Episode: 314
Title: HPR0314: LVM2
Source: https://hub.hackerpublicradio.org/ccdn.php?filename=/eps/hpr0314/hpr0314.mp3
Transcribed: 2025-10-07 16:04:47

---

😋
Hi there. My name is Kevin Benco and welcome to Hacker Public Radio. I wish to discuss
a brief and basic overview of logical volume management or LVM. Please note some disclaimers.
First off, I'm coming from a Debian Genial Linux frame of reference and there is a possibility
that some of the command line details and some of the commands that I'm going to discuss
may differ slightly in other distributions. Also note the current LVM package is LVM2 and
when I use the term LVM, I am indeed referring to LVM2. What is logical volume management? LVM is
to paraphrase it in my own words an abstraction layer between the real live actual physical device
partitions and the GNU Linux operating system. When I say it's an abstraction layer, I mean to say
that you've got the real live physical partitions and your LVM is a set of tools that allows you to
create and manipulate, let's say, virtual partitions on top of these physical partitions and the
operating system sees and mucks around with the virtual partitions plus the term virtual and
virtualization is kind of trendy now so we'll stick with that definition. For a brief overview
of our brief overview, with logical volume management we must first consider the physical
partitions on the real live physical device. Now each of these real live physical partitions to be
used in LVM must be identified in the partition table as an LVM partition. Each real live physical
partition must be prepared as an LVM physical volume. Now an LVM volume group is composed of one
or more LVM physical volumes and each LVM volume group consists of zero or more LVM logical volumes.
Each LVM logical volume is a container that the GNU Linux operating system will view as a
device partition onto which a file system can be created. And for all intents and purposes,
to the best of my knowledge, an LVM logical volume can be treated as if it were a real live
virtual device partition. It will become clear hopefully as we go on. Let us consider the
process for setting up LVM on an existing GNU Linux system. Personally I have no experience
setting up LVM during install and since I run Debian I don't foresee ever having to do a
reinstall on my system until the heat death of the universe. But I really can't conceive
that it's any more difficult to set up LVM on an install than it is to set up LVM on an existing
system. And given today's installers that come with the distributions it might be a little easier
to set up LVM on an install. Anyway, for examples here I am going to use what I've got going
on my system to some extent. First we need some partitions on real-life physical devices to mess
around with. On my particular setup I am using the following partitions for LVM
slash dev slash HDE2 slash dev slash HDE3 and slash dev slash HDF1. So by using FDISC
when I create these partitions in the partition table I need to set the system ID flags for these
partitions to be LVM partitions. And with FDISC that's 8E. So using FDISC I set these partitions
to be identified as 8E Linux LVM partitions. Then I am going to use the PVCreate command
PV stands for physical volume. I am going to use the PVCreate command to initialize the partitions
for LVM. I would guess that the PVCreate command amongst other things makes entries into some sort of
LVM database that identifies or notes or records the drive and partition specifications in some
manner. Back about two years ago I borked my LVM stuff to some degree when I was messing around
with it and I had to actually go in and edit the database file manually and thank God for text
files. Anyway the syntax of the PVCreate command is PVCreate followed by a list of one or more
physical device partitions. So in this particular case I enter the command PVCreate
slash dev slash HDE2 slash dev slash HDE3 and slash dev slash HDF1. I then need to create some volume
groups. As I said earlier each volume group consists of one or more physical volumes
and each volume group also contains zero or more logical volumes. We may, if we desire, have a volume
group extend across physical volumes on more than one physical device. Physical hard drive
isn't that cool. I will be creating two volume groups for my example. In an astounding
flash of creativity and divine inspiration I've decided to call these two volume groups LVM
group and alternate LVM. Let me give you a moment to settle down from this blinding flash of
creativity. Thus to create the first to create the volume group called LVM group I enter the
command VG create space LVM group, space slash dev slash HDE2, space slash dev slash HDE3.
This creates the volume group called LVM group and it contains the physical volumes dev HDE2
and dev HDE3. Any other for the other volume group I use VG create alternate LVM dev HDF1.
These commands have created the two volume groups LVM group and alternate LVM. Right now all I
have is a pair of volume groups with nothing in them. All about all I can do right now is use the
VGS command to query the volume groups. I enter the VGS command amongst other things the
command will tell me the number of physical volumes composing each volume group the number of
logical volumes in each volume group the size of each volume group and the amount of free space
in each volume group. Let's face it you might not in a particular volume group you might not
want to use all the the free space available for your logical volumes. So there's this pool of
free space that you can give you as you will amongst the logical volumes. Now let us create some
logical volumes to actually do something with going to be using the LV create command and the syntax
is LV create dash dash size followed by the size of the logical volume dash dash name followed by
the name of the volume and the name of the volume group you're you're putting this logical volume in.
Now in my particular situation in my particular in this particular example I wish to create
logical volumes named swap home and music in the volume group called LV group and logical volumes named swap
and source in the volume group named alternate LVM. So I enter LV create dash dash size 1G name swap
LVM group. This creates a logical volume named swap of a size of one gigabyte
in the volume group called LVM group. Similarly LV create dash dash size 13G name home LVM group
creates a volume group sorry create a logical volume called named home in the volume group named
LVM group and it's 13 gigabytes big and LV create dash dash size 47G dash dash name music LVM group
creates a logical volume named music in the volume group named LVM group and it is 47 gigabytes big.
Similarly into the commands LV create dash dash size 1G dash dash name swap alternate LVM
and LV create dash dash size 1G dash dash name source alternate LVM. This is going to create the
appropriately named logical volumes in the alternate LVM group and both of them are one gigabyte big.
So as of right now these logical volumes are just containers they don't have any file systems in
can't do squat with them. So we've got physical volumes that refer to physical partitions on devices.
We've got one or more physical volumes grouped together to form a volume group
and inside this volume group we've got several logical volumes. Now as far as the computer
or as far as the operating system is concerned these logical volumes are the same thing as
partitions and right now these partitions these logical volumes don't have anything in them so they're
just un-initialized partitions right now. We got to throw some file systems in there.
Now since each of these logical volumes is considered as a partition by the operating system
we can refer to the logical volumes the same way we refer to partitions in the following manner.
Slash dev slash the name of the volume group slash the name of the logical volume.
So for example my home the logical volume named home I used my example I can I will refer to that
as slash dev slash LVM group slash home that's the name of the partition as far as the operating
system is concerned. Now using the examples above we're going to create the appropriate
file systems in the above logical volume containers as follows. First a disclaimer and don't laugh at
me. I am using EXT2 and EXT3 exclusively and swap partition stuff exclusively. I'm not going
to mess around with the XT4 yet. I don't use riser file system. I am old fashioned EXT2,
EXT3. So once you're done laughing at me I'll continue. So we're guys since I've got two logical
volumes set aside for swap partitions and use the make swap command on both of them. MKSwap slash dev slash
LVM group slash swap and MKSwap slash dev slash alternate LVM slash swap. This as expected creates
swap partitions in creates swap file systems like this inside these partitions.
Man using our example and continuing with our example above I'm going to throw some or create
some EXT3 file systems on the remaining three logical volumes. Make MKE2FS dash j dev LVM group home
make E2FS dash j dev LVM group music and make E2FS dash j dev alternate LVM source.
So inside of our logical volume containers I've created an appropriate swap file systems and
EXT3 file systems. Next we need to mount the logical volumes in our directory tree by making
the appropriate entries in the Etsy FS tab file. I'm going to assume that because you're
screwing around with logical volume management and partitions and whatnot that you're pretty okay
with editing the Etsy FS tab file. I would like to make one note on something in the FS tab file
with respect to logical volume management with respect to logical volumes actually.
In the field where the first field in the Etsy FS tab where it we put down the name of the
partition. I've seen it done two different ways and it's probably at least a third way to put
it to denote the logical volume partition. I don't see any difference between them with regards to
how the operating system deals with them so it probably doesn't make a difference but I'm not too
sure. Now the first way I've seen it designating it and designing the logical volume is to use our
example above using the home logical volume slash dev slash LVM group slash home. I've also seen
it used the noted as slash dev slash mapper slash LVM group dash home. I've used them both I don't
see any difference they both work. Additionally one can use UID numbers in order to designate the
logical volumes in the Etsy FS tab file. UID stands for universally unique identifier.
Sadly in my unworthiness I have very little experience using UID numbers in the Etsy FS tab file and
if you use UID you know more about it than I do I'm sorry I am unworthy I suck. Thus far we've
created the I've created the abstraction layer that allows us that has allowed us to create
several logical volumes in several volume groups. We've also created file systems in these logical
volumes and we have the ability to create mount points at the Etsy FS tab file and to plug our
or to mount our new file systems into our directory tree. Now as an aside my own personal reasons
for even getting into logical volume management were that once upon a time in the days of your
I partitioned my hard drive using regular plain old physical partitions and I set up a partition
for my home directory and everything was good and then I started accumulating
audio files and I was in danger dreadful danger of overrunning of running out of space in my
home partition. For a brief point for a brief piece of time I for brief period of time I even had
public directory in the root file system to put on my music in but that was a bad idea I think
that the throw extraneous crap like that in root partition. So I needed some solution and I sat
down and I tried to figure out a way to partition my hard drive so that this kind of thing wouldn't
happen again. I asked myself well how would I partition this hard drive that will allow me to
have enough space to do anything I wanted that it would allow me to flex ability to
or if I accumulated started messing around with video and stuff like that like my wife does.
It would allow me to with my car with a to set it set up a set of partitions that would
be always be big enough no matter what and you know I just couldn't think of any way to do it
and that's how I and I remember hearing with this LMV stuff and what the hell is this LMV stuff
so I looked into it and that's why I'm using LVN because it gives me the the flexibility
I wanted to grow at first my home partition and also shrink other partitions to make room for
the larger home partition so in order to give myself this flexibility I eventually basically LVN
most of my my file system. I still have to real live hard drive non LVN partitions that's my boot
partition and my root partition but I've got LVN set up for two slot partitions on two separate
drives and I have got LVN set up for one of my local backup on the on my local backup that I do
my hourly backups for my home directory on the physical hard drive my I have a public directory
in under a home directory and I got a music partition and audio books partition a podcast
partition the partition in which I do all my compiling my wallpapers I even got LVN set partitions
for my temp directory my usr directory and my var directory yeah maybe I'm a little crazy with
that but it gives me a lot of flexibility to screw around with partition sizes and resize things
and since I'm kind of crazy and I don't think he's sort of crazy so some kind of weird about
also for some reason I really haven't figured out yet I keep my logical volumes all with about 90%
capacity this gives me some free space left over in the volume groups that I could mess around with
if I need to if I need to slap a quick gigabyte on this partition real quick I can do that
if I need to grow a particular partition by a few hundred megabytes or a gigabyte or so
I can do that without worrying about it because I got in a free space left over in the volume groups
how do you both set up works for me it allows me the flexibility to shrink and grow partitions
to suit my own evil purposes the way I got this set up with keeping all my partitions
at all my LVN partitions at 90% capacity requires that I check available
file system sizes are the available space I got on the file systems and the available space
I've got in my volume groups but hi that's okay for me this sort of thing might not be to anyone
else's liking you might decide that if you use LVN you want to have a more oh conservative setup
than I've got whatever LVN allows you the flexibility to do all this kind of stuff
now I've been referring to shrinking and growing logical volumes and file systems without mentioning
the process yet in a normal non LVN setup where all my partitions are just partitions on devices
if I wanted to shrink a file system I'd have to use the first up to shrink the file system using
the appropriate resizing command resize to ask yeah resize to FS for EXT2 and XT3 then I'd have to
resize the partition by way of FDisk write the changes to the partition table and hope it worked
similarly to grow a file system I'd have to use FDisk to make the changes to the partition table
and then use the resize command to grow the file system up to the size of the partition
seems to me one of the shortcomings of this is the fact that I will be needing in order to grow a
file system I need there to be some unused or unallocated space between the partitions
even using a graphical tool like g-parked I'm sure we can really quick think of
circumstances in which we don't have the unallocated space between partitions or between the
partitions that are going to let us directly grow that partition or we do have an allocated free
space somewhere else and we've got to do the whole juggle shrink and grow thing and you know
what I don't I can just think how that could be either not possible or not easing logical volume
management makes this process of resizing logical volumes and file systems really much more flexible
with how VM when we grow a file system first we grow we make the logical volume larger
then we resize a file system up to the size of the logical volume when we shrink a file system
we first resize the file system down smaller to where we want it and we resize the logical volume
till it matches the size of the file system I'm going to mention this several times
you must be extremely careful when shrinking the logical volume if you shrink the logical
volume to be smaller than the file system you will cry ask me how I know this
more notes sorry I'm like I said I'm coming from an EXT2 and EXT3 frame of reference I'm old
fashion here if I have an EXT2 file system in a logical volume if I want to grow it or shrink it
I need to unmount the file system run a file system check with a dash f flag on the logical volume
before I can do anything to file system so if I got EXT2 need to unmount
do a file system force a file system check on it before I can before I can even think about
resizing things if I have an EXT3 file system inside of a logical volume I can grow the file system
in line without having to unmount or file system check or anything however if I want to shrink
an EXT3 file system my first have to unmount do a file system check or force a file system check
and then shrink it and again I have to mention if you shrink the logical volume to be smaller than
the file system you will have a bad day continuing with our examples above suppose I have just acquired
a large number of org files containing yodeling music
and I want to add that yodeling music to my music collection which has its own partition
but you know suppose my music my home public music partition also known as
slash dev slash lvm group slash music suppose that partition is too small and you know I really
want that yodeling music I need to grow that partition by one gigabyte because you can never have
too much yodeling music can you so first thing I check that my volume group has no free space to
let me grow it using the VGS command I check using the VGS command I check on the free space and say
let's just suppose I do have a gigabyte of free space available so growing and growing an
EXT3 file system enter LV resize dash dash size plus one g and the name of the logical volume
slash dev slash LVM group slash music so I've resized the logical volume the container to be
one gigabyte bigger than it used to be then I resize a file system up to the size of the container
using the resize to FS slash dev slash LVM group slash music of note with the LV resize command
my size can be plus some size minus some size or just a plain old size and if I just enter a
plain old size it will be that absolute it will resize it to that value in this particular
in this particular example I use my size of plus one g basically saying add a gigabyte
if for example however I had um EXT2 file system on that music partition first I don't
mount it to a file system check enter LV resize resize e2 FS and then remount it but because I'm
using LV at EXT3 don't worry about unmounting things to grow a logical volume or a file system
this is good for suppose I'm making some changes and installing a bunch of packages and I've
got my usr directory on its own partition and I notice that while I'm running out of space there
I can if I got the XT3 on there if I have an XT3 file system in there I can enter a quick
LV resize command and I'll resize e2 FS command and if I type quick enough I can get those things
resized up to something bigger that I can fit my stuff on new stuff on anyway back to our example
suppose I have noted that my free space in my volume group is insufficient to grow that music
partition but I really really really want that yodeling music so I need to check if I have any
extraneous free space in any file systems that are in logical volumes in that volume group
so that I can reallocate some of that extraneous free space into the volume group is
unallocated into the volume groups free space so that I can have enough free space left over to
grow the music partition so I check the file system sizes by doing a df-h or capital H actually
to see if I have any file systems with extraneous free space and suppose I note for example that I
have oh five gigabytes of free space in my home file system my slash home file system and then I
recall in an amazing fit of sanity I've deleted my three gigabyte collection of LLCAT pictures
why because I realize that no one needs three gigabytes of LLCATs no I need three K of LLCATs
whatever so I can free up that extraneous free space in the file system shrink the logical volume
to fit the file system and I'll have that reallocated space that a new space back in the volume
group free space pool to let me grow some other file system for example or to let me grow some
other logical volume for example and I need to mention if you shrink the logical volume to be
smaller than the file system it will screw up your day oh some notes first if you shrink if
you're shrinking file systems the df command if you're displaying the size of the file system
and human readable format it's going to do some rounding take this into account please also
for as I know most file systems okay let's say the only file systems I know are the EXT2 and the
XT3 and these file systems set aside a default of 5% of the file system for the root user don't
forget to take that into account so when you're resizing a file system the df command rounds off
when you're in human when you're displaying it in human readable format and don't forget that 5%
that it's been set aside for the root user and now let us resize a file system down let us shrink
a file system first we unmount the file system and if we have other file systems mounted on that
file system we need to unmount those also then we force a file system check on that file system
let's say we do e2fsek-f slash-lvm group slash home then we do a resize then we resize that file
system down so we say resize e2fse slash-lvm home slash home sorry slash-lvm group slash home
let me enter some number call it new size right new size then we do an LV resize
LV resize dash dash size and we enter that number we used that size we used for the resize to fs
then we enter the name of the logical volume slash-lvm group slash home
important when we use an LV resize command we need to double check a few things
when we're shrinking a logical volume LV resize will warn you that since you're shrinking the L
logical volume you could screw up your file system have I mentioned that if you shrink the
logical volume to be smaller than the file system you will cry so please please please please
double check first that when you resize the file system it resize successfully if it didn't
resize successfully because it ran out of space you got to get it to resize successfully so you
got to start yutzing around with it until you get it to resize successfully so there's enough
space to resize it down to so you make certain that when you resize that file system down when you
shrunk that file system it did it successfully next double check the value you entered
for LV resize but the size of the logical new logical volume is the same exact value you entered
for your successful resizing of the file system again double check when you shrunk the file system
was it successful and the number you the size you entered to shrink the file system had better be
the same size that you enter to shrink the logical volume have I mentioned that if you shrink
the logical volume to be smaller than the file system you will cry so if you double check this
and you're absolutely certain that your logical volume is going to be resized no smaller than
your file system enter you enter yes or enter y for yes and it'll and LV resize will do it
note that the size of a logical volume defaults to be a multiple of four megabytes I think
thus the LV create command will always will if necessary round up to the nearest multiple of four
megabytes if this happens LV create will inform you of this so you might have a couple extra
megabyte you know so when you enter the LV create command it'll say oh that around up to the nearest
four megabytes and not being one to waste disk space I'd hit it within a I'd resize the file
system again once I resize the logical volume to get pick up those anywhere between zero and
four megabytes extra that it rounded up to oh when I'm all done remount what I had just
unmounted and everything's happy also of notes suppose you've got a file you've got a
you know Linux a junior Linux setup we've got some logical volume management going on you've got
a bunch of logical volumes and for some reason you need to use a live CD to oh do some rescue
or something now some rescue CDs will have LVM functionality this is good however depending
on the setup of a particular live CD even though it has the LVM functionality when your live CD
came online it might not have bothered to identify any logical volumes you've got laying around
for example I'm going to like a two-year-old canopics CDI use sometimes when I break things it has
LVM functionality but it doesn't bother to check my logical volumes when it when it comes online
so I need to use the command VG scan to scan for volume groups now I just enter I just enter the
command VG scan things where things spin things grind a little bit and it recognizes the volume
groups and then I can mount them in the appropriate mount points and walk around with them if I need to
well you know I think that about does it form my attempted basic and brief overview of logical
volume management hopefully from the examples I've given it can sort of see how LVM is
pretty powerful and flexible tool for managing partitions for creating and managing virtual partitions
shall we say please note I've only brushed the surface I've really scratched the surface of
the flexibility in LVM as there are many more commands and many more command line options
that'll allow the system administrator a lot more flexibility in creating and managing
physical volumes volume groups and logical volumes well I guess that's about it my name is Kevin
Benco and this was my attempt at hacker public radio thank you for your time and have a swell day
thank you for listening to hacker public radio hpr responses by ferro.net so head on over to car0.18 for all the