Playing with LVM ( a.k.a. Logical Volume Manager for Linux ) on any existing disk

Playing with LVM ( a.k.a

Playing with LVM ( a.k.a. Logical Volume Manager for Linux ) on any existing disk

Let's start with what LVM is; in short it's away to manage disk volumes in more of a user friendly way, whether they are whole hard disk, disk partitions, or SAN disk. Logical Volume Manager gives the Administrator much more flexibility in allocating, re-sizing, and moving storage around. With that being said, the greatest advantage is having the ability to add additional disk space with relative ease, which with Moore's Law and Kryder's Law [1] floating around makes life a little easier on your System Administrator.

Now that you are all interested in using LVM, let's bust out a new hard-drive or re-partition our hard-disk and have at it. Woo! Hang on a minute! That sounds like a pain just to play around with something. That's what I thought too, so here is a way to use your existing partitions using loop devices and empty disk images to play around with and get use to the commands.

I have personally tested this on RHEL 4 and 5, but I don't see why this couldn't be done on any current version of Linux.

1.) First create a couple of empty dd images that will be used as loop devices.

shell>dd if=/dev/zero of=/tmp/diskvol0.img bs=1024 count=102400

102400+0 records in

102400+0 records out

shell>dd if=/dev/zero of=/tmp/diskvol1.img bs=1024 count=102400

102400+0 records in

102400+0 records out

shell>dd if=/dev/zero of=/tmp/diskvol2.img bs=1024 count=102400

102400+0 records in

102400+0 records out

2.) Now set the images up as loop devices using the losetup command.

shell>losetup /dev/loop0 /tmp/diskvol0.img

shell>losetup /dev/loop1 /tmp/diskvol1.img

shell>losetup /dev/loop2 /tmp/diskvol2.img

3.) Now use the pvcreate command to initialize the loop devices for use by LVM.

shell> pvcreate /dev/loop0

Physical volume "/dev/loop0" successfully created

shell> pvcreate /dev/loop1

Physical volume "/dev/loop1" successfully created

shell> pvcreate /dev/loop2

Physical volume "/dev/loop2" successfully created

4.) Create a volume group with vgcreate.

shell> vgcreate volumegroup0 /dev/loop0 /dev/loop1 /dev/loop2

Volume group "volumegroup0" successfully created

5.) Display the volume group information, so we can see the VG Size and see how big to make the volume.

shell> vgdisplay

--- Volume group ---

VG Name volumegroup0

System ID

Format lvm2

Metadata Areas 3

Metadata Sequence No 1

VG Access read/write

VG Status resizable

MAX LV 0

Cur LV 0

Open LV 0

Max PV 0

Cur PV 3

Act PV 3

VG Size 288.00 MB

PE Size 4.00 MB

Total PE 72

Alloc PE / Size 0 / 0

Free PE / Size 72 / 288.00 MB

VG UUID 0mAIRf-Gzz0-R4zP-K0OH-u33B-R31w-jvVbC2

6.) Create a logic volume with lvcreate.

shell> lvcreate -L288M -nvolume0 volumegroup0

Logical volume "volume0" created

7.) Now you can format the volume with mkfs.

shell> mkfs.ext3 /dev/volumegroup0/volume0

mke2fs 1.35 (28-Feb-2004)

Filesystem label=

OS type: Linux

Block size=1024 (log=0)

Fragment size=1024 (log=0)

73728 inodes, 294912 blocks

14745 blocks (5.00%) reserved for the super user

First data block=1

Maximum filesystem blocks=67633152

36 block groups

8192 blocks per group, 8192 fragments per group

2048 inodes per group

Superblock backups stored on blocks:

8193, 24577, 40961, 57345, 73729, 204801, 221185

Writing inode tables: done

Creating journal (8192 blocks): done

Writing superblocks and filesystem accounting information: done

This filesystem will be automatically checked every 34 mounts or

180 days, whichever comes first. Use tune2fs -c or -i to override.

8.) Now you can mount it.

shell> mount /dev/volgroup0/volumename /mnt

Now that you have gotten started, I suggest that you play around with some of the other options, such as adding and removing disks, deleting groups and volumes, moving and splitting volume groups.

References and more information:

http://www.tldp.org/HOWTO/LVM-HOWTO/

http://www.redhat.com/magazine/009jul05/features/lvm2/

Notes:

[1] "Moore's Law" describes an important trend in the history of computer hardware: that the number of transistors that can be inexpensively placed on an integrated circuit is increasing exponentially, doubling approximately every two years.

A similar law (sometimes called "Kryder's Law") has held for hard disk storage cost per unit of information. Disk storage over the past decades has actually sped up more than once, corresponding to the utilization of error correcting codes, the magneto-resistive effect and the giant magneto-resistive effect. The current rate of increase in hard drive capacity is roughly similar to the rate of increase in transistor count. Recent trends show that this rate has been maintained into 2007.

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