LVM Quick Command Reference For Linux And Unix

Hello again,

Today, as promised in yesterday's post (where we came full circle from getting started with LVM, and using monitoring and display commands to removing those same LVM devices), today we've got a little quick command reference of the most commonly used LVM commands.

You may have noticed that I named Linux and Unix in the title of this post. That's because HP-UX's implementation of LVM is very similar to Linux's, and this reference basically works for both of them :) If you're interested in specifics on most of these commands, please refer to our previous posts. We'll note commands in this reference that we "haven't" covered yet.

And, off we go. Hopefully this will help out if you ever need a one-stop-shop for that LVM command you've been looking for. In future posts, we'll definitely be looking at more advanced LVM concepts. For instance, we haven't even begun to look at things like setting up RAID or any form of advanced manipulation of the physical volumes, logical volumes and volume groups we created. You have our promise that posts on theses subjects will be forthcoming. Just maybe not tomorrow ;)

1. LVM Basic relationships. A quick run-down on how the different parts are related

Physical volume - This consists of one, or many, partitions (or physical extent groups) on a physical drive.
Volume group - This is composed of one or more physical volumes and contains one or more logical volumes.
Logical volume - This is contained within a volume group.

2. LVM creation commands (These commands are used to initialize, or create, new logical objects) - Note that we have yet to explore these fully, as they can be used to do much more than we've demonstrated so far in our simple setup.

pvcreate - Used to create physical volumes.
vgcreate - Used to create volume groups.
lvcreate - Used to create logical volumes.

3. LVM monitoring and display commands (These commands are used to discover, and display the properties of, existing logical objects). Note that some of these commands include cross-referenced information. For instance, pvdisplay includes information about volume groups associated with the physical volume.

pvscan - Used to scan the OS for physical volumes.
vgscan - Used to scan the OS for volume groups.
lvscan - Used to scan the OS for logical volumes.
pvdisplay - Used to display information about physical volumes.
vgdisplay - Used to display information about volume groups.
lvdisplay - Used to display information about logical volumes.

4. LVM destruction or removal commands (These commands are used to ensure that logical objects are not allocable anymore and/or remove them entirely) Note, again, that we haven't fully explored the possibilities with these commands either. The "change" commands in particular are good for a lot more than just prepping a logical object for destruction.

pvchange - Used to change the status of a physical volume.
vgchange - Used to change the status of a volume group.
lvchange - Used to change the status of a logical volume.
pvremove - Used to wipe the disk label of a physical drive so that LVM does not recognize it as a physical volume.
vgremove - Used to remove a volume group.
lvremove - Used to remove a logical volume.

5. Manipulation commands (These commands allow you to play around with your existing logical objects. We haven't posted on "any" of these commands yet - Some of them can be extremely dangerous to goof with for no reason)

pvextend - Used to add physical devices (or partition(s) of same) to a physical volume.
pvreduce - Used to remove physical devices (or partition(s) of same) from a physical volume.
vgextend - Used to add new physical disk (or partition(s) of same) to a volume group.
vgreduce - Used to remove physical disk (or partition(s) of same) from a volume group.
lvextend - Used to increase the size of a logical volume.
lvreduce - Used to decrease the size of a logical volume.

You may note that some of these commands vary slightly from LVM1 to LVM2 and also between the pure Linux implementation and HP-UX's Unix version. We'll be looking at the differences, and similarities, between all of these in future posts, for sure.

Until then. Enjoy :)

, Mike

How To Get Started With Logical Volume Management In Linux

Hey again,

Today, since past posts regarding disk management software have mostly been limited to Veritas Volume Manager, I thought we'd look at LVM on Linux (In a future post, we'll check out the differences between LVM 1 and 2, but for now I figured we could keep it simple and broad).

If you use any of the "major player" Linux flavours out there, you've probably noticed (over the last year or so) that standard installs show all your disks listed out in df, like:

/dev/mapper/vg01-lvol01 246M   75.2M    158M 32%    /dir

rather than the old-style:

/dev/hdd1      246M   75.2M    158M    32%    /dir

that you've (if you're anything like me and only have two hard drives on the home PC you're running Linux on ;) grown to love.

And, while I think it's fantastic that Linux has grown into a mostly-user-friendly operating system with GUI's to make all of the extended partitioning and volume management more accessible, I also think there's something to be said for knowing how to be able to set all that stuff up on your own. You never know when your main display (or X-windows session) will die on you and, at some point, you may end up having to do some disk repair on the lonely old command line.

With that in mind, we'll step through the most basic LVM setup: Getting from /dev/hdd to /dev/mapper/vg01-lvol01 from the command prompt.

Our basic (and huge assumption) here is that we've got a primary drive to work from (although we could do this just as easily from a rescue, or live, CD-ROM if we needed to perform this work on our primary disk during the Linux installation process or a freakish-nightmare of a fix)

First, we'll want to use fdisk to create a "new" "primary" partition that consists of the entire disk (512Mb in size). Barring a primer on fdisk, we'll want to set up our disk to have one primary partition that consists of all cylinders (first and last should be the default start and end values) and is set to the "Linux LVM" partition type.

Once we're done with the stuff we'd "normally" do (for the most part) to set up any other type of partition table on our /dev/hdd disk, we can begin to play around with the LVM commands. Note that if you chose any partition type other than LVM when setting up your partition, using the following commands will, at best, result in error messages and accomplish nothing :)

If you've ever used HP-UX before, a lot of these commands will seem familiar. There's a reason, but the legalities of discussing it (and possibly getting bits of it wrong) fall outside the scope of my pocketbook ;)

The first thing we'll do is create a "physical volume," like so:

host # pvcreate /dev/hdd1

Yes, it was a physical volume already. The logical-physical relationship sometimes doesn't hold up to scrutiny outside of the "rules" ;)

Next, well create a "volume group." This is necessary, because "logical volumes" run in packs... pardon my lack of propriety ;) We could use the "-s" option here to define the "physical extent" size, but we'll leave that alone. To say the least, specifying this requires some research and planning. If you get that number wrong, things can go South later, and you'll have to do some ugly backtracking. We just want to define a "volume group" so we have something to work with. We'll call it vg01, for lack of a more original name.

host # vgcreate vg01 /dev/hdd1

Next, we'll create a "logical volume" (we'll name this one "lvol01" to keep things consistent) of 246 Megabytes size. That should fit nicely inside vg01 (which we're basically using to represent the entire partition /dev/hdd1). We could make our "logical volume" as large as the entire partition and/or "volume group", but that would defeat the purpose of this whole exercise. The aim, of course, is to end up with flexible volumes so we can resize them later if we need to.

host # lvcreate -L 246M -n lvol01 vg01

This is where the /dev/mapper stuff comes into play (it never made sense to me the first time I saw it... really. It's because of my past experience with HP-UX ;) The "lvcreate" command creates a symlink between /dev/vg01/lvol01 and the character device /dev/mapper/vg01-lvol01.

And, next, all we have to do is format that bad boy:

host # mkfs -t ext3 /dev/vg01/lvol01

Note that, above, you can set a reserved block count (based on percentage) using the -m flag. I would usually set this to "-m 5" so that 5% of the allocated space would be reserved for root (although you can set it as low as 1). This way, if someone jams your filesystem up to 100% and no one can get any work done, you can still log in as root and fix the issue. -v is another useful flag if you want to see verbose output. You can insert these two flags anywhere in the above command line except at the very end (after the logical device name) or in between the -t flag and its argument. Common sense, I know, but I feel compelled to over-explain ;)

Now, we'll make a mount point, or, really, just make a new directory off of the root filesystem (although you can do overlays if you wanna get creative ;)

host # mkdir /dir

and then we'll mount our logical volume up and check it out with df:

host # mount -t ext3 /dev/vg01/lvol01 /dir

host # df -h /dir
...
/dev/mapper/vg0-lvol0 246M   75.2M    158M 32%    /dir

Tomorrow, or maybe the day after that, we'll take a look at some more ways to manipulate your existing volumes, add more volumes, do mirroring and lots of other fun stuff that LVM allows you to do.

For now, we'll just thank our deity-of-choice that this all worked out ;)

Cheers,

, Mike