Rebuilding a Removed / Failed RAID 10 Array in CentOS / Rocky Linux

Tuesday, February 22nd, 2022

Replace Hard Drive in a RAID 10 Array and Sync the RAID 10 Array to the New Hard Drive

I had the hardest time rebuilding a RAID 10 array after replacing a hard drive.  I didn't fail the old hard drive before removing it from the array, and sometimes, this may not be an option.  What happened in my case is the data center replaced the hard drive that I had shipped to them directly from an eBay seller.  I was hoping that the RAID array would rebuild itself onto the new drive (as I have seen happen before in some circumstances).  However, that may not happen if the replacement drive still has its old RAID array or partition information present, and then, it might be difficult to actually get the RAID array to sync to the new drive. 

In my case, I run LVM (Logical Volume Manager) for my partitions.  This complicates the RAID setup, and I found that mdadm commands didn't work as expected.  If this situation occurs, it is best to boot Rocky Linux or CentOS in recovery mode using a Rocky Linux ISO or CentOS ISO.  Once the recovery system loads, drop to a shell without mounting any file systems.  Next, you will need to deactivate your LVM volume group:

vgchange -a n my_volume_group # deactivate

Next, examine your md RAID array by running the following command:

cat /proc/mdstat

After running that command, I identied my RAID devices as md126 and md127.  /dev/md127 is considered the parent even though /dev/md126 is where everything is. 

I can get more information about the RAID array by running the below commands:

mdadm --detail /dev/md126
mdadm --detail /dev/md127

Let's fail and remove any removed (no longer existing) drives using this command:

mdadm /dev/md126 --remove failed
mdadm /dev/md126 --remove detached
mdadm /dev/md127 --remove failed
mdadm /dev/md127 --remove detached

Next, we need to identify the hard drive we want to add / replace the removed drive in the array:


From running the above command, I noticed that the new drive was /dev/sde, so I needed to wipe its old RAID configuration (if there is any) and then add it to the RAID array.

wipefs /dev/sde
mdadm --add /dev/md127 /dev/sde

Check to see if the syncing process has started:

cat /proc/mdstat

You may or may not need to run the below command to get the RAID device to start syncing to the new drive:

mdadm --grow /dev/md126 --raid-devices=4

Helpful Links:

CentOS LVM and Software RAID Partitioning Instructions

Sunday, May 30th, 2021

Installing and Configuring CentOS to Host KVM Virtual Machines


When configuring a fresh install of CentOS for a KVM host machine (the main server that hosts all of the virtual machines), I like to run a GUI to make managing some of the virtual machines easier.  Thus, during install, choose the options for CentOS with Minimal GUI:

RAID 10 LVM Partitions

When configuring the hard drive partitions, set it up to use RAID 10 LVM SOFTWARE RAID:

Create volume group called "vms" without the quotes that is setup as RAID 10 (set volume group space to be as large as possible).

Set the "/" partition to 100GB XFS LVM (RAID10).

Set the "swap" partition to 32GB.

Only setup those two partitions.  The remaining space in the RAID 10 volume group "vms" will be used for KVM containers (and the remaining space does NOT need to be assigned to any mount points).

That's all.

Increasing KVM Guest Hard Disk (Hard Drive) Space

Sunday, May 30th, 2021

Increasing KVM Guest Hard Disk (Hard Drive) Space

Increasing the hard drive space in a KVM guest can be rather tricky.  The first step is to shutdown (completely turn off) the guest machine by running the below command from the guest system:

sudo shutdown -h now

Once the guest machine has been turned off (verify it is off by using sudo virt-manager on the host machine to see if it's no longer running), on the host machine, resize the LVM partition by running the following command (and adjust the size as necessary):

sudo lvextend -L+78G /dev/vg_vps/utils

If you need help identifying the name of the disk your guest has been assigned, run this command from the host:

sudo virsh domblklist {VIRSH_NAME_OF_VIRTUAL_MACHINE}

For my example, I would use this command:

sudo virsh domblklist utils

From the host machine, download the GParted live ISO image for your system's architecture (x86 or x64).  Start virt-manager:

sudo virt-manager

Assign a CD drive to the virtual machine you're expanding the hard drive space for, and assign / mount the GParted ISO to it.  Change the boot order so that the KVM guest boots from the CD first.  Save your settings and start the KVM guest virtual machine.  Boot into GParted Live.  GParted will run automatically.  Use GParted to expand the partitions so that they make use of the added storage based on your own preferences.  Apply the resize operation.  Exit GParted and shutdown the virtual machine so that it's off again. Remove the CD drive from the boot options from virt-manager, and then start the KVM guest again. 

If Guest Doesn't Use LVM Partitioning

If your KVM guest virtual machine hasn't been configured to use LVM, the added hard drive space should already be available to your system.  Verify it has been expanded by again running the df -h command.  You're done!

If Guest Uses LVM

Let the OS boot.  From the guest, the file system needs to be resized itself.  You can do this by running the following command to see the current space allocated to your system's partitions:

df -h

You'll see a bunch of output similar to:

Filesystem                  Size  Used Avail Use% Mounted on
udev                        2.9G     0  2.9G   0% /dev
tmpfs                       597M  8.3M  589M   2% /run
/dev/mapper/utils--vg-root  127G   24G   98G  20% /
tmpfs                       3.0G     0  3.0G   0% /dev/shm
tmpfs                       5.0M     0  5.0M   0% /run/lock
tmpfs                       3.0G     0  3.0G   0% /sys/fs/cgroup
/dev/vda1                   720M   60M  624M   9% /boot
tmpfs                       597M     0  597M   0% /run/user/1000

You'll notice that the added hard drive space doesn't show up on any of the partitions.  However, it is available to be assigned to these partitions.  To assign additional space, you will need to resize it using these commands (run from the guest virtual machine… the machine you're resizing):

lvextend /dev/mapper/utils--vg-root -L +78G
resize2fs /dev/mapper/utils--vg-root

Obviously, you need to substitute the name of the LVM partition with the one from your system shown in your output of the df -h command.

Resources if Offline if Offline

Adding SAS RAID Drivers to CentOS 8 and Red Hat Linux During Installation

Friday, April 30th, 2021

Adding SAS RAID Drivers to CentOS 8 and Red Hat Linux During Installation

CentOS 8 and Red Hat Linux 8 removed a lot of built in RAID controller and SAS drivers.  As such, you'll need to identify your SAS RAID controller card model number, and then during the installation of CentOS 8 or Red Hat, you will need to follow these instructions (modifying them for your hardware).

If for some reason the link above is no longer available, I saved and archived a copy which can be read here.

Add El Repo Permanently

As updates are released to CentOS 8 / Rocky Linux / Red Hat 8, the kernel will often be upgraded.  To make sure the SAS drives are updated as well, you'll need to configure your system to pull updates from El Repo automatically by using the following commands:

sudo rpm --import
sudo yum install
sudo yum update -y

In case the above instructions no longer work, this guide should help.

Disable NetworkManager Wait Online Service

Prevent the boot from being halted on startup by network connection checks by running the below command:

sudo systemctl mask NetworkManager-wait-online.service

CentOS – Using NAT with KVM Guests

Wednesday, July 17th, 2019

CentOS – Using NAT with KVM Guests

Please note that all commands in this guide must be run on the main HOST machine (the physical machine).  They should not be run on KVM guests (virtual machines).

If your server has a limited number of IPv4 addresses, it might be best to setup and run some virtual machines that are configured to use the default NAT network interface that KVM provides.  This will allow you to run multiple virtual machines that share the same IP address.  Think of it as setting up a home network with multiple devices with certain ports forwarded to specific devices for incoming connections.

First, create the virtual machines that are going to use NAT using virt-manager as you normally would.  In the virtual machine configuration wizard, assign the default NAT network interface named "virbr0".  After the virtual machines have been created, shut down the virtual machines.  Now, we'll assign these virtual machines static LAN IP addresses so that we can port forward certain ports and always have them reach the proper virtual machines. 

The first thing we need to do is get the MAC address of each virtual machine.  Write down the name of the virtual machine and its MAC address, as we'll need this information later on when we edit the NAT interface and assign static LAN IP addresses to our virtual machines.  Run this command to retrieve the MAC address for a specific virtual machine.

virsh dumpxml VM_NAME | grep -i '<mac'

Using the MAC address information from the VMs we want to use NAT with, edit the default NAT interface by running the below command. 

virsh net-edit default

If for some reason the NAT interface is not named default, you can find it by running the below command:

virsh net-list

After the <range /> entry, assign the static LAN IP addresses similar to the following:

      <range start='' end=''/>
      <host mac='52:54:00:ff:4a:2a' name='vm1' ip=''/>
      <host mac='52:54:00:bb:35:67' name='vm2' ip=''/>
      <host mac='52:54:00:aa:d9:f2' name='vm3' ip=''/>

Save the file with your desired values and quit the editor.  Restart the NAT interface by running the below commands:

virsh net-destroy default
virsh net-start default

Now, you'll need to setup your iptables port forwarding rules.  Adjust the below rules as necessary (changing the port numbers to the ones you want to use) and then save them so that they persist:

iptables -I FORWARD -o virbr0 -d -j ACCEPT
iptables -t nat -I PREROUTING -p tcp --dport 39989 -j DNAT --to
iptables -I FORWARD -o virbr0 -d -j ACCEPT
iptables -t nat -I PREROUTING -p tcp --dport 39990 -j DNAT --to
iptables -I FORWARD -o virbr0 -d -j ACCEPT
iptables -t nat -I PREROUTING -p tcp --dport 39991 -j DNAT --to
iptables -t nat -A POSTROUTING -s -j MASQUERADE
iptables -A FORWARD -o virbr0 -m state --state RELATED,ESTABLISHED -j ACCEPT
iptables -A FORWARD -i virbr0 -o br0 -j ACCEPT
iptables -A FORWARD -i virbr0 -o lo -j ACCEPT
service iptables save

Congrats, your virtual machines are now using NAT, have been assigned static LAN IP addresses, and iptables rules on the host server have been configured to port forward specific ports to each NAT VM.

Persistently Saving NAT Port Forward Rules

The only solution I found that would persistently save my NAT forwarding rules is to create a libvirt hook bash script as mentioned here

service iptables stop
iptables -F
service iptables save
service iptables start
mkdir -p  /etc/libvirt/hooks
nano  /etc/libvirt/hooks/qemu

The contents of the "/etc/libvirt/hooks/qemu" file should look similar to the following:

# IMPORTANT: Change the "VM NAME" string to match your actual VM Name.
# In order to create rules to other VMs, just duplicate the below block and configure
# it accordingly.
if [ "${1}" = "vm1" ]; then   # Update the following variables to fit your setup
   if [ "${2}" = "stopped" ] || [ "${2}" = "reconnect" ]; then
    /sbin/iptables -D FORWARD -o virbr0 -d  $GUEST_IP -j ACCEPT
    /sbin/iptables -t nat -D PREROUTING -p tcp --dport $HOST_PORT -j DNAT --to $GUEST_IP:$GUEST_PORT
   if [ "${2}" = "start" ] || [ "${2}" = "reconnect" ]; then
    /sbin/iptables -I FORWARD -o virbr0 -d  $GUEST_IP -j ACCEPT
    /sbin/iptables -t nat -I PREROUTING -p tcp --dport $HOST_PORT -j DNAT --to $GUEST_IP:$GUEST_PORT
if [ "${1}" = "vm2" ]; then   # Update the following variables to fit your setup
   if [ "${2}" = "stopped" ] || [ "${2}" = "reconnect" ]; then
        /sbin/iptables -D FORWARD -o virbr0 -d  $GUEST_IP -j ACCEPT
        /sbin/iptables -t nat -D PREROUTING -p tcp --dport $HOST_PORT -j DNAT --to $GUEST_IP:$GUEST_PORT
   if [ "${2}" = "start" ] || [ "${2}" = "reconnect" ]; then
        /sbin/iptables -I FORWARD -o virbr0 -d  $GUEST_IP -j ACCEPT
        /sbin/iptables -t nat -I PREROUTING -p tcp --dport $HOST_PORT -j DNAT --to $GUEST_IP:$GUEST_PORT
if [ "${1}" = "vm3" ]; then   # Update the following variables to fit your setup
   if [ "${2}" = "stopped" ] || [ "${2}" = "reconnect" ]; then
        /sbin/iptables -D FORWARD -o virbr0 -d  $GUEST_IP -j ACCEPT
        /sbin/iptables -t nat -D PREROUTING -p tcp --dport $HOST_PORT -j DNAT --to $GUEST_IP:$GUEST_PORT
   if [ "${2}" = "start" ] || [ "${2}" = "reconnect" ]; then
        /sbin/iptables -I FORWARD -o virbr0 -d  $GUEST_IP -j ACCEPT
        /sbin/iptables -t nat -I PREROUTING -p tcp --dport $HOST_PORT -j DNAT --to $GUEST_IP:$GUEST_PORT

Save and exit.  Make the script executable.

chmod +x /etc/libvirt/hooks/qemu

If you're attempting to forward incoming traffic (only traffic originating from the outside), be sure to exclude the main virbr0 interface so that requests originating from the DHCP LAN aren't forwarded to itself.  This is helpful for port forwarding DNS (port 53).  Incoming requests from outside of the network will be forwarded to the specified server, but outgoing DNS requests made from the server will NOT be forwarded back to itself.  Basically, you want to forward all traffic on port 53 to that specific server except for traffic that originates from that server.  So, be sure to add this line if that is the case:

/sbin/iptables -t nat -I PREROUTING \! -i virbr0 -p udp --dport 53 -j DNAT --to $GUEST_IP:$GUEST_PORT

More information can be found here:

Reboot the host server.

Old Instructions for Persistent Saving (Non-Working)

If your iptables forwarding rules are not persisted after the host machine is rebooted or shutdown, run the following commands:

sudo -i
yum install -y iptables-services
systemctl stop firewalld
systemctl disable firewalld
systemctl enable iptables
nano /etc/sysconfig/iptables-config 

Change the below values to "yes":


Save and exit.  Reboot the server.

If you're still having issues, try this (will clear your existing iptables rules):

iptables-save > iptables_bk
service iptables stop
iptables -F
<run iptables NAT rules here>
<run any other iptables rules you want>
service iptables save
service iptables start

More Detailed Guide

Change the Default Editor to nano in Linux

Saturday, April 27th, 2019

Use nano as the Default Editor

If you hate vi like I do, you can configure Linux to always default to using the nano editor.

Simply add the following to the bottom of the /etc/bashrc file:

export EDITOR="nano"

Save the file.  nano is now the default editor.  When you use

sudo crontab -e

The nano editor will now be used by default.

Configuring Let’s Encrypt Certbot on CentOS 7 with lighttpd

Saturday, April 27th, 2019

Configuring Let's Encrypt Certbot on CentOS 7 with lighttpd

Installing Certbot

First, install certbot by using the below commands:

sudo yum -y install epel-release
sudo yum install certbot

certbot is python based program that allows you to request SSL certificates for your domains. 

Request a Certificate

Use the below command to request a certificate (adjust paths and replace the domain as necessary):

sudo certbot certonly --webroot -w /var/www/vhosts/test/httpdocs -d

A certificate has now been stored in /etc/letsencrypt/live.  Create a combined certificate format by using the below command (replacing with your real domain):

/bin/cat /etc/letsencrypt/live/ /etc/letsencrypt/live/ > /etc/letsencrypt/live/ && /bin/chmod 777 /etc/letsencrypt/live/ && /sbin/service lighttpd restart

Certificate Renewal Cronjobs

You may want to create a cronjob to renew the certificate and a cronjob for regenerating the combined format certificate since the underlying certificate file can change (such as when it's renewed):

sudo crontab -e

Insert the below cronjobs:

0 1 * * 1 /usr/bin/certbot renew --quiet
5 1 * * 1 /bin/cat /etc/letsencrypt/live/ /etc/letsencrypt/live/ > /etc/letsencrypt/live/ && /bin/chmod 777 /etc/letsencrypt/live/ && /sbin/service lighttpd restart

Save your crontab configuration. 

Setting Up Lighttpd to Use SSL Certificate

Edit your default-enabled lighttpd configuration file in /etc/lighttpd/vhosts.d to look similar to the following (replacing with your real domain and adjusting various file paths)

$HTTP["host"] == "" {
  var.server_name = "" = server_name  server.document-root = vhosts_dir + "/test/httpdocs"
  #accesslog.filename          = vhosts_dir + "/test/log" + "/access.log"
$SERVER["socket"] == ":80" {
  server.document-root = vhosts_dir + "/test/httpdocs"
$SERVER["socket"] == ":443" {
    ssl.engine           = "enable"
    ssl.pemfile          = "/etc/letsencrypt/live/"
    server.document-root = vhosts_dir + "/test/httpdocs" = "/etc/letsencrypt/live/" # Root CA = "" # Domain Name OR Virtual Host Name

Here's how you can set a different document root for specific https (port 443) virtual hosts:

$SERVER["socket"] == ":443" {
    ssl.engine           = "enable"
    ssl.pemfile          = "/etc/letsencrypt/live/"
    server.document-root = vhosts_dir + "/test/httpdocs/" = "/etc/letsencrypt/live/" # Root CA = "" # Domain Name OR Virtual Host Name
    $HTTP["host"] =~ "(^|www\.)" {
        server.document-root = vhosts_dir + "/test/httpdocs/subdir"

Save and restart the lighttpd service.

sudo service lighttpd restart

Congrats, SSL is now available on your domain, and your Let's Encrypt certificate has been configured and will be renewed automatically by your cronjob.

CentOS 7 – Easiest Way to Configure LVM KVM Pool for Virtual Machines

Saturday, April 27th, 2019

Configuring LVM in CentOS

When installing CentOS 7, be sure to only partition the hard drive with about 100GB of space for the OS file system itself.  Leave the rest of the drive unpartitioned.  After CentOS has been successfully installed, run gparted via a terminal using the below command:

sudo gparted

Create a new "LVM2 PV" file system based partition on the drive's remaining space like so:

Now, create the LVM volume group by using the below command and replacing /dev/md126p3 with the new partition's path label:

sudo vgcreate vms /dev/md126p3

Now, launch virt-manager by running the below command:

sudo virt-manager

Go to "Edit" –> "Connection Details" –> click on the "Storage" tab.  Click on the "+" icon on the bottom left.  You're now creating a storage pool.  Give it a name like "vms" which is short for virtual machines.  Select "logical: LVM Volume Group" for the type.  Here's a screenshot:

In "Target Path" select the volume group that you created named vms (which you did earlier using the "vgcreate" command).  Do NOT check the "Build Pool" checkbox, and leave the "Source Path" field blank.  Here's a screenshot of what it should look like:

Click on "Finish".  You're done, and you can now create LVM storage containers for your KVM configured pool named vms.

Here's a good LVM KVM Pool guide from RedHat that includes more information (though it's not as simple as following this guide).