Linux Sysadmin

A Simple Introduction to Working with LVM (mirror)

Note: I refer to this article all the time, so I’ve decided to mirror it here on my site. It comes from the now-defunct Debian Administration website.

A simple introduction to working with LVM

Posted by Steve on Wed 28 Jun 2006 at 21:22

The logical volume manager allows you to create and manage the storage of your servers in a very useful manner; adding, removing, and resizing partitions on demand. Getting started with LVM can be a little confusing to newcomer so this guide intends to show the basics in a simple manner.

There several pieces of terminology that you’ll need to understand to make the best use of LVM. The most important things you must know are:

  • physical volumes
    • These are your physical disks, or disk partitions, such as /dev/hda or /dev/hdb1. These are what you’d be used to using when mounting/unmounting things. Using LVM we can combine multiple physical volumes into volume groups.
  • volume groups
    • A volume group is comprised of real physical volumes, and is the storage used to create logical volumes which you can create/resize/remove and use. You can consider a volume group as a “virtual partition” which is comprised of an arbitary number of physical volumes.
  • logical volumes
    • These are the volumes that you’ll ultimately end up mounting upon your system. They can be added, removed, and resized on the fly. Since these are contained in the volume groups they can be bigger than any single physical volume you might have. (ie. 4x5Gb drives can be combined into one 20Gb volume group, and you can then create two 10Gb logical volumes.)

Logically these are stacked from top to bottom like this:

Dia source file

Creating A Volume Group

To use LVM you need to take at least one partition, initialise it for use with LVM and then include it in a volume group. Why would you do this? Well it would let you create new partitions on the fly, and make better use of your space.

In my case I have a laptop with the following setup:

    Name        Flags      Part Type  FS Type          [Label]        Size (MB)
    hda1        Boot        Primary   Linux ext3       [/]              8000.01 
    hda2                    Primary   Linux swap / Solaris              1000.20
    hda3                    Primary   Linux                            31007.57

Here I have a 7Gb root partition which contains my Debian GNU/Linux installation. I also have a 28Gb partition which will be used by LVM. I’ve chosen this setup so that I can create a dedicated /home partition using LVM – and if I need more space I can extend it.

In this example hda1, hda2, and hda3 are all physical volumes. We’ll initialize hda3 as a physical volume:

root@lappy:~# pvcreate /dev/hda3

If you wanted to combine several disks, or partitions you could do the same for those:

root@lappy:~# pvcreate /dev/hdb
root@lappy:~# pvcreate /dev/hdc

Once we’ve initialised the partitions, or drives, we will create a volume group which is built up of them:

root@lappy:~# vgcreate skx-vol /dev/hda3

Here “skx-vol” is the name of the volume group. (If you wanted to create a single volume spanning two disks you’d run “vgcreate skx-vol /dev/hdb /dev/hdc“.)

If you’ve done this correctly you’ll be able to see it included in the output of vgscan:

root@lappy:~# vgscan
  Reading all physical volumes.  This may take a while...
  Found volume group "skx-vol" using metadata type lvm2

Now that we have a volume group (called skx-vol) we can actually start using it.

Working with logical volumes

What we really want to do is create logical volumes which we can mount and actually use. In the future if we run out of space on this volume we can resize it to gain more storage. Depending on the filesystem you’ve chosen you can even do this on the fly!

For test purposes we’ll create a small volume with the name ‘test’:

root@lappy:~# lvcreate -n test --size 1g skx-vol
Logical volume "test" created

This command creates a volume of size 1Gb with the name test hosted on the LVM volume group skx-vol.

The logical volume will now be accessible via /dev/skx-vol/test, and may be formatted and mounted just like any other partition:

root@lappy:~# mkfs.ext3 /dev/skx-vol/test
root@lappy:~# mkdir /home/test
root@lappy:~# mount /dev/skx-vol/test  /home/test

Cool, huh?

Now we get onto the fun stuff. Let us pretend that the test partition is full and we want to make it bigger. First of all we can look at how big it is at the moment with lvdisplay:

root@lappy:~# lvdisplay 
  --- Logical volume ---
  LV Name                /dev/skx-vol/test
  VG Name                skx-vol
  LV UUID                J5XlaT-e0Zj-4mHz-wtET-P6MQ-wsDV-Lk2o5A
  LV Write Access        read/write
  LV Status              available
  # open                 0
  LV Size                1.00 GB
  Current LE             256
  Segments               1
  Allocation             inherit
  Read ahead sectors     0
  Block device           254:0

We see it is 1Gb in size (no suprise really!) before we go on to resize the volume remember we should unmount it first:

root@lappy:~# umount  /home/test/
root@lappy:~# lvextend -L+1g /dev/skx-vol/test 
Extending logical volume test to 2.00 GB
Logical volume test successfully resized

(It is possible to resize ext3 filesystems whilst they’re mounted, but I’d still suggest doing it offline as that is less scary.)

Looking at lvdisplay again we can see the volume was resized:

root@lappy:~# lvdisplay 
  --- Logical volume ---
  LV Name                /dev/skx-vol/test
  VG Name                skx-vg
  LV UUID                uh7umg-7DqT-G2Ve-nNSX-03rs-KzFA-4fEwPX
  LV Write Access        read/write
  LV Status              available
  # open                 0
  LV Size                2.00 GB
  Current LE             512
  Segments               1
  Allocation             inherit
  Read ahead sectors     0
  Block device           254:0

The important thing to realise is that although the volume has been resized the ext3 filesystem on it has stayed unchanged. We need to resize the filesystem to actually fill the volume:

root@lappy:~# e2fsck -f /dev/skx-vol/test 
root@lappy:~# resize2fs /dev/skx-vol/test

Remount the logical volume and you’ll discover it is now only half full instead of completely full!

If you get bored of the volume and its contents you can remove it with the lvremove command:

root@lappy:~# lvremove /dev/skx-vol/test
Do you really want to remove active logical volume "test"? [y/n]: y
Logical volume "test" successfully removed

Other userful commands include lvrename to change the name, and lvreduce to reduce the size of a volume.

Mounting Logical Volumes

In the previous section we showed how you could mount a logical volume, with a command like this:

mount /dev/skx-vol/test  /home/test

If you want your partition to be mounted at boot-time you should update your /etc/fstab to contain an entry like this:

/dev/skx-vol/home    /home       ext3  noatime  0 2
/dev/skx-vol/backups /backups    ext3  noatime  0 2


If you’re worried about losing details about your volumes in the event of problems do not worry. The current state of the LVM setup upon your machine is maintained in the event of errors.

Running pvdisplay will allow you to see which physical volume(s) make up your volume group. In the case of our example we only used /dev/hda3, but if you’re using more volumes it might be useful to take a look at them with the pvdisplay command:

root@lappy:~# pvdisplay 
  --- Physical volume ---
  PV Name               /dev/hda3
  VG Name               skx-vol
  PV Size               28.88 GB / not usable 0   
  Allocatable           yes 
  PE Size (KByte)       4096
  Total PE              7392
  Free PE               5280
  Allocated PE          2112
  PV UUID               WyXQtL-OdT6-GnGd-edKF-tjRU-hoLA-RJuQ6x

If we ever lost this information we could find it contained in the file /etc/lvm/backup/skx-vol.

Similarly if we wanted to know which logical volumes we’d created we could example the directory /etc/lvm/archive. This contains numbered files containing backups of the operations we’ve conducted.

As an example we created the “test” volume, which we went on to resize. Here is the first section of /etc/lvm/archive/

# Generated by LVM2: Sat Jun 10 12:35:57 2006

contents = "Text Format Volume Group"
version = 1

description = "Created *before* executing 'lvcreate -n test --size 1g skx-vg'"

creation_host = "lappy" 
# Linux lappy 2.6.8-2-686 #1 Sat Jan 8 16:50:08 EST 2005 i686


When it comes to using LVM effectively it is worth considering the filesystem that you wish to use upon your logical volumes.

If you choose a filesystem which doesn’t support resizing then increasing the size of your LVM volumes would be pointless. Here is a brief list of the resizable filesystems:

filesystem      increase while mounted  increase while unmounted  decrease

ext2fs          yes                     yes                       yes
ext3fs          yes                     yes                       yes
ReiserFS        yes                     yes                       yes
JFS             no                      no                        no
XFS             yes                     no                        no

Note that some filesystems can be increased in size, but cannot be reduced.

If I’ve missed one you’re familiar with please do let me know.

Closing Comments

If you’re ready to make the jump to LVM and don’t have a lot of space handy for allocating to LVM then it might make sense to reinstall your system. The Debian installer has excellent support for creating LVM setups.

We’ve not really covered advanced usage in this introduction but there is a lot of readable and useful documentation available if you’re prepared to search for it. The most obvious starting point is the LVM howto.

Linux Tech

DNS-based mitigation for Samsung SwiftKey keyboard vulnerability

I was just listening to the discussion of the Samsung SwiftKey keyboard vulnerability from Security Now! episode 513, and I came up with a simple DNS-based mitigation that a user could implement to protect themselves.

The Vulnerability

Without any user interaction, the user’s phone makes a plaintext http GET request to a SwiftKey update server, and this request can be hijacked and malicious code injected into the phone by any man-in-the-middle bad actor. According to NowSecure, the discoverer of the vulnerability, the request looks like this:


DNS-based Mitigation

With a rooted Android phone, a user could edit their /etc/hosts file to redirect the hostname of the update server ( to localhost, preventing the http GET request from ever leaving the phone. In other words, the user is hijacking the request to the update server before a bad guy gets the opportunity to do the same.

With a non-rooted phone, there are DNS Resolver apps that can be installed that do the same kind of redirection to localhost.

Will this kind of mitigation work? Since I don’t have an Android phone to test against, this is just a thought experiment for myself.

Linux Python Sysadmin

Backup Your Dropbox Files with rdiff-backup

The Problem

Teresa and I use a single Dropbox account to share files between our computers. I also use the same account to store (and sync) plain-text notes on my iPad and iPhone (I use the apps PlainText and iA Writer). In case things go wrong with these apps, the syncing, or with Dropbox itself, I want to backup my Dropbox files and keep past snapshots of the backups so I can go back in time.

The Solution

rdiff-backup can do this. It is a command-line tool written in Python that:

…backs up one directory to another, possibly over a network. The target directory ends up a copy of the source directory, but extra reverse diffs are stored in a special subdirectory of that target directory, so you can still recover files lost some time ago. The idea is to combine the best features of a mirror and an incremental backup.

To make this all happen, I have Dropbox installed, signed-in, and running on my Linux desktop/server, which runs Ubuntu 11.04 Natty with Gnome 2.

Install rdiff-backup thusly:

    # aptitude install rdiff-backup

I use the directory /backup/ to hold all my backup targets, so I can run rdiff-backup like this:

    $ rdiff-backup  \
        --exclude $HOME/Dropbox/.dropbox \
        --exclude $HOME/Dropbox/.dropbox.cache \
        $HOME/Dropbox /backup/Dropbox

Every time I run rdiff-backup like this, it creates a new snapshot of my Dropbox files. Old snapshots are kept until I decide to purge them (if at all). To purge any snapshots older than two months, for example, I run this command:

    $ rdiff-backup --force --remove-older-than 2M /backup/Dropbox

I run the above two commands in an @hourly crontab script to keep this all happening automatically.

Browsing Past Snapshots

rdiff-backup has its own commands for digging into the files stored in the past snapshots, but it requires exactly knowing the filenames and backup times. Another tool, rdiff-backup-fs solves this problem by mounting the rdiff-backup backup directory as a FUSE filesystem, allowing me to grep and find my way through a directory tree of all snapshots.

After installing FUSE and rdiff-backup-fs, I mount my Dropbox snapshot tree with this command:

    $ rdiff-backup-fs ~/mnt /backup/Dropbox

Note that the order of the arguments for mounting source and target are backwards compared to the canonical mount command.

A long listing of my 10-oldest snapshots looks like this:

    $ ls -lF ~/mnt/ | head -10
    total 0
    dr-xr-xr-x 1 root root 4096 2013-03-10 13:52 2013-01-06T05:00:01/
    dr-xr-xr-x 1 root root 4096 2013-03-10 13:52 2013-01-06T06:00:01/
    dr-xr-xr-x 1 root root 4096 2013-03-10 13:52 2013-01-06T07:00:01/
    dr-xr-xr-x 1 root root 4096 2013-03-10 13:52 2013-01-06T08:00:01/
    dr-xr-xr-x 1 root root 4096 2013-03-10 13:52 2013-01-06T09:00:01/
    dr-xr-xr-x 1 root root 4096 2013-03-10 13:52 2013-01-06T10:00:01/
    dr-xr-xr-x 1 root root 4096 2013-03-10 13:52 2013-01-06T11:00:01/
    dr-xr-xr-x 1 root root 4096 2013-03-10 13:52 2013-01-06T12:00:01/
    dr-xr-xr-x 1 root root 4096 2013-03-10 13:52 2013-01-06T13:00:01/

I can then explore all my snapshots at once with any tools wish.

When done, I unmount the rdiff-backup-fs filesystem with:

    $ /bin/fusermount -u ~/mnt

Which editor should I learn?

On, Rory McCann asked, “What’s the best terminal editor to suggest to a Unix newbie? i.e. not vi or Emacs.”

This answer, which purposefully ignores the original poster’s restriction, says it best:

My take is still Emacs or vi. Even for a beginner.


Because time invested in learning an editor is productive only as long as you keep using that editor. All those less expressive options are poor choices for the long run, and will be abandoned eventually. At which point the time spent learning them is wasted, and the user still has to learn Emacs or vi.

In other words, the best (most expressive) tool for the job is one of Emacs or vi, and so you’ll eventually switch to one of them. It ultimately doesn’t matter which one you choose, but you would be smart to invest yourself into learning one of them.

For the record, I’m a vim user, and I love using it.


My First Bug Report

I recently submitted my first bug report to the Debian Project, regarding mod_dav and apache2. It was accepted by the maintainers of the relevant packages, and they’re taking the necessary steps to fix it.

So I’m proud of my little contribution. 🙂 Yay for me! Yay for Debian!

Linux Python Sysadmin

An awesome blog about Python, Linux, and System Administration

I recently discovered an awesome blog about Python, Linux, and System Administration. Chris Siebenmann is a sysadmin for the University of Toronto Unix Systems Group. I could spend hours reading his archives. Smart guy. Articles are short and informative to the max. Hope you enjoy it:

Linux Sysadmin

MTR as a combined traceroute and ping tool

mtr combines the functionality of the traceroute and ping programs in a single network diagnostic tool.” And it’s really cool.

Like ping, it sends “echo” packets from your machine to the target machine to measure latency and packet loss along the network path, but it continuously displays updated statistics in real time as it operates.

Like traceroute, it shows the names or IP addresses of each machine along the network path, also updating these statistics for each machine.

Here’s some (frozen) sample output from the ncurses mode (terminal mode) mtr:

                                My traceroute  [v0.71]
exobox (                                             Thu Dec 21 16:15:01 2006
Keys:  Help   Display mode   Restart statistics   Order of fields   quit
                                             Packets               Pings
 Host                                      Loss%   Snt   Last   Avg  Best  Wrst StDev
 1.                                         0.0%    20    0.4   0.3   0.2   0.7   0.1
 2.                                         0.0%    20    1.6   1.4   1.0   2.7   0.4
 3.                                         0.0%    19    1.3   1.3   1.0   2.7   0.4
 4.                               0.0%    19    1.1   1.2   1.0   2.7   0.4
 5.                           0.0%    19    1.4   2.6   1.2  16.0   3.3
 6.             0.0%    19    1.4   1.6   1.4   2.5   0.3
 7.                           0.0%    19  199.3  12.0   1.3 199.3  45.4
 8.                             0.0%    19    1.7  35.7   1.3 186.5  56.2
 9.                           0.0%    19    1.8   2.0   1.5   4.8   0.8
10.                            0.0%    19  284.6 292.2 278.3 305.8   8.7
11.        15.8%    19  286.2 291.9 278.6 325.2  11.5
12.        15.8%    19  313.2 323.8 311.3 340.5   8.6
13.          5.3%    19  319.6 331.1 314.1 404.6  20.3
14.         11.1%    19  696.0 718.7 693.3 797.2  33.7
15.         36.8%    19  347.4 354.9 342.5 368.7   8.6
16.        11.1%    19  421.5 423.6 411.1 438.2   7.8
17.        11.1%    19  433.9 438.9 422.4 514.1  22.0
18.                     33.3%    19  423.1 432.7 418.7 463.0  11.4
19.        27.8%    19  405.8 414.7 398.2 434.6  11.6
20.                         23.5%    18  404.8 418.1 399.9 485.8  21.9

Looking at the Avg column (units in ms), the above output shows that my network packets pass through Beijing Telecom’s routers to the U.S., then to the U.K., and finally to their destination in the Netherlands. A large latency increase occurs between lines 9 and 10 (presumably leaving P.R. China), and another between lines 13 and 16 (U.S. to U.K.).

mtr has some interesting display modes besides the above, where it shows the latency of each packet graphically according to a dynamic scale. In this way, the above points of really large latency can be easily detected.

mtr can be obtained from the mtr website, or it can be installed in Debian/Ubuntu by:

# apt-get install mtr-tiny


# apt-get install mtr

for the ncurses or X11 versions, respectively, although mtr-tiny appears to be installed by default in the Debian and Ubuntu machines I have tested. So you may already have it.


Ubuntu Open Week Begins Tonight

If you’re curious or interested in getting involved in the Ubuntu community, “Ubuntu Open Week” begins tonight. It’s “a week of IRC tutorials and sessions designed to encourage more and more people to join our diverse community”. More information is here:

The times in the calendar are UTC, so just add 8 hours for China. For example, the first session on Monday on the “Ubuntu Desktop Team – Sebastien Bacher” at 15:00 UTC will actually occur at 23:00 tonight, Beijing time. Of notable interest is the “Ask Mark” session on Tuesday, featuring Ubuntu founder Mark Shuttleworth.

By the way, I plan on attending some of the earlier-in-the-evening sessions, but once it gets too late I’ll just let my IRC client log the rest of them while I’m sleeping. 😉

Linux Palm Tech

How to convert CHM files under Linux

CHM files, known as Microsoft Compressed HTML Help files, are a common format for eBooks and online documentation. They are basically a collection of HTML files stored in a compressed archive with the added benefit of an index.

Under Linux, you can view a CHM file with the xchm viewer. But sometimes that’s not enough. Suppose you want to edit, republish, or convert the CHM file into another format such as the Plucker eBook format for viewing on your Palm. To do so, you first need to extract the original HTML files from the CHM archive.

This can be done with the CHMLIB (CHM library) and its included helper application extract_chmLib.

In Debian or Ubuntu:

$ sudo apt-get install libchm-bin
$ extract_chmLib book.chm outdir

where book.chm is the path to your CHM file and outdir is a new directory that will be created to contain the HTML extracted from the CHM file.

In other Linuxes, you can install it from source. First download the libchm source archive from the above website. I couldn’t get the extract_chmLib utility to compile under the latest version 0.38, so I used version 0.35 instead.

$ tar xzf chmlib-0.35.tgz
$ cd chmlib-0.35/
$ ./configure
$ make
$ make install
$ make examples

After doing the “make examples“, you will have an executable extract_chmLib in your current directory. Here is an example of running the command with no arguments and the output it produces:

$ ./extract_chmLib
usage: ./extract_chmLib <chmfile> <outdir>

After running the utility to extract the HTML files from your CHM file, the extracted files will appear in <outdir>. There won’t be an “index.html” file, unfortunately. So you’ll have to inspect the filenames and/or their contents to find the appropriate main page or Table of Contents.

Now the HTML is yours to enjoy!


I got help in writing this article from here and here.

FreeBSD Linux Sysadmin

A Hacker’s Vacation

[Head in the clouds]I took this week off work to enjoy a Hacker’s Vacation. That is, I’m planning to spend a lot of time hacking on my computer.

It’s more than that, actually. I desperately need some time to put my life back in order and catch up on things that I’ve been neglecting, such as housework, email, this website, hard drive spring cleaning, my Tchou Tchou’s website, the Swing website, a new server, and various little projects I have going on. Slowly, I’m getting parts of it all done. I’ll have to carry on some of the tasks later, but at least this week will give me a good foundation to work with.

The biggest thing I want to hack on is my brain. As I mentioned above, I’ve got a new server and I need to spend some time learning how it works. I’m intimately familiar with FreeBSD, but since it’s a virtual hosting solution, I’m constrained at this point to use Debian GNU/Linux on the new server. Since I’ve been using Ubuntu (which is based on Debian) on my desktop for over a year, it is fairly easy to manage. But there are lots of server-related configurations and tasks that I need to nail down for good security and management.

For general Linux information goodness, I’m following a set of tutorials from the IBM Developer Network entitled the Linux Professional Institute (LPI) exam prep, described as a “series of tutorials to help you learn Linux fundamentals and prepare for system administrator certification”. I’m not intending to write the exams—just learn the material. I’m finding that the tutorials give very good background information, covering things in enough detail to explain the process. I can then, of course, delve into the man pages and other documentation to learn more.

I’m enjoying it so far.