1. What is LVM?
LVM is a Logical Volume Manager for the Linux operating system. It provides a higher-level of the disk storage than the traditional view of disks and partitions. This gives the system administrator much more flexibility in allocating storage to applications and users. Storage volumes created under the control of LVM can be resized and moved around almost at will, although this may need some upgrading of file system tools.
There are two version of LVM for Linux:
1) LVM 2 – The latest and greatest version of LVM for Linux
LVM 2 is almost completely backward compatible with volumes created with LVM 1. The exception to this is snapshots;
LVM 2 uses the device mapper kernel driver. Device mapper support is in the 2.6 kernel tree and there are patches available for current 2.4 kernels.
2) LVM 1 – The version that is in the 2.4 series kernel
LVM 1 is a mature product that has been considered stable for a couple of years. The kernel driver for LVM 1 is included in the 2.4 series kernels, but this does not mean that your 2.4.x kernel is up to date with the latest version of LVM.
2. Anatomy of LVM
Fig 1 shows what is in LVM.
1) Our hard drives /dev/sda, /dev/sdb … may include several partitions name /dev/sda1, /dev/sda2, /dev/sdb1 …;
2) Each physical partitions is a physical volume (PV) in LVM, PV is divided chunks of data, known as physical extents (PE);
3) Each volume group (VG) is composed of several PVs in LVM physical side;
4) Each volume group (VG) is composed of several logical volume (LV) in file system side, LV is split into chunks of data, known as logical extents (LE);
5) Each LV is mounting as a file system such as /usr, /local.
Fig 1 anatomy of LVM
3. LVM Common Task
1) Display
Pvdisplay, vgdisplay, lvdisplay, pvs, vgs, lvs
2) Initializing disks or disk partitions
# pvcreate /dev/sda1
3) Creating a volume group
# vgcreate my_volume_group /dev/sda1 /dev/sda2
4) Activating a volume group
# vgchange –a y my_volume_group
5) Removing a volume group
# vgchange –a n my_volume_group
# vgremove my_volume_group
6) Adding physical volumes to a volume group
# vgextend my_volume_group /dev/sda1
7) Removing physical volumes from a volume group
# vgreduce my_volume_group
8) Creating a logical volume
# lvcreate –L 1500 –ntestlv testvg
9) Removing a logical volume
# umount /dev/myvg/homevol
# lvremove /dev/myvg/homevol
10) Extending a logical volume
# lvextend –L 12G /dev/myvg/homevol
After you have extended logical volume it is necessary to increase the file system size to match, but default, most file system resizing tools will increase the size of the file system to be the size of the underlying logical volume, so you don’t need to worry about specifying the same size for each of the two commands.
# umount /dev/myvg/homevol
# resize2fs /dev/myvg/homevol
# mount /dev/myvg/homevol /home
11) Others
# lvresize 60G /dev/vg/lv -> # resize2fs 60G /dev/vg/lv
# pvmove /dev/sda1 /dev/sda2
4. All LVM commands
[PV]
pvchange pvcreate pvmove pvresize pvscan
pvck pvdisplay pvremove pvs pv.sh
[VG]
vgcfgbackup vgconvert vgextend vgreduce vgscan
vgcfgrestore vgcreate vgimport vgremove vgsplit
vgchange vgdisplay vgmerge vgrename
vgck vgexport vgmknodes vgs
[LV]
lvdisplay lvmdiskscan lvm.static lvresize
lvchange lvextend lvmdump lvreduce lvs
lvconvert lvm lvmsadc lvremove lvscan
lvcreate lvmchange lvmsar lvrename
5. Step-by-step to shrink a LVM physical volume
1) Boot into rescue mode with installation CD, skip mounting of the system partitions;
2) Activating volume group
# lvm vgchange –a y
3) Fsck your single, large root file system on LogVol00
#e2fsck –f /dev/VolGroup00/LogVol00
4) Shrink your(ext3) file system inside the LogVol00 container BEFORE shrinking its container
# resize2fs –p /dev/VolGroup00/LogVol00 50G
5) Shrink the LogVol container to match
# lvm lvresize /dev/VolGroup/LogVol00 –size 50G
6) There is now enough free physical space available (lvm pvs) to shrink the physical volume by that much in order to make room for a new partition of that size or smaller. Unfortunately, the current implementation of “pvresize” is lacking – from the man page: “pvresize will refuse to shrink physical volume if it has allocated extents after where its new end would be. In the future, it should relocate these elsewhere in the volume group if there is sufficient free space, like pvmove does.”
Because of this limitation, or we can say “this bug”, it’s necessary to temporarily remove your LoVol01 (swap LV), which is still physical allocated at the very end of your PV, then add it back such that its newly located immediately after the smaller LogVol00 nearer to the start of the PV (and partition).
Assuming your swap is “2G”, for example, do:
# lvm pvdisplay -m
# lvm lvremove /dev/VolGroup00/LogVol01
# lvm lvcreate –size 2G –name LogVol01 VolGroup00
# mkswap /dev/VolGroup00/LogVol01
7) Shrink the physical volume
First determine how much you can shrink it, by running # lvm pvs to see “PFree”, then
# lvm pvresize /dev/sda1 –setphysicalvolumesize 50G
If this step fails, it's probably because you're trying to shrink too small, and/or there are still allocated extents -- other than swap, which you moved in the step above -- after where the new tail end would be. A future version of pvresize will automatically move those to earlier free space as needed. (As an alternative, if you have lots of free space on another drive, you could pvcreate there, then pvmove temporarily, then pvresize, and pvmove back.)
8) Finally – Shrink the partition containing the shrunk PV to match its new size.
Display the PV sizes and take note of the smaller PSize:
# Lvm pvs for example it’s 65.00G
Display the partition and take note of the start sector and partition number of the partition with “lvm” in the “Flags” column:
#parted /dev/sda print
For example, it prints
Minor Start End Type Filesystem Flags
1 0.031 101.975 primary ext3 boot
2 101.975 76785.127 primary lvm
So we have free space 76.785127-65.00=11G(we need a safety margin of >=64MB)
9) Recreate our partitions
WARNING: Point of no return – have backups of your partitions point such as 101.975
So now we need to nuke the old, large partition from the partition table, and immediately recreate it with the exact same start sector, but with a newer, shorter end sector.
Nuke it # parted /dev/sda rm 2 (remove sda2)
Recreate it smaller + safety margin.
# parted /dev/sda mkpart primary 101.975 65785.127
# parted /dev/sda set 2 lvm on
# parted /dev/sda print
#fdisk –l /dev/sda
10) Optional step
You can use pvresize, lvresize and resize2fs onece more if you would not like to waste some space produced by above steps.
11) Verify you didn’t make a major oops truncating something by running fsck again
# fsck –fC /dev/VolGroup00/LogVol00
12) All done. You now have some unallocated space ourside of LVM to play with, enjoy! J