 | Level: Introductory Daniel Robbins (drobbins@gentoo.org), President/CEO, Gentoo Technologies, Inc.
01 Mar 2001 In this article, Daniel introduces you to the concepts behind Linux LVM (Logical Volume Management) and shows you how to get the latest kernel patches and tools installed on your system. LVM allows you to create logical volumes out of the physical storage resources on your machine. However, unlike physical volumes, the logical volumes can be expanded 0and shrunk while the system is still running, providing Linux system administrators with the storage flexibility that they've until now only dreamed of.
In this series, I'm going to show you how to install and use the new Logical
Volume Management support built-in to the Linux 2.4 kernel. If you've never
used a form of LVM before, you're in for a treat; it's a wonderful technology.
Before we actually get LVM up and running, I'm going to explain exactly what it
is and how it works. Then, we'll be ready to test out LVM and get the most out
of it.
LVM intro
If you're like me, then your experience with UNIX and Linux began on a PC
platform, rather than on large, commercial UNIX servers and workstations. On
the basic PC, we've always had to deal with partitioning our hard drives. PC
people are generally well-acquainted with tools such as fdisk, which are used
to create and delete primary and extended partitions on hard disks. Hard disk
partitioning is an annoying but accepted part of the process of getting an
operating system up and running.
Hard drive partitioning can be annoying because to do a good job you really
need to accurately estimate how much space you'll need for each partition. If
you make a poor estimation, your Linux system could possibly be crippled -- to
fix the problem, it's possible that you might even need to perform a full
system backup, wipe your hard drives clean, and then restore all your data to a
new (and presumably better) partition layout. Ick! These are exactly the
kinds of situations that sysadmins try their best to avoid in the first place.
While partitions were once static storage regions, thankfully, we now have a
proliferation of PC repartitioning tools (PowerQuest's Partition Magic product
is one of the most popular). These tools allow you to boot your system with a
special disk and dynamically resize your partitions and filesystems. Once you
reboot, you have newly resized partitions, hopefully getting you out of your
storage crunch. These partition resizing tools are great and solve the problem
storage management for some. But are they perfect? Not exactly.
Tools like Partition Magic are great for workstations, but aren't really
adequate for servers. First of all, they require you to reboot your system.
This is something most sysadmins desperately try to avoid doing. What if you
simply can't reboot your machine every time your storage needs change, such as
if your storage needs change dramatically on a weekly basis? What happens if
you need to expand a filesystem so that it spans more than one hard drive, or
what do you do if you need to dynamically expand or shrink a volume's storage
capacity while allowing Apache to continue to serve Web pages? In a
highly available, dynamic environment, a basic partition resizer just won't
work. For these and other situations, Logical Volume Management is an
excellent (if not perfect) solution.
Enter LVM
Now, let's take a look at how LVM solves these problems. To create an LVM
logical volume, we follow a three-step process. First, we need to select the
physical storage resources that are going to be used for LVM. Typically, these
are standard partitions but can also be Linux software RAID volumes that we've
created. In LVM terminology, these storage resources are called "physical
volumes". Our first step in setting up LVM involves properly initializing
these partitions so that they can be recognized by the LVM system. This
involves setting the correct partition type if we're adding a physical
partition, and running the pvcreate command.
Once we have one or more physical volumes initialized for use by LVM, we can
move on to step two -- creating a volume group. You can think of a
volume group as a pool of storage that consists of one or more physical volumes.
While LVM is running, we can add physical volumes to the volume group or even
remove them. However, we can't mount or create filesystems on a volume group
directly. Instead, we can tell LVM to create one or more "logical volumes" using
our volume group storage pool:
A volume group is created out of physical volumes
Creating an LVM logical volume is really easy, and once it's created we can go
ahead and put a filesystem on it, mount it, and start using the volume to store
our files. To create a logical volume, we use the "lvcreate" command,
specifying the name of our new volume, the size we'd like the volume to be, and
the volume group that we'd like this particluar logical volume to be part of.
The LVM system will then allocate storage from the volume group we specify and
create our new volume, which is now ready for use. Once created, we can put an
ext2 or ReiserFS filesystem on it, mount it, and use it as we like.
Creating two logical volumes from our existing volume group
Extents
Behind the scenes, the LVM system allocates storage in equal-sized "chunks",
called extents. We can specify the particular extent size to use at volume
group creation time. The size of an extent defaults to 4Mb, which is perfect
for most uses. One of the beauties of LVM is that the physical storage
locations of the extents used for one of our logical volumes (in other words, what disk
they're stored on) can be dynamically changed while our logical volume is
mounted and in use! The LVM system ensures that our logical volumes continue
to operate perfectly while allowing the administrator to physically change
where everything is stored.
Of course, since everything is created out of equally-sized extents, it's
really easy to allocate some additional extents for an already-existing
logical volume -- in other words, dynamically "grow" the volume:
Adding additional extents from our volume group, expanding the size of our logical volume
One the logical volume has been expanded, you can then expand your ext2 or
ReiserFS filesystem to take advantage of this new space. If you use a program
such as resize_reiserfs, this filesystem expansion can also happen while the
volume is mounted and being used! Truly amazing -- with LVM and online
filesystem expansion utilties, it's no longer necessary to reboot your system
or even drop to runlevel 1 to change your storage configuration.
The only time you need to shut down your system is when you need to add new
physical disks. Once new disks have been added, you then can add these new
physical volumes to your volume group(s) to create a fresh supply of extents.
Setting up LVM
OK, let's get LVM installed. LVM consists of two parts: a kernel part and a
suite of user-space tools. To start, head over to the main LVM page (see Resources later in this article) and download the most
recent version of the LVM tarball (currently lvm_0.9.1_beta3.tar.gz) you can
find. The LVM tarball contains all the user-space tools, as well as a bunch of
kernel patches. Here are where things get interesting.
If you already have a 2.4-series kernel installed, you may already have LVM
support available on your system, and if not, it's a simple matter to recompile
your kernel to enable LVM support. However, you may not want to use the LVM
support included with your stock (or distribution-supplied) 2.4 kernel. If you
want to use the latest LVM version, you'll want to apply patches from the LVM
tarball to your current 2.4 kernel source tree. Here's how to do it.
To start, enter your kernel source directory (/usr/src/linux) and create a
directory called "extras". Then, enter this directory and extract your LVM
tarball:
# cd /usr/src/linux
# mkdir extras
# cd extras
# tar xzvf /path/to/location/of/lvm_0.9.1_beta3.tar.gz
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Once you've done that, you'll notice a new directory in extras called "LVM"
that contains another directory named after the version of LVM that you just
unpacked. Enter these two directories to get to the LVM sources:
Listing 1: Getting to the LVM sources
You'll see several text files, scripts, and source directories. You'll find the
installation instructions in the "INSTALL" file; I'll guide you through this
process. First, we'll want to run the configure script, as follows:
# ./configure --prefix=/ --mandir=/usr/man
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Patching
After executing this command, the Makefiles will be created and configured to
install all the LVM tools in /sbin and the man pages in /usr/man. If your man
pages are in /usr/share/man (as per FHS 2.1), then adjust the above path
accordingly. And, if your kernel sources aren't in /usr/src/linux, then add a
"--with-kernel_dir=/path/to/usr/src/linux" option to the line as well. Once the
configure script completes, we're ready to install the tools and generate a
patch for your current kernel. Let's patch the kernel first. Enter the
PATCHES directory:
Now, we're going to type "make". The makefile will generate a patch
specifically for our particular 2.4-series kernel sources:
The patch will be named lvm-[lvmversion]-[kernelversion].patch. For example,
since I'm using version 0.9.1_beta3 of LVM and kernel 2.4.0-ac11, my patch is
called lvm-0.9.1_beta3-2.4.0-ac11.patch. You'll find it in your current
directory. Now, it's time to apply the patch. To do this, you'll want to
change directories to the location of your kernel sources and use the patch
command as shown in Listing 2:
Listing 2: The patch
command
While the LVM INSTALL documentation doesn't mention it, I typically pass the
"-l" option to patch. This option allows the patch program to compensate for
any changes in whitespace (such as minor indentation changes) that would
ordinarily cause some hunks of the patch to fail. If the above command
completes without any "FAILED" lines, then you're ready to install the
user-space tools. If not, you'll need to scan your /usr/src/linux directory for
..rej files and insert the rejected hunks into the sources by hand using a text
editor -- ick! However, in most all situations the patch will apply cleanly
and you'll be ready to go.
Configuring, compiling, and installing
OK, you now have a kernel that's been patched so that it has the most current
LVM code available. Now, you'll want to configure your kernel so that LVM
support is enabled. I recommend that you compile LVM support directly into
the kernel rather than configuring it to compile as a module. Fire up your
favorite Linux kernel configuration method:
# cd /usr/src/linux
# make menuconfig
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You'll find the LVM options under the "Multi-device support (RAID and LVM)"
section. Once you enable the first option:
[*] Multiple devices driver support (RAID and LVM)
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....you'll see the following option, which you should also enable:
<*> Logical volume manager (LVM) support
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Depending on your LVM version, there may be other LVM-related options that
you'll want to enable as well. Once you're done, save your kernel
configuration and perform your standard kernel compilation routine and reboot.
Congratulations -- you now have kernel LVM support enabled; now, we need to get
the user-space tools compiled and installed. This step is easy:
# cd /usr/src/linux/extras/LVM/0.9.1_beta3
# make
# make install
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There's just one more step, and it's optional. If you're going to be doing
more than just testing out LVM, you'll want to add the following lines to your
startup rc scripts:
/sbin/vgscan
/sbin/vgchange -a y
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These lines will scan for all available volume groups and activate them. Then,
add the following line to your shutdown rc script, and make sure that it
executes after all filesystems have been unmounted:
If you're just testing out LVM, then you can skip these steps. Just remember
that after every reboot, you'll need to type "vgscan" and "vgchange -a y" as
root before your logical volumes are available for use.
That's it for this article. Next article, I'll show you how to create
your own logical volumes and unleash the power of LVM. I'll see you then!
Resources - Download the LVM tarball from Sistina Software.
- The impatient will want to check out Heinz Mauelshagen's LVM HOWTO, which shows you how to set up physical volumes, volume groups, and logical volumes. This is something we'll cover in my next article.
- There's also an interesting HOWTO that shows you how to set up your root filesystem on a logical volume.
- Andreas Dilger is involved with the Linux LVM project and has a nice-looking
online ext2 filesystem resizer.
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ReiserFS is an excellent filesystem (especially in combination with LVM). If you're using ReiserFS, you'll want to grab the reiserfs-utils tarball, which contains a program called "reiserfs_resize" -- allowing online resizing of ReiserFS filesystems.
- For more information on setting up Linux software RAID volumes, see Part 1 and Part 2 of Daniel's developerWorks series on software RAID.
- For a refresher, see the developerWorks tutorial on compiling the Linux kernel.
About the author | | | Residing in Albuquerque, New Mexico, Daniel Robbins is the
President/CEO of Gentoo Technologies,
Inc., the creator of Gentoo Linux, an advanced Linux for the
PC, and the Portage system, a next-generation ports system for Linux.
He has also served as a contributing author for the Macmillan books
Caldera OpenLinux Unleashed, SuSE Linux Unleashed, and Samba Unleashed.
Daniel has been involved with computers in some fashion since the
second grade, when he was first exposed to the Logo programming
language as well as a potentially dangerous dose of Pac Man. This
probably explains why he has since served as a Lead Graphic Artist at
SONY Electronic Publishing/Psygnosis. Daniel enjoys spending
time with his wife, Mary, and his new baby daughter, Hadassah. |
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