Allow Linux VM running on KVM paravirtualized environment to support disk hotplugging

In case you have a Linux OS VM running on KVM paravirtualized environment and found that it doesn't response to the new virtual disk you added (you dont see a new disk on dmesg or nothing coming up from fdisk -l), you will need to make sure virtio_blk driver is in place.


[root@vm ~]# lsmod | grep -i virtio
virtio_net             15665  0
virtio_balloon          4281  0
virtio_blk              5087  3
virtio_pci              6733  0
virtio_ring             7169  4 virtio_net,virtio_balloon,virtio_blk,virtio_pci
virtio                  4824  4 virtio_net,virtio_balloon,virtio_blk,virtio_pci


Indeed, you should be seeing /dev/vd* instead of /dev/sd* if you have picked correct OS type during VM initialization. In case you are seeing /dev/sd*, probably you didnt pick correct OS type and that caused virtio associated driver not being loaded.

[root@vm ~]# fdisk -l /dev/vda

Disk /dev/vda: 21.5 GB, 21474836480 bytes
16 heads, 63 sectors/track, 41610 cylinders
Units = cylinders of 1008 * 512 = 516096 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x0006fe0a

   Device Boot      Start         End      Blocks   Id  System
/dev/vda1   *           3         409      204800   83  Linux
Partition 1 does not end on cylinder boundary.
/dev/vda2             409       41611    20765696   8e  Linux LVM
Partition 2 does not end on cylinder boundary.

Garbage / Missing fonts in RHEL Virt-manager

Virt-Manager is a X application which is capable to manage RHEL's XEN, QEMU/KVM, LXC service. Similar to other X application, it could be launched remotely via X11 Forwarding.

However, it happens that garbage fonts will be shown if the host is not installed with appropriate font package. To fix this, package dejavu-lgc-sans-fonts have to be installed.

[root@server ~]# yum -y install dejavu-lgc-sans-fonts

Disable X windows on Ubuntu 10.04

In CentOS / RHEL, people would change option on initdefault from /etc/inittab to control whether X11 should be started. In Ubuntu, it is totally different, there is no such iniitab file on Ubuntu. To stop Ubuntu to start X11 (i.e. behave more or less the same as runlevel 3 in CentOS), one have to modify the /etc/init/gdm.conf and edit the line to disable gdm service on runlevel 2.

Here is the line

# stop on runlevel [0126]  ## This means GDM will start on run level 1, 2 and 6.

stop on runlevel [016] ## This means GDM will only start on run level 1 and 6. And Ubuntu's default run level is 2, i.e GDM wont start on default run level.

After that, a system reboot will bring your ubuntu installation to text mode.

Recover corrupted LVM root partition by fsck

So you have a Linux machine that is installed with LVM and you have assigned root partition to sit on that LVM pool. One day your machine is crashed and the root partition is corrupted. It failed to boot properly and asked you to fill in root password to run fsck. Sadly, you lost the root password and you need a recovery CD (or installation CD) to boot and recover the disk.

Now your machine is booted and you found that your just couldn't run fsck against a LVM partition.

root@test:/# fdisk -l

Disk /dev/vda: 21.5 GB, 21474836480 bytes
16 heads, 63 sectors/track, 41610 cylinders
Units = cylinders of 1008 * 512 = 516096 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00014ef8

   Device Boot      Start         End      Blocks   Id  System
/dev/vda1   *           3         389      194560   83  Linux
/dev/vda2             391       41609    20773888   8e  Linux LVM
So rescue CD by default just won't automatically activate the LVM (and its underlaying volumes). What you need to do is to activate the LVM partition. and then run fsck on the volumes.

### Run lvm from rescue CD
bash-4.1# lvm

### This will scan and list the PV
lvm> pvscan 
   PV /dev/vda2   VG volgroup01   lvm2 [19.78GiB/ 0     free]
   Total: 1 [19.78 GiB] / in use: 1 [19.78 GiB] / in no VG: 0 [0    ]

### This will scan and list the VG
lvm> vgscan
   Reading all physical volumes. This may take a while...
   Found volume group "volgroup01" using metadata type lvm2

### This will list and scan the LV (the meat is here)
lvm> lvscan
   inactive                 '/dev/volgroup01/root'    [11.78 GiB] inherit
   inactive                 '/dev/volgroup01/swap'  [8.00 GiB] inherit

### And then execute lvchange
bash-4.1# lvchange -ay /dev/volgroup01/root

### So quit the lvm
lvm > exit
Adding dirhash hint to filesystem

### now time to run fsck
bash-4.1# fsck -y /dev/volgroup01/root

Install QEMU-KVM in RHEL / CentOS

Here is the command to install QEMU-KVM packages in RHEL / CentOS, 

yum -y install qemu-kvm qemu-kvm-tools libvirt

or these command will install the packages for associated virtualization stuff.

yum -y groupinstall "Virtualization"
yum -y groupinstall "Virtualization Client"
yum -y groupinstall "Virtualization Platform"
yum -y groupinstall "Virtualization Tools"

Bandwidth testing with iperf

To test bandwidth between 2 linux servers, we would use a tools call iperf. Installing iperf from repository is easy, what we need to run is

[root@server ~]# yum -y install server.

Or,

You may want to run apt-get install iperf on debian or ubuntu.

Once we install the package on both machines, we will need to pick one node to run as server

[root@server ~]# iperf -s
------------------------------------------------------------
Server listening on TCP port 5001
TCP window size: 85.3 KByte (default)
------------------------------------------------------------


Once service is running, we could run iperf from the client machine to connect to server.
[root@client ~]# iperf -c 192.168.0.1
------------------------------------------------------------
Client connecting to 192.168.0.1, TCP port 5001
TCP window size: 16.0 KByte (default)
------------------------------------------------------------
[  3] local 192.168.0.2 port 37476 connected with 192.168.0.1 port 5001
[ ID] Interval       Transfer     Bandwidth
[  3]  0.0-10.0 sec  1.38 GBytes  1.19 Gbits/sec

Now the bandwidth is 1.19Gbit/sec, which is pretty much correct on my VM.

There are some other common options could be used. For e.g. using -d would perform a bi-directional test, i.e testing the bandwidth from source-to-target and then target-to-source. Using -n $some_number would define the size to transfer.

Below example will send approximately 1000Mb of data to test.

[root@client ~]# iperf -n 1000000000 -c 192.168.0.1 
------------------------------------------------------------
Client connecting to 192.168.0.1, TCP port 5001
TCP window size: 16.0 KByte (default)
------------------------------------------------------------
[  3] local 192.168.0.2 port 39375 connected with 192.168.0.1 port 5001
[ ID] Interval       Transfer     Bandwidth
[  3]  0.0- 7.8 sec   954 MBytes  1.03 Gbits/sec

Allow X11-Forwarding to work in Linux

In RHEL / CentOS

- Install package xorg-x11-xauth

- Have below options set in /etc/ssh/sshd_config and followed by sshd restart

[root@dumphost ~]# grep X11Forwarding /etc/ssh/sshd_config
X11Forwarding yes

[root@dumphost ~]# grep AllowTcpForwarding /etc/ssh/sshd_config
AllowTcpForwarding yes

Setting up NFS v4 in RHEL6 / CentOS6 with user authentication

Prior to NFS v4, NFS authentication are made via hosts authentication.

Starting from NFS v4,

To allow user id mapping to work in NFS v4, one have to edit /etc/idmapd.conf to match with the DNS domain name of associated hosts.

RHEL / CentOS nic mapping via udev.d

To re-arrange the eth numbering of a nic device, one have to edit /etc/udev/rules.d/70-persistent-net.rules and follow by a system reboot. 


[root@dumphost ~]# cat /etc/udev/rules.d/70-persistent-net.rules
# This file was automatically generated by the /lib/udev/write_net_rules
# program, run by the persistent-net-generator.rules rules file.
#
# You can modify it, as long as you keep each rule on a single
# line, and change only the value of the NAME= key.

# PCI device 0x14e4:0x1639 (bnx2) (custom name provided by external tool)
SUBSYSTEM=="net", ACTION=="add", DRIVERS=="?*", ATTR{address}=="98:4b:e1:66:09:ee", ATTR{type}=="1", KERNEL=="eth*", NAME="eth1"

# PCI device 0x14e4:0x1639 (bnx2) (custom name provided by external tool)
SUBSYSTEM=="net", ACTION=="add", DRIVERS=="?*", ATTR{address}=="98:4b:e1:66:29:40", ATTR{type}=="1", KERNEL=="eth*", NAME="eth2"

# PCI device 0x14e4:0x1639 (bnx2) (custom name provided by external tool)
SUBSYSTEM=="net", ACTION=="add", DRIVERS=="?*", ATTR{address}=="98:4b:e1:66:29:42", ATTR{type}=="1", KERNEL=="eth*", NAME="eth3"

# PCI device 0x14e4:0x1639 (bnx2) (custom name provided by external tool)
SUBSYSTEM=="net", ACTION=="add", DRIVERS=="?*", ATTR{address}=="98:4b:e1:66:09:ec", ATTR{type}=="1", KERNEL=="eth*", NAME="eth0"

RHEL / CentOS change default settings of nfs client

In CentOS 6 / RHEL6,  nfs client is defaulted to mount as version 4. For any reason one would want to mount nfs as version 3, here is the options to do so

[root@dumphost ~]# showmount -e nfshost
Export list for nfshost:
/nfs *
[root@dumphost ~]# mount -o vers=3 nfshost:/nfs /mnt
[root@dumphost ~]# mount
nfshost/nfs on /mnt type nfs (rw,vers=3,addr=10.1.0.2)

In any case you want version 3 be set as default, uncomment the option Defaultvers from /etc/nfsmount.conf and change it from 4 to 3.

[root@dumphost ~]# grep  Defaultvers  /etc/nfsmount.conf
# Defaultvers=4
Defaultvers=3