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Admin / Linux / LVM Quick Reference

This document is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY, either expressed or implied. While every effort has been taken to ensure the accuracy of the information documented herein, the author(s)/editor(s)/maintainer(s)/contributor(s) assumes NO RESPONSIBILITY for any errors, or for any damages, direct or consequential, as a result of the use of the information documented herein.

Literaturempfehlungen

Wichtig: vor dem Murksen kommt das Lesen! Man muß schließlich wissen, was man da eigentlich treibt... Man-Pages, Info-Dateien, HOWTOS, READMEs, etc. Falls manche Befehle unverständlich sind wegen ihren Kommandozeilenparamentern, dann ist es an der Zeit, die man-Page zu lesen...

Logical Volume Manager HOWTO, also try your local copy in /usr/share/doc/howto/en/html/LVM-HOWTO.html.

LVM HOWTO

LVM-Administration

Prepare partition /dev/sdb1 for being a member in the LVM system. Two steps:

Set the partition system id to 0x8e ("Linux LVM"):

shell$ fdisk /dev/sdb
Command (m for help): p

Disk /dev/sdb: 255 heads, 63 sectors, 555 cylinders
Units = cylinders of 16065 * 512 bytes

   Device Boot    Start       End    Blocks   Id  System
/dev/sdb1             1       555   4458006   8e  Linux LVM

Command (m for help): t
Partition number (1-4): 1
Hex code (type L to list codes): 8e

Command (m for help): w
[...]
shell$

Check it:

shell$ fdisk -l /dev/sdb

Disk /dev/sdb: 255 heads, 63 sectors, 555 cylinders
Units = cylinders of 16065 * 512 bytes

   Device Boot    Start       End    Blocks   Id  System
/dev/sdb1             1       555   4458006   8e  Linux LVM

Sign the partition for use in the LVM system ("make it a Physical Volume"):
```
shell$ pvcreate /dev/sdb1
```

Now we need to tell the LVM system which devices/partitions are part of the LVM system.

If LVM has never been used on your system before, it is lacking its database files /etc/lvmtab and /etc/lvmtab.d/*. You can search for them in your filesystem, or the first invocation of one of the vg*-commands will tell you so.

So we create the database:

shell$ vgscan --verbose

Now the LVM system knows where to store its meta information. So we can start throwing partitions into the LVM system. As this would create a mess, LVM provides the concept of Volume Groups (VGs); You can have one or more VGs where you can throw partitions into. And you can name these VGs as you like (and rename them afterwards, too).

We create one VG named "test_vg" and stick the partition /dev/sdb1 into it:

shell$ vgcreate test_vg /dev/sdb1

You may have prepared further partitions, i.e. /dev/sdc6; stick it into VG "test_vg", too:

shell$ vgextend test_vg /dev/sdc6

Now we have gathered all/most of our unused/available partitions in one great pool, a VG called "test_vg".

Now it would be fine to create one huge "virtual partition" from this pool, create a filesystem on it, mount it somewhere, be happy and go to sleep.

But stop. - Your sleep wouldn't last long, because of the following experiences in Real Life:

Your users did/could not, did not want to, were not able to estimate/forecast their disk space requirements. They just wanted "all the rest" in /where/they/thought/they/need-it/. Now they want more space in /somewhere/else/, and they are far from spending "all the rest" in /where/they/thought/they/need-it/... Now you are locked out, because all your disk space is allocated in the pool under /where/they/thought/they/need-it/...

But LVM has foreseen this and provides the solution. It lets you split the each VG into one ore more Logical Volumes (LVs). These LVs are variable in size, and they are the new "partitions" where you can create filesystems on and mount them as usual.

Let's do that now, within VG "test_vg" create two LVs named "test00_lv" (1600MB in size) and "test01_lv" (500MB in size):

shell$ lvcreate --size 1600M --name test00_lv test_vg
shell$ lvcreate --size  500M --name test01_lv test_vg

This creates the following device special files:

shell$ ls -l /dev/test_vg
total 46
crw-r-----    1 root     disk     109,   0 Mar 15 21:24 group
brw-rw----    1 root     disk      58,   0 Mar 15 21:28 test00_lv
brw-rw----    1 root     disk      58,   1 Mar 15 21:28 test01_lv

We use /dev/test_vg/test0[10]_lv as "partitions", create (ext2-) filesystems on them and mount them (also make an entry in /etc/fstab):

shell$ mkfs -t ext2 /dev/test_vg/test00_lv
shell$ mkfs -t ext2 /dev/test_vg/test01_lv

shell$ mount /dev/test_vg/test00_lv /where/they/thought/they/need-it/
shell$ mount /dev/test_vg/test01_lv /somewhere/else/

Now go to sleep...

Suddenly (after your winter sleep) the users ring your phone, wanting more disk space under /somewhere/else/... - This is the hour where LVM shows its strength.

You hurry to your glass teletype to see how you can shift disk space space from /where/they/thought/they/need-it/ (/dev/test_vg/test00_lv) to /somewhere/else/ (/dev/test_vg/test01_lv). Type in the following commands:

shell$ vgdisplay
--- Volume group ---
VG Name               test_vg
VG Access             read/write
VG Status             available/resizable
VG #                  0
MAX LV                256
Cur LV                2
Open LV               1
MAX LV Size           255.99 GB
Max PV                256
Cur PV                2
Act PV                2
VG Size               6.33 GB
PE Size               4 MB
Total PE              1621
Alloc PE / Size       525 / 2.05 GB
Free  PE / Size       1096 / 4.28 GB
VG UUID               BKIj2D-wOBk-JAXO-SZsu-j5MD-3hCH-2LTIvo

You see that VG "test_vg" has a total size of 6.33 GB ("VG Size").

shell$ lvdisplay /dev/test_vg/test0[01]_lv
--- Logical volume ---
LV Name                /dev/test_vg/test00_lv
VG Name                test_vg
LV Write Access        read/write
LV Status              available
LV #                   1
# open                 0
LV Size                1.56 GB
Current LE             400
Allocated LE           400
Allocation             next free
Read ahead sectors     120
Block device           58:0

--- Logical volume ---
LV Name                /dev/test_vg/test01_lv
VG Name                test_vg
LV Write Access        read/write
LV Status              available
LV #                   2
# open                 0
LV Size                500 MB
Current LE             125
Allocated LE           125
Allocation             next free
Read ahead sectors     120
Block device           58:1

You see that /dev/test_vg/test00_lv has size 1600 MB ("LV Size), /dev/test_vg/test01_lv has size 500 MB ("LV Size").

Now you notice that calculating in MB is inaccurate. Further you need to know that you can only resize LVs by a minimum unit called "Logical Extends" (LEs); these are internally mapped to "Physical Extends" (PEs) on the VG/PV-level. Their sizes are equal and can be seen from the output from the vgdisplay-command, "PE Size" (here: 4 MB).

Let's do some calulation:

shell$ bc -q
total_pe=1621     # from vgdisplay, "Total PE"
le_test00_lv=400  # from lvdisplay, "Current LE"
le_test01_lv=125  # from lvdisplay, "Current LE"

rest = total_pe - le_test00_lv - le_test01_lv
rest
1096
^D
shell$

You have two options here:

throw some of rest=1096 LEs at /dev/test_vg/test01_lv (/somewhere/else/), or
shift some LEs from /dev/test_vg/test00_lv to /dev/test_vg/test01_lv.

We discuss option 1 first, because it is easier.

At this point we must notice that we have filesystems on "virtual partitions" (LVs), and if we resize the underlying partitions, we destroy the filesystems! Two cases:

extend the LV: the filesystem won't notice, so nothing won here (maybe even the filesystem will be corrupted!).
shrink the LV: the filesystem will be corrupted! Dont't do that!

So what to do? - Two ways:

Copy all data out from your LV, resize it, recreate filesystem, mount it again, copy data in.
Tell the filesystem developers about the needed capability to "resize" their filesystem. - Fortunately they have implemented this feature in some of the fileystems available to Linux.

Here is a list (date: Sam Mär 16 14:19:54 MET 2002), which filesystems support resizability:

FS type	resizability	via command
ext2	unmounted	e2fsadm
reiserfs	unmounted	lvextend/lvreduce,resize_reiserfs

todo @@@@@@@@@@, list to complete, others to come

So we want to resize /dev/test_vg/test01_lv (/somewhere/else/); Let's assume we have a non-resizable filesystem on it:

Copy all your data out via cp, tar, cpio or your favourite backup tool. (Make shure your backup is readable...)
```
shell$ umount /dev/test_vg/test01_lv
```
Resize your LV /dev/test_vg/test01_lv by 700MB:
```
shell$ lvextend --size +700M /dev/test_vg/test01_lv --verbose
```
These 700MB are taken from the unallocated PEs (rest=1096) in VG "test_vg".

shell$ mkfs -t ext2 /dev/test_vg/test01_lv

shell$ mount /dev/test_vg/test01_lv /somewhere/else/

Copy all your data in again via cp, tar, cpio or your favourite backup tool. (Now you know, whether your backup was readable...)

A lot of work... Now assume we we have a unmounted-resizable filesystem on /dev/test_vg/test01_lv, like ext2:

shell$ umount /dev/test_vg/test01_lv
shell$ e2fsadm --size +700M /dev/test_vg/test01_lv --verbose
shell$ fsck -t ext2 -f /dev/test_vg/test_lv
shell$ mount /dev/test_vg/test01_lv /somewhere/else/

Now extend /dev/test_vg/test01_lv with reiserfs (reiserfsprogs-3.x.0j, RTFM 'man resize_reiserfs'!):

shell$ umount /dev/test_vg/test01_lv
shell$ lvextend --size +700M /dev/test_vg/test01_lv --verbose
shell$ resize_reiserfs -s +700M /dev/test_vg/test01_lv 
shell$ mount /dev/test_vg/test01_lv /somewhere/else/

Now shrink /dev/test_vg/test01_lv with reiserfs (reiserfsprogs-3.x.0j, RTFM 'man resize_reiserfs'!):

shell$ umount /dev/test_vg/test01_lv
shell$ resize_reiserfs -s -700M /dev/test_vg/test01_lv
shell$ lvreduce --size -700M /dev/test_vg/test01_lv --force 
shell$ mount /dev/test_vg/test01_lv /somewhere/else/

Still a lot of work... Now assume we we had a mounted-resizable filesystem on /dev/test_vg/test01_lv, like @@@non-existant-fs???@@@@:

shell$ resize_@@@non-existant-fs???@@@@ -s +700M /dev/test_vg/test01_lv

FAQ

Q: How can I join disk space?

Use LVM...

Q: Which filesytem type should I use?

Use a filesystem that conforms to the following criteria:

stable
journalling (if fsck-time matters)
fast (if performance matters)
resizable, ideally mounted-resizable (if there exists one?)

Q: How can I shift disk space between LVs?

@@@@@@@@@ TODO

Q: How can I shift disk space between VGs?

You can only shift PVs between VGs. Shifting LEs/PEs between VGs wouldn not work because PE are mapped to LE, and each PE belongs to one PV.

Q: How can I find all devices, which I could use for LVM?

shell$ lvmdiskscan

Q: How can I find all devices, which are marked for use by LVM (partition system id)?

shell$ lvmdiskscan --lvmpartition

Q: How can I find all devices, which are prepared to be used by LVM?
(I mean: ..., which have been treated with pvcreate?)

shell$ lvmdiskscan | \
grep -i "0x8e" | \
while read foo1 foo2 device rest ; do echo $device ; done | \
xargs pvdisplay --colon

Q: How can I find all devices, which are actually used by LVM?
(I mean: ..., which have been added to a VG via vgcreate/vgextend?)

shell$ vgdisplay --verbose | grep "PV Name"

Q: How can I remove a PV from a VG (especially when PEs/LEs have been allocated by one or more LVs)?

Move all allocated PEs from the PV to other PVs (in the same VG), thus freeing the PV you want to split off:
```
shell$ pvmove ...
```
Reduce the VG by removing the PV:
```
shell$ vgreduce ...
```

Q: How can I rename a VG?

Only VGs with _no_ open LVs can be deactivated! (See next question.) Then

shell$ vgchange --available n test_vg
shell$ vgrename test_vg vg00

Q: How can I see, whether a VG has open LVs? And if it has:
which of the LVs are open (=mounted)? And how can I close them?

shell$ vgdisplay --verbose | grep -e "Open LV" -e "LV Name" -e "# open"
Open LV               1
LV Name                /dev/test_vg/test00_lv
# open                 0
LV Name                /dev/test_vg/test01_lv
# open                 1

Now close the open LV /dev/test_vg/test01_lv:

shell$ umount /dev/test_vg/test01_lv
shell$ vgdisplay --verbose | grep -e "Open LV" -e "LV Name" -e "# open"
Open LV               1
LV Name                /dev/test_vg/test00_lv
# open                 0
LV Name                /dev/test_vg/test01_lv
# open                 0

Now you can do things like vgchange ... test_vg.

Q: How can I see, which process keeps a mounted filesystem busy?
("umount: /mnt/rest/test01_lv: device is busy")

Use lsof .

Q: Can I use entire disks?

Yes, you can. You do not have to create partitions on your disks; you can do pvcreate /dev/sdb.

Q: How to move data from one physical device to another while online?

@@@@@@@@@ TODO

Q: Can I use a LV as root partition/filesystem?

I expect the answer to be 'No', because the bootloader LILO needs to know, where the kernel image is located. I believe, that the kernel image must be located within one partition. With LVM it could happen, that kernel image gets spread over different partitions or even disks. - Is there someone out there who knows better? Please let me know.

Q: Can I use LVs as raw partitions?

@@@@@@@@@ TODO

© 2002 ich
Created: Sam Mär 16 18:26:22 MET 2002
Last updated:
2007-09-13T13:34:30+0200
EOF

Admin / Linux / LVM Quick Reference

Content:

Introduction

Disclaimer

Literaturempfehlungen

LVM-Administration

FAQ

Introduction

Disclaimer

Literaturempfehlungen

LVM-Administration

FAQ

Q: How can I join disk space?

Q: Which filesytem type should I use?

Q: How can I shift disk space between LVs?

Q: How can I shift disk space between VGs?

Q: How can I find all devices, which I could use for LVM?

Q: How can I find all devices, which are marked for use by LVM (partition system id)?

Q: How can I find all devices, which are prepared to be used by LVM?
(I mean: ..., which have been treated with pvcreate?)

Q: How can I find all devices, which are actually used by LVM?
(I mean: ..., which have been added to a VG via vgcreate/vgextend?)

Q: How can I remove a PV from a VG (especially when PEs/LEs have been allocated by one or more LVs)?

Q: How can I rename a VG?

Q: How can I see, whether a VG has open LVs? And if it has:
which of the LVs are open (=mounted)? And how can I close them?

Q: How can I see, which process keeps a mounted filesystem busy?
("umount: /mnt/rest/test01_lv: device is busy")

Q: Can I use entire disks?

Q: How to move data from one physical device to another while online?

Q: Can I use a LV as root partition/filesystem?

Q: Can I use LVs as raw partitions?

Admin / Linux / LVM Quick Reference

Q: How can I join disk space?

Q: Which filesytem type should I use?

Q: How can I shift disk space between LVs?

Q: How can I shift disk space between VGs?

Q: How can I find all devices, which I could use for LVM?

Q: How can I find all devices, which are marked for use by LVM (partition system id)?

Q: How can I find all devices, which are prepared to be used by LVM? (I mean: ..., which have been treated with pvcreate?)

Q: How can I find all devices, which are actually used by LVM? (I mean: ..., which have been added to a VG via vgcreate/vgextend?)

Q: How can I remove a PV from a VG (especially when PEs/LEs have been allocated by one or more LVs)?

Q: How can I rename a VG?

Q: How can I see, whether a VG has open LVs? And if it has: which of the LVs are open (=mounted)? And how can I close them?

Q: How can I see, which process keeps a mounted filesystem busy? ("umount: /mnt/rest/test01_lv: device is busy")

Q: Can I use entire disks?

Q: How to move data from one physical device to another while online?

Q: Can I use a LV as root partition/filesystem?

Q: Can I use LVs as raw partitions?

Q: How can I find all devices, which are prepared to be used by LVM?
(I mean: ..., which have been treated with pvcreate?)

Q: How can I find all devices, which are actually used by LVM?
(I mean: ..., which have been added to a VG via vgcreate/vgextend?)

Q: How can I see, whether a VG has open LVs? And if it has:
which of the LVs are open (=mounted)? And how can I close them?

Q: How can I see, which process keeps a mounted filesystem busy?
("umount: /mnt/rest/test01_lv: device is busy")