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OPS235 Lab 4 - CentOS6

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Logical Volume Management (Continued) and User / Group Management


In this lab you're going to learn how to:

  • Add a virtual hard disk and expand your vm's existing file system using LVM
  • Administer (add, remove, modify) users on a Linux system.
  • Save time while adding new users using a template of start-up files.
  • Create and manage groups on a Linux system.

Required Materials (Bring to All Labs)

  • CentOS 6.5 x86_64 Live DVD
  • CentOS 6.5 x86_64 Installation DVD1
  • SATA Hard Disk (in removable disk tray)
  • USB Memory Stick
  • Lab Logbook


Linux Command Online Reference

Each Link below displays online manpages for each command (via

LVM Information Utilities: LVM Management Utilities: Additional Utilities:

Resources on the web

Additional links to tutorials and HOWTOs: Please read this page to get an overview of LVM:

  • Logical Volume Management (Note: It is recommended to return to this guide as a reference when performing the next several investigations)

Storage Setup (prior to starting Lab 4)

At this point, having completed Lab 3 you should have roughly the following disk storage setup:
(Note: These results are output from the vgs, lvs, and pvs commands within each virtual machine. You will be using these commands in the following lab investigations)

  • centos1
 This VM has a GUI and no free space in the VG.

 VG         #PV #LV #SN Attr   VSize VFree
 vg_centos1   1   2   0 wz--n- 9.80G    0

 LV      VG         Attr   LSize  
 lv_root vg_centos1 -wi-ao   8.83G                                     
 lv_swap vg_centos1 -wi-ao 992.00M                                     

 PV         VG         Fmt  Attr PSize PFree
 /dev/vda2  vg_centos1 lvm2 a-   9.80G    0
  • centos2
 This VM has a GUI and free space in the VG.

 VG         #PV #LV #SN Attr   VSize VFree
 vg_centos2   1   4   0 wz--n- 19.51g 4.73g

 LV      VG         Attr   LSize   
 lv_archive vg_centos2 -wi-ao---- 1.00g                                            
 lv_home    vg_centos2 -wi-ao---- 4.00g                                            
 lv_root    vg_centos2 -wi-ao---- 7.81g                                            
 lv_swap    vg_centos2 -wi-ao---- 1.97g                                  

 PV         VG         Fmt  Attr PSize PFree
 /dev/vda2  vg_centos2 lvm2 a--  19.51g 4.73g
  • centos3
 This VM has no GUI installed.

 VG         #PV #LV #SN Attr   VSize VFree
 vg_centos3   2   3   0 wz--n- 16.50g    0 

 LV      VG         Attr   LSize  
 lv_home vg_centos3 -wi-ao----   1.00g                                            
 lv_root vg_centos3 -wi-ao----  14.54g                                            
 lv_swap vg_centos3 -wi-ao---- 992.00m                                    

 PV         VG         Fmt  Attr PSize PFree
 /dev/vda3  vg_centos3 lvm2 a--  14.51g    0
 /dev/vdb   vg_centos3 lvm2 a--   2.00g    0
  • c6host (i.e. "disk pack")
 /dev/sdc3        20G  3.7G   15G  20% /
 /dev/sdc2        29G  4.7G   23G  18% /home
 /dev/sdc1        97G   48G   45G  52% /var/lib/libvirt/images

Logical Volume Management (Continued)

Recovering VMs
Most of these investigations will take place in you virtual machines. If you make a significant mistake, your virtual machine may not boot. Remember that you created backups of your virtual machines in Lab 3, and you can restore them if something goes wrong.


Please read this page to get an overview of LVM:

Investigation 1: How are LVMs are managed using Command-Line Tools

  1. You are going to repeat the same LVM management operations (as your did with the centos2 VM in lab3), but you will using command-line tools in the centos3 VM. Since the centos3 VM only operates in command-line mode, you will need to refer to the "Logical Volume Management" link above.
  2. Write down the exact commands used at each step, and record appropriate command output:
    1. Determine the current LVM configuration using the pvs, vgs, and lvs command.
    2. Grow the home filesystem to 2G using the command lvextend and resize2fs.
    3. Create a new 2G LV containing an ext4 filesystem and mount it at /archive (use lvcreate, mkfs, mount, edit the file /etc/fstab, and then reboot to confirm automatic mount).
    4. Copy the contents of /etc into /archive.
    5. Shrink lv_archive to 1G (use umount, resize2fs, lvreduce, and mount)

Investigation 2: How can a PV be added to an existing VG using Command-Line Tools?

  1. Add an additional 2 GB virtual disk to your centos1 system, we will use it as an additional physical volume.
  2. Do the rest of this investigation in the command line.
  3. You should now have both /dev/vda and /dev/vdb.
  4. Record the size of the volume group and the amount of free space (Hint: use a command that you learned in a previous lab).
  5. Use fdisk to partition /dev/vdb with a single partition that fills the whole disk.
  6. Check the messages printed when fdisk exits -- you may need to reboot the system in order for the new partition table to take effect.
  7. Mark vdb1 as a physical volume usable by LVM.
  8. Add the new physical volume to the existing volume group.
  9. Verify that the volume group is larger and has more free space.
The next part of this investigation requires some research, thought, and creativity.

Using that additional space, create a separate filesystem for /home:

  1. Create the logical volume lv_home (1G ext4)
  2. Find a way to move the contents of /home onto it.
  3. Change your system configuration so that the new filesystem is mounted on /home from now on.
SELinux relabelling may be required
Your system may report that the files are not present in the new /home filesystem when they are clearly there. This is due to the Security-Enhanced Linux system (SELinux) preventing access to the files, because the files were tampered with (moved) and are no longer recognized as home directory content. You can fix this problem by restoring the file context labels so that SELinux accepts the files as valid home directory content: restorecon -r /home

Investigation 3: How can I use LVM to manage storage on my disk pack?

On your c6host (i.e. "disk pack"), using your choice of the GUI and/or command-line tools:

  1. Create a new logical partition (NOT a logical volume!) - minimum 5G, leaving at least 10G free space in the extended partition.
  2. Add that partition as a PV into the existing VG.
  3. Grow the root filesystem to fill the available space.

User/Group Management

Investigation 4: The /etc/passwd file

  1. Look at the /etc/passwd file.
  2. Make note of the contents of that file.
  3. Read about the file:
  4. Make sure you know what information each field contains.
  5. Why do you think there are so many users?
  6. Look at the names of the users. What do you think these user names represent? Are they people?
  7. What is the numeric user ID (UID) of the root user?
  8. The user IDs of real users (people) are different from the user IDs of system accounts. What is the pattern?

Investigation 5: Adding users

Use centos1
Perform these steps in the centos1 system.
  1. Read the man page for the useradd command.
  2. Create a new user account for each of your pod mates, using their learn account name as a user name. Give each user a password.
  3. Grep the /etc/passwd file for each of the new users.
    • What is the home directory of each user?
    • What group is each user in?
    • What else do you know about each user?
    • Where are the passwords stored?
  4. Look at the man page for /etc/shadow using the command man 5 shadow
    • Grep the /etc/shadow file for each of the new users.
    • Make note of this information.
  5. Create two new dummy users, ops235_1 and ops235_2.
  6. Investigate the home directory of one of your new users.
    • What files are there? Be sure to include hidden files.
    • What do you think these files are used for?
    • How does the operating system determine which files are created in a new home account? The answer can be found here:
    • Look at the files (including hidden files) in the template directory referred to in the article. Compare them to what is in a home directory for a new user. What do you notice?
    • Create a new file in this directory with the following command: touch foo
    • Create a new user named foobar, with the option to automatically create a home directory.
    • Look at the contents of foobar's home directory. What do you notice?
  7. Be sure to record your observations in your lab notes.
Use centos3
Perform these steps in the centos3 virtual machine.
  1. Add your matrix account user to centos3.

Investigation 6: Managing Groups

Use centos1
Perform these steps in the centos1 virtual machine.
  1. Read the man page for the groupadd and groupdel commands.
  2. Note which option allows you to set the Group ID number (GID) when you create a new group.
  3. Examine the file /etc/group
    • Which values of GID are reserved for system accounts?
    • Which values of GID are reserved for non-system user accounts?
    • What is the lowest available GID number for non-system users?
    • What is the default group name of a new user?
    • Add a new group named ops235 with a GID of 600.
    • You are angry at some irresponsible users on your system.
      • Add a new group named idiots.
      • Look at /etc/group and note the GID of idiots.
      • What GID is given to a new group if if you do not specify it?
      • Your anger has subsided. Delete the idiots group.
      • Look at /etc/group again and note the change.

Be sure to record your observations in your lab notes.

Investigation 7: Deleting users

Use centos1
Perform these steps in the centos1 virtual machine.
  1. Read the man page for the userdel command. Note which option automatically removes the users home directory when that user is deleted.
  1. Delete the user ops235_1 using the command userdel ops235_1
  2. Delete the user ops235_2 using the same command with the option which removes the home directory of the user.
  3. Check the contents of the /home directory. What do you notice?
  4. Check the contents of the /etc/group file. What do you notice?

Be sure to record your observations in your lab notes.

Investigation 8: Modifying users

Use centos1
Perform these steps in the centos1 virtual machine.
  1. Read the man page for the usermod command. Note which options change the user's full name, primary group, supplementary groups, and shell.
  1. Add each of your new users to the group ops235 (in other words, add ops235 to each user as a supplementary group).
  2. Examine /etc/group. What has changed?
  3. Use the usermod command to associate each of your pod mates' full name to their user name. With each change, examine their entries in the /etc/passwd file. What has changed?
  4. Be sure to record your observations in your lab notes.

Completing the lab

Time for a new backup!
If you have successfully completed this lab, make a new backup of your virtual machines. Remember to also make a backup of the new second virtual disk drive on centos1 -- you now have two virtual disks on centos1, and therefore two image files, and therefore will need two backup files.

Arrange proof of the following on the screen:

  1. Two PVs on centos1.
  2. Separate /home filesystem (on an LV) in centos1.
  3. Account created on centos3 matching your Matrix account.
  4. List contents of /etc/group file (ops235 group).
  5. List contents of /etc/passwd file (created accounts).
  6. Fresh backup of the virtual machines.

Preparing for the Quizzes

  1. What is a VG? PV? LV?
  2. What is the total size of the "main" VG on your system?
  3. How do you create a LV?
  4. How do you delete an LV?
  5. How would you add the disk partition /dev/sdb7 to your volume group "main"?
  6. How would you increase the size of the root filesystem by 50 MB?
  7. What is the purpose of /etc/fstab?
  8. What is the purpose of /etc/shadow?