
  5. Run the df command.  How much free space shows for your filesystem?  Why is this less than 10 MB?
  6. Change your current directory to the root of the new filesystem, using “cd /mnt/myfs”.  Run the command “ls”.  What files and/or directories were created automatically?  What is their purpose?
  7. Create a file on your filesystem using vi (or some other method):
        cd /mnt/myfs
        vi foo # add a few lines to the file
    

    Now run the df command again.  Is the output any different?  If so why?

    Finally, run the command “ls -li”.  What is the inode number of your new file (“foo”)?

  8. Next try to unmount the filesystem by running this command:
        umount /dev/loop0
    

    What happened?  You cannot unmount a filesystem if it is in use.  If the current working directory of any process (such as your shell) is any directory in that filesystem, or if any process has any file open from that filesystem (such as vi) then the device will be considered busy

    Use the commands fuser and lsof to see what files are in use and what processes are using them.  (See the man pages for details on these commands.)  Run the commands “fuser -cuv /mnt/myfs” and “lsof -Rw +D /mnt/myfs”.  What was the output?  What is the meaning of the options used?

  9. Try to unmount the filesystem again, after changing the current working directory to your home directory:
        cd
        umount /dev/loop0
    

    What happened this time?

  10. Run the fsck command on your filesystem.  The fsck command is actually a common front-end for a family of related commands, one for each type of file system.  What is the actual command that is run?  (Use the man pages to determine this.)

    Run this command and note the output:
        fsck -CVfp /dev/loop0
    
    What is the purpose (or meaning) of each of these options used with fsck?
  11. Now we will corrupt the image deliberately, and then re-run fsck.  There are many ways to corrupt the image, such as using a tool such as hexedit on the image file.  But it is hard to know exactly where to corrupt the image, in a way that matters and that can be repaired.

    Instead you will use a tool “debugfs” that can be used to examine and edit image files.  You will change the link count field of your new file from “1” to “2”.  You will need to know your file's inode number in order to run this command.  In the command line below, the inode number for the file foo was “13”.  If you inode number is different, use that instead.  Once you have unmounted the filesystem (the previous step), run this command (including the quotes and the angle-brackets):

        debugfs -wR 'sif <13> links_count 2' /dev/loop0
    

    (This command is part of the “e2fsprogs” package; you can install it as root with “yum install e2fsprogs”.)  Now re-run the same fsck command as before.  What was the output this time?

  12. The tune2fs command is used to examine and optionally modify the superblock of ext[234] filesystems.  Use the man page to determine how to use this command to list the default values used in your filesystem.  What are the default values for ext4 filesystems for:
    • max mount counts (how many mounts are allowed before a fsck check is forced)
    • interval between checks (How much time is allowed to pass before a fsck check is forced)
    (Note this information is also reported when the filesystem was created.)
  13. To break the association between your image file and the loop device use:
         losetup -d /dev/loop0
    
  14. To have your filesystem mounted automatically at boot time, you must edit the /etc/fstab file.  Each line of this file contains what to mount, the mount point (where to mount it), the mount options to use, and other information.  To automatically mount your filesystem at boot time add this entry to the /etc/fstab file:
        /tmp/myfs.img    /mnt/myfs     ext4   owner,loop  0 0
    
    What is the meaning of the fields and options used?  (Read the man page for fstab to find out.)  What is the name of the similar file for Solaris?  Are the fields the same in the Solaris file as for the Linux fstab file?  If not what are the differences?
  15. Try out your changes to fstab and verify they work:
        mount /mnt/myfs
    
    Note that even without the /etc/fstab line, the command
        mount -t ext4 -o loop /tmp/myfs.img /mnt/myfs
    
    will work.  In this case mount will automatically associate /tmp/myfs.img with an unused loop device, and umount will automatically break the connection.  (Earlier we merely took advantage of the fact that /dev/loop0 was already associated with /tmp/myfs.img, so different mount options were used.)

    Files in /tmp will eventually be automatically deleted, either during a reboot or from a cron job.  What will eventually happen if you don't remove the line from /etc/fstab?
  16. Suppose you had a large number of CDs (and a very large hard disk).  How can you make all the CD contents available on the system, as a jukebox?
  17. Using the techniques learned above, explore some of the image files on your Install/Live CDWhat are the pathnames of some of the image files?  Why might there be more than one image file?  (The file command can be helpful to determine the type of these image files, so you can use the right tools to examine them.)

To be turned in:

The answers and journal entries from the above list describing the steps you have taken to create and use a filesystem.

You can submit your project as email to .  Please see your syllabus for more information about submitting projects.

 


Footnotes:

  1. Although the concepts apply to Unix systems as well as Linux systems, some commands and kernel features (such as ramfs, ext4 tools, and loop devices) are different or not available on most Unix systems.  For Solaris, see the “lofiadm” command to work with the loop devices, “/dev/lofi/num”, where num is 1, 2, 3, ...      Back
  2. Creating a true RAM disk is rarely done anymore, except when booting (see “man initrd”).  This is because the virtual memory system is so efficient.  To create a real ram disk is easy provided you kernel is configured to include the “ramfs” filesystem.  Then:
            mkdir /mnt/ramdisk
            mount -t ramfs none /mnt/ramdisk
    
    That's it!  You can now create files there.  All files will be lost when the ram disk is unmounted.       Back