Mount a Linux filesystem on a SAN from multiple nodes at the same time

Posted on in Categories CentOS, FAQ, File system, Gentoo Linux, Hardware, High performance computing, Linux, Linux Scalability, RedHat/Fedora Linux, Storage last updated May 12, 2006

If you try to mount an ext3 Linux filesystem on a SAN from multiple nodes at the same time you will be in serious deep trouble.

SAN based storage allows multiple nodes to connect to same devices at the same time. Ext3/2 are not cluster aware file system. They can lead to a disaster such as kernel panic, server hang, corruption etc.

You need to use something which supports:

  1. Useful in clusters for moderate scale out and shared SAN volumes
  2. Symmetrical Parallel Cluster File System, Journaled
  3. POSIX access controls

Both GFS (RedHat Global File System) and Lustre (a scalable, secure, robust, highly available cluster file system) can be used with SAN based storage allows multiple nodes to connect to same devices at the same time.

Many newbie get confused as Linux offers a number of file systems. This paper (Linux File System Primer) discusses these file systems, why there are so many, and which ones are the best to use for which workloads and data.

No Route to Host error and solution

Posted on in Categories Debian Linux, Gentoo Linux, Linux, Networking, RedHat/Fedora Linux, Tips, Troubleshooting, Ubuntu Linux last updated May 5, 2006

I am getting error that read as No Route to Host. I am trying to ping my ISP gateway as well as DNS server but I am getting this error. How do I solve this problem?

This problem indicate networking conflicts or some sort of networking configuration problem.

Here are things to check:

Can you ping to your local router interface (such as

Make sure your card (eth0) is properly configured with correct IP address and router address. Use ifconfig command to configure IP address and route command to setup correct router address. If you prefer to use GUI tools:

  • redhat-config-network – Works on Red Hat and Fedora Linux/Cent OS.
  • network-admin – Debian and Other Linux distribution use this GUI too

Use above two GUI tools to setup correct IP address, DNS address and router address.

b) Make sure firewall is not blocking your access

iptables is default firewall on Linux. Run following command to see what iptables rules are setup:
# /sbin/iptables -L -n

You can temporary clear all iptables rules so that you can troubleshoot problem. If you are using Red Hat or Fedora Linux type command:
# /etc/init.d/iptables save
# /etc/init.d/iptables stop

If you are using other Linux distribution type following commands:
# iptables -F
# iptables -X
# iptables -t nat -F
# iptables -t nat -X
# iptables -t mangle -F
# iptables -t mangle -X

c) Finally make sure you are using a router and not a proxy server. Proxy servers are good for Internet browsing but not for other work such as ftp, sending ICMP request and so on.

See also:

Black Screen While Starting X window

Posted on in Categories Debian Linux, Linux, Linux desktop, RedHat/Fedora Linux, Suse Linux, Sys admin, Tips, Troubleshooting, Ubuntu Linux, X server last updated April 2, 2006

My friend is new to Linux, everything was fine until he made some changes to X, which result into this problem. Now whenever he types command startx to start X he get a blank screen. Finally, he contacted me me via yahoo and asked me to get rid rid of this problem.

Step # 1: Reset blank X screen

First, get out of Blank screen by pressing combination of keys Ctrl+Alt+Backspace. If this fails, reboot system.

Step # 2: Reconfigure X server display

At shell, prompt type the following command to reconfigure X display.

If you are using Red Hat /Fedora / CentOS Linux type command:
# redhat-config-xfree86
If you are using Debian Linux type command:
# dpkg-reconfigure xserver-xfree86
OR login as the root user and type command:
# XFree86 -configure

Step # 3: Test new setup

Type startx to test your new settings:
# startx
OR if you used XFree86 –configure command to reconfigure X, then test it with following command:
# XFree86 -xf86config /etc/X11/

See also:

How to: Linux flush or remove all iptables rules

Posted on in Categories Debian Linux, Howto, Iptables, Linux, Networking, RedHat/Fedora Linux, Ubuntu Linux last updated June 20, 2005

Here is a small script that does this. Debian or Ubuntu GNU/Linux does not comes with any SYS V init script (located in /etc/init.d directory). You create a script as follows and use it to stop or flush the iptables rules. Please don’t type rules at the command prompt. Use the script to speed up work.

Warning: All the commands must be executed with root privileges.

Procedure for Debian / Ubuntu Linux (Generic method)

First, create /root/fw.stop script using text editor such as vi:

echo "Stopping firewall and allowing everyone..."
## Failsafe - die if /sbin/iptables not found
[ ! -x "$ipt" ] && { echo "$0: \"${ipt}\" command not found."; exit 1; }
$ipt -F
$ipt -X
$ipt -t nat -F
$ipt -t nat -X
$ipt -t mangle -F
$ipt -t mangle -X
$ipt iptables -t raw -F
$ipt -t raw -X

Make sure you can execute the script:
# chmod +x /root/fw.stop

Run the script as root user:
# /root/fw.stop

How do I verify that my firewall rules are flushed out?

Type the following command:
# iptables -L -n -v
Sample outputs:

Chain INPUT (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination         
Chain FORWARD (policy ACCEPT 0 packets, 0 bytes) pkts bytes target prot opt in out source destination
Chain OUTPUT (policy ACCEPT 0 packets, 0 bytes) pkts bytes target prot opt in out source destination

A note for RedHat (RHEL), CentOS and friends Linux user

Please note that RedHat Enterprise Linux (RHEL), Fedora and Centos Linux comes with pre-installed rc.d script, which can be used to stop the firewall, enter:
# /etc/init.d/iptables stop
# service iptables stop
Sample outputs:

A note about firewalld on CentOS 7/Fedora (latest)/RedHat Enterprise Linux 7.x+ user

Type the following command to stop and flush all rules:
# systemctl stop firewalld