Posts Tagged ‘LVM’

Relocating LVs with snapshots

Monday, February 2nd, 2009

Linux LVM is a wonderful thing. It is scalable, flexible, and truly, almost enterprise-class in every details. It lacks, of course, at IO performance for LVM snapshots, but this can be worked-around in several creative ways (if I haven’t shown here before, I will sometime).

What it can’t do is dealing with a mixture of Stripes, Mirrors and Snapshots in a single logical volume. It cannot allow you to mirror a stripped LV (even if you can follow the requirementes), it cannot allow you to snapshot a mirrored or a stripped volume. You get the idea. A volume you can protect, you cannot snapshot. A volume with snapshots cannot be mirrored or altered.

For the normal user, what you get is usually enough. For storage management per-se, this is just not enough. When I wanted to reduce a VG – remove a disk from an existing volume group,¬† I had to evacuate it from any existing logical volume. The command to perform this actions is ‘pvmove‘ which is capable of relocating data from within a PV to other PVs. This is done through mirroring each logical volume and then removing the origin.

Mirroring, however, cannot be performed on LVs with snapshots, or on an already mirrored LV, so these require different handling.

We can detect which LVs reside on our physical volume by issuing the following command

pvdisplay -m /dev/sdf1

/dev/sdf1 was only an example. You will see the contents of this PV. So next, performing

pvmove /dev/sdf1

would attempt to relocate every existing LV from this specific PV to any other available PV. We can use this command to change the disk balance and allocations on multi-disk volume groups. This will be discussed on a later post.

Following a ‘pvmove‘ command, all linear volumes are relocated, if space permits, to another PVs. The remaining LVs are either mirrored or LVs with snapshots.

To relocate a mirrored LV, you need to un-mirror it first. To do so, first detect using ‘pvdisplay‘ which LV is belongs to (the name should be easy to follow) and then change it to non-mirrored.

lvconvert -m0 /dev/VolGroup00/test-mirror

This will convert it to be a linear volume instead of a mirror, so you could move it, if it still resides on the PV you are to remove.

Snapshot volumes are more complicated, due to their nature. Since all my snapshots are of a filesystem, I could allow myself to use tar to perform the action.

The steps are as follow:

  1. tar the contents of the snapshot source to nowhere, but save an incremental file
  2. Copy the source incremental file to a new name, and tar the contents of a snapshot according to this copy.
  3. Repeat the previous step for each snapshot.
  4. Remove all snapshots
  5. Relocate the snapshot source using ‘pvmove
  6. Build the snapshots and then recover the data into them

This is a script to do steps 1 to 3. It will not remove LVs, for obvious reasons. This script was not tested, but should work, of course ūüôā

None of the LVs should be mounted for it to function. It’s better to have harder requirements than to destroy data by double-mounting it, or accessing it while it is being changed.

# Get: VG Base-LV, snapshot name, snapshot name, snapshot name...
# Example:
# ./backup VolGroup00 base snap1 snap2 snap3
# Written by Ez-Aton

if [ "[email protected]" -le 3 ]
   echo "Parameters: $0 VG base snap snap snap snap"
   exit 1
shift 2

function check_not_mounted () {
   # Check if partition is mounted
   if mount | grep /dev/mapper/${VG}-${1}
      return 0
      return 1

function create_base_diff () {
   # This function will create the diff file for the base
   mount /dev/${VG}/${BASE} $MNT
   if [ $? -ne 0 ]
      echo "Failed to mount base"
      exit 1
   cd $MNT
   tar -g $TARGET/${BASE}.tar.gz.diff -czf - . > /dev/null
   cd -
   umount $MNT

function create_snap_diff () {
   mount /dev/${VG}/${1} $MNT
   if [ $? -ne 0 ]
      echo "Failed to mount base"
      exit 1
   cp $TARGET/${BASE}.tar.gz.diff $TARGET/$1.tar.gz.diff
   cd $MNT
   tar -g $TARGET/${1}.tar.gz.diff -czf $TARGET/${1}.tar.gz .
   cd -
   umount $MNT

function create_mount () {
   # Creates a temporary mount point
   if [ ! -d /mnt/$$ ]
      mkdir /mnt/$$

if check_not_mounted $BASE
   echo "$BASE is mounted. Exiting now"
   exit 1
for i in [email protected]
   if check_not_mounted $i
      create_snap_diff $i
      echo "$i is mounted! I will not touch it!"

The remaining steps should be rather easy – just mount the newly created snapshots and restore the tar file on them.

Quick provisioning of virtual machines

Friday, February 1st, 2008

When one wants to achieve fast provisioning of virtual machines, some solutions might come into account. The one I prefer uses Linux LVM snapshot capabilities to duplicate one working machine into few.

This can happen, of course, only if the host running VMware-Server is Linux.

LVM snapshots have one vast disadvantage – performance. When a block on the source of the snapshot is being changed for the first time, the original block is being replicated to each and every snapshot COOW space. It means that a creation of a 1GB file on a volume having ten snapshots means a total copy of 10GB of data across your disks. You cannot ignore this performance impact.

LVM2 has support for read/write snapshots. I have come up with a nice way of utilizing this capability to my benefit. An R/W snapshot which is being changed does not replicate its changes to any other snapshot. All changes are considered local to this snapshot, and are being maintained only in its COOW space. So adding a 1GB file to a snapshot has zero impact on the rest of the snapshots or volumes.

The idea is quite simple, and it works like this:

1. Create adequate logical volume with a given size (I used 9GB for my own purposes). The name of the LV in my case will be /dev/VGVM3/centos-base

2. Mount this LV on a directory, and create a VM inside it. In my case, it’s in /vmware/centos-base

3. Install the VM as the baseline for all your future VMs. If you might not want Apache on some of them, don’t install it on the baseline.

4. Install vmware-tools on the baseline.

5. Disable the service “kudzu”

6. Update as required

7. In my case I always use DHCP. You can set it to obtain its IP once from a given location, or whatever you feel like.

8. Shut down the VM.

9. In the VM’s .vmx file add a line like this:

uuid.action = “create”

I have added below (expand to read) two scripts which will create the snapshot, mount it and register it, including new MAC and UUID.

Press below for the scripts I have used to create and destroy VMs

# This script will replicate vms from a given (predefined) source to a new system
# Written by Ez-Aton,
# Arguments: name

test_can_do () {
# To be able to snapshot, we need a set of things to happen
if [ -d $DIR/$TARGET ] ; then
echo “Directory already exists. You don’t want to do it…”
exit 1
if [ -f $VG/$TARGET ] ; then
echo “Target snapshot exists”
exit 1
if [ `vmrun list | grep -c $DIR/$SRC/$SRC.vmx` -gt “0” ] ; then
echo “Source VM is still running. Shut it down before proceeding”
exit 1
if [ `vmware-cmd -l | grep -c $DIR/$TARGET/$SRC.vmx` -ne “0” ] ; then
echo “VM already registered. Unregister first”
exit 1

do_snapshot () {
# Take the snapshot
lvcreate -s -n $TARGET -L $SNAPSIZE $VG/$SRC
if [ “$RET” -ne “0” ]; then
echo “Failed to create snapshot”
exit 1

mount_snapshot () {
# This function creates the required directories and mounts the snapshot there
mkdir $DIR/$TARGET
if [ “$RET” -ne “0” ]; then
echo “Failed to mount snapshot”
exit 1

alter_snap_vmx () {
# This function will alter the name in the VMX and make it the $TARGET name
cat $DIR/$TARGET/$SRC.vmx | grep -v “displayName” > $DIR/$TARGET/$TARGET.vmx
echo “displayName = “$TARGET”” >> $DIR/$TARGET/$TARGET.vmx

register_vm () {
# This function will register the VM to VMWARE
vmware-cmd -s register $DIR/$TARGET/$SRC.vmx

if [ -z “$1” ]; then
echo “Arguments: The target name”
exit 1

# Parameters:
SRC=centos-base         #The name of the source image, and the source dir
PREFIX=centos             #All targets will be created in the name centos-$NAME
DIR=/vmware               #My VMware VMs default dir
SNAPSIZE=6G              #My COOW space
VG=/dev/VGVM3           #The name of the VG

exit 0

# This script will remove a snapshot machine
# Written by Ez-Aton,
# Arguments: machine name

does_it_exist () {
# Check if the described VM exists
if [ `vmware-cmd -l | grep -c $DIR/$TARGET/$SRC.vmx` -eq “0” ]; then
echo “No such VM”
exit 1
if [ ! -e $VG/$TARGET ]; then
echo “There is no matching snapshot volume”
exit 1
if [ `lvs $VG/$TARGET | awk ‘{print $5}’ | grep -c $SRC` -eq “0” ]; then
echo “This is not a snapshot, or a snapshot of the wrong LV”
exit 1

ask_a_thousand_times () {
# This function verifies that the right thing is actually done
echo “You are about to remove a virtual machine and an LVM. Details:”
echo “Machine name: $TARGET”
echo “Logical Volume: $VG/$TARGET”
echo -n “Are you sure? (y/N): ”
read RES
if [ “$RES” != “Y” ]&&[ “$RES” != “y” ]; then
echo “Decided not to do it”
exit 0
echo “”
echo “You have asked to remove this machine”
echo -n “Again: Are you sure? (y/N): ”
read RES
if [ “$RES” != “Y” ]&&[ “$RES” != “y” ]; then
echo “Decided not to do it”
exit 0
echo “Removing VM and snapshot”

shut_down_vm () {
# Shut down the VM and unregister it
vmware-cmd $DIR/$TARGET/$SRC.vmx stop hard
vmware-cmd -s unregister $DIR/$TARGET/$SRC.vmx

remove_snapshot () {
# Umount and remove the snapshot
umount $DIR/$TARGET
if [ “$RET” -ne “0” ]; then
echo “Cannot umount $DIR/$TARGET”
exit 1
lvremove -f $VG/$TARGET
if [ “$RET” -ne “0” ]; then
echo “Cannot remove snapshot LV”
exit 1

remove_dir () {
# Removes the mount point
rmdir $DIR/$TARGET

if [ -z “$1” ]; then
echo “No machine name. Exiting”
exit 1

DIR=/vmware                #VMware default VMs location
VG=/dev/VGVM3            #The name of the VG
PREFIX=centos              #Prefix to the name. All these VMs will be called centos-$NAME
SRC=centos-base           #The name of the baseline image, LVM, etc. All are the same


exit 0


1. Very fast provisioning. It takes almost five seconds, and that’s because my server is somewhat loaded.

2. Dependable: KISS at its marvel.

3. Conservative on space

4. Conservative on I/O load (unlike the traditional use of LVM snapshot, as explained in the beginning of this section).


1. Cannot streamline the contents of snapshot into the main image (LVM team will implement it in the future, I think)

2. Cannot take a snapshot of a snapshot (same as above)

3. If the COOW space of any of the snapshots is full (viewable through the command ‘lvs‘) then on boot, the source LV might not become active (confirmed RH4 bug, and this is the system I have used)

4. My script does not edit/alter /etc/fstab (I have decided it to be rather risky, and it was not worth the effort at this time)

5. My script does not check if there is enough available space in the VG. Not required, as it will fail if creation of LV will fail

You are most welcome to contribute any further changes done to this script. Please maintain my URL in the script if you decide to use it.


Linux LVM performace measurement

Sunday, June 10th, 2007

Modern Linux LVM offers great abilities to maintain snapshots of existing logical volumes. Unlike NetApp “Write Anywhere File Layout” (WAFL), Linux LVM uses “Copy-on-Write” (COW) to allow snapshots. The process, in general, can be described in this pdf document.

I have issues several small tests, just to get real-life estimations of what is the actual performance impact such COW method can cause.

Server details:

1. CPU: 2x Xion 2.8GHz

2. Disks: /dev/sda – system disk. Did not touch it; /dev/sdb – used for the LVM; /dev/sdc – used for the LVM

3. Mount: LV is mounted (and remains mounted) on /vmware


1. No snapshot, Using VG on /dev/sdb only:

# time dd if=/dev/zero of=/vmware/test.2GB bs=1M count=2048
2048+0 records in
2048+0 records out

real 0m16.088s
user 0m0.009s
sys 0m8.756s

2. With snapshot on the same disk (/dev/sdb):

# time dd if=/dev/zero of=/vmware/test.2GB bs=1M count=2048
2048+0 records in
2048+0 records out

real 6m5.185s
user 0m0.008s
sys 0m11.754s

3. With snapshot on 2nd disk (/dev/sdc):

# time dd if=/dev/zero of=/vmware/test.2GB bs=1M count=2048
2048+0 records in
2048+0 records out

real 5m17.604s
user 0m0.004s
sys 0m11.265s

4. Same as before, creating a new empty file on the disk:

# time dd if=/dev/zero of=/vmware/test2.2GB bs=1M count=2048
2048+0 records in
2048+0 records out

real 3m24.804s
user 0m0.006s
sys 0m11.907s

5. Removed the snapshot. Created a 3rd file:

LVM Snapshots with MySQL

Saturday, December 2nd, 2006

Nowadays, when LVM2 is common and is actually the default in installation of RedHat based distributions, using its snapshot capabilities can save lots of grief when files are deleted or when you need to revert to a day in the past – both for your files and for your MySQL DB.

I have created a script which is based on the following assumptions:

1. Inside /etc/samba/smb.conf there is a directive such as: include /etc/samba/smb.conf

2. There is a single LV containing all the system’s data. It doesn’t occupy all the physical disk (or, for the matter, the entire VG space). Free space is 10-20% of disk size

3. Specific share directives are located inside /etc/samba/smb.conf.snapshot.full. An empty file /etc/samba/smb.conf.snapshot.empty exists.

4. I do not trust all places to hold a password for their MySQL (although it is advised!). This script assumes such password doesn’t always exist

5. The script mounts the snapshot read-only just after creating an empty file with the date of the snapshot inside its root.

The script is attached here. take-snapshot.txt

Single-Node Linux Heartbeat Cluster with DRBD on Centos

Monday, October 23rd, 2006

The trick is simple, and many of those who deal with HA cluster get at least once to such a setup – have HA cluster without HA.

Yep. Single node, just to make sure you know how to get this system to play.

I have just completed it with Linux Heartbeat, and wish to share the example of a setup single-node cluster, with DRBD.

First – get the packages.

It took me some time, but following Linux-HA suggested download link (funny enough, it was the last place I’ve searched for it) gave me exactly what I needed. I have downloaded the following RPMS:









I was required to add up the following RPMS:




I have added DRBD RPMS, obtained from YUM:


kernel-module-drbd-2.6.9-42.EL-0.7.21-1.c4.i686.rpm (Note: Make sure the module version fits your kernel!)

As soon as I finished searching for dependent RPMS, I was able to install them all in one go, and so I did.

Configuring DRBD:

DRBD was a tricky setup. It would not accept missing destination node, and would require me to actually lie. My /etc/drbd.conf looks as follows (thanks to the great assistance of

resource web {
protocol C;
incon-degr-cmd “echo ‘!DRBD! pri on incon-degr’ | wall ; sleep 60 ; halt -f”; #Replace later with halt -f
startup { wfc-timeout 0; degr-wfc-timeout 120; }
disk { on-io-error detach; } # or panic, …
syncer {
group 0;
rate 80M; #1Gb/s network!
on p800old {
device /dev/drbd0;
disk /dev/VolGroup00/drbd-src;
address; #eth0 network address!
meta-disk /dev/VolGroup00/drbd-meta[0];
on node2 {
device /dev/drbd0;
disk /dev/sda1;
address; #eth0 network address!
meta-disk /dev/sdb1[0];

I have had two major problems with this setup:

1. I had no second node, so I left this “default” as the 2nd node. I never did expect to use it.

2. I had no free space (non-partitioned space) on my disk. Lucky enough, I tend to install Centos/RH using the installation defaults unless some special need arises, so using the power of the LVM, I have disabled swap (swapoff -a), decreased its size (lvresize -L -500M /dev/VolGroup00/LogVol01), created two logical volumes for DRBD meta and source (lvcreate -n drbd-meta -L +128M VolGroup00 && lvcreate -n drbd-src -L +300M VolGroup00), reformatted the swap (mkswap /dev/VolGroup00/LogVol01), activated the swap (swapon -a) and formatted /dev/VolGroup00/drbd-src (mke2fs -j /dev/VolGroup00/drbd-src). Thus I have now additional two volumes (the required minimum) and can operate this setup.

Solving the space issue, I had to start DRBD for the first time. Per Linux-HA DRBD Manual, it had to be done by running the following commands:

modprobe drbd

drbdadm up all

drbdadm — –do-what-I-say primary all

This has brought the DRBD up for the first time. Now I had to turn it off, and concentrate on Heartbeat:

drbdadm secondary all

Heartbeat settings were as follow:


use_logd on #?Or should it be used?
udpport 694
keepalive 1 # 1 second
deadtime 10
initdead 120
bcast eth0
node p800old #`uname -n` name
crm yes
auto_failback off #?Or no
compression bz2
compression_threshold 2

I have also created a relevant /etc/ha.d/haresources, although I’ve never used it (this file has no importance when using “crm=yes” in I did, however, use it as a source for /usr/lib/heartbeat/

p800old IPaddr:: drbddisk::web Filesystem::/dev/drbd0::/mnt::ext3 httpd

It is clear that the virtual IP will be in my class A network, and DRBD would have to go up before mounting the storage. After all this, the application would kick in, and would bring up my web page. The application, Apache, was modified beforehand to use the IP, and to search for DocumentRoot in /mnt

Running /usr/lib/heartbeat/ on the file (no need to redirect output, as it is already directed to /var/lib/heartbeat/crm/cib.xml), and I was ready to go.

/etc/init.d/heartbeat start (while another terminal is open with tail -f /var/log/messages), and Heartbeat is up. It took it few minutes to kick the resources up, however, I was more than happy to see it all work. Cool.

The logic is quite simple, the idea is very basic, and as long as the system is being managed correctly, there is no reason for it to get to a dangerous state. Moreover, since we’re using DRBD, Split Brain cannot actually endanger the data, so we get compensated for the price we might pay, performance-wise, on a real two-node HA environment following these same guidelines.

I cannot express my gratitude to, which is the source of all this (adding up with some common sense). Their documents are more than required to setup a full working HA environment.