How to open ports in Red Hat Linux

1. Edit vi /etc/sysconfig/iptables
2. Add -A RH -INPUT -m state --state NEW -m tcp -p tcp --dport port number -j ACCEPT
Change 'port number' to desired port number
3. save and exit
4. restart iptables with 'service iptables restart'

when we mounting chroot /mnt/sysimage we are getting below error, chroot: cannot run command '/bin/sh' :No such file or directory

# ls /bin/sh 

If the command returns no file or directory again, then run

# ls /bin/bash

If this command returns the path of /bin/bash then recreate the symlink from /bin/sh to /bin/bash as such:

# ln -s /bin/sh /bin/bash

Why do I get this error '/dev/st0: inappropriate ioctl for device'

When I run the command mt -f /dev/st0 status, it produces /dev/st0:inappropriate ioctl for device. How do I fix this issue?

How do I open doc files from command line in linux

Doc files can be opened following default tools.

1. gnome-open
2. oowriter
3. xdg-open


For example:

#gnome-open test.doc
#oowriter test1.doc
#xdg-open test2.doc

How to measure linux performance

Check with dd command in unix/linux systems... for example

#time dd if=/dev/zero of=/dev/null bs=1024k count=10000000
10000000+0 records in
10000000+0 records out
10485760000000 bytes (10 TB) copied, 359.587 seconds, 29.2 GB/s

real    5m59.615s
user    0m4.403s
sys     5m54.958s

#top
#ps

CIFS VFS: cifs_mount failed w/return code = -101

CIFS VFS: Error connecting to socket. Aborting operation
 CIFS VFS: cifs_mount failed w/return code = -101

how to redirect strace to text file

how to redirect strace to text file

#strace -o filename command

For example:

strace -o strace1.txt  ls  

read and edit initrd image contents

1. Make temporary directory

#mkdir /tmp/initrd
2. Copy initrd image to destination directory
#cp /source/directory/initrd-version.img /tmp/initrd
3. unzip and use cpio
#gzip -dc initrd-2.6.18-164.6.1.el5.img | cpio -ivd

blocked FC remote port time out

logs shows 'blocked FC remote port time out' errors

SIOCADDRT: No such device or SIOCADDRT: Network is unreachable

1. Check 'ifconfig ethx' to determine ip address has been assigned. If not assign ip address

Write protecting the kernel read-only data

There was a bug in previous version, please update to the latest version. That should get rid of these error messages

upgrade from RHEL4 to RHEL5

Upgrading between major versions is not support. Install a required version from scratch.

linux bonding does not double bandwidth

For 1G Nic, the maximum theoretical speed limit is 128MB/s.
In real world you get much lesser, adding the protocol overhead like we have scp, wget and other protocols.
If you are currently using mode 0.
In mode 0 there is chances for packet re-ordering. Which can delay the speed.
There may be slight improvement if you use mode 4.
With mode 4 the incoming connection is controlled by switch. So there will be no packet re-order issues or packet drops.
Even if you use mode 4, you cannot achieve 2G speed for a single connection. You can enjoy 2G bandwidth for multiple connections.


There is a assumption that configuration of bonding with multiple network cards gives double bandwidth. That is not at all true.

GFS2 lockdump analysis

G:  s:EX n:2/1fda70 f:Dy t:EX d:UN/132055446000 l:0 a:0 r:5
 H: s:EX f:H e:0 p:1405 [httpd] gfs2_write_begin+0x61/0x33e [gfs2]
 H: s:EX f:W e:0 p:31819 [umount] gfs2_write_inode+0x57/0x152 [gfs2]

The content of the file is a series of lines. Each line starting with G: represents one glock, and the following lines, indented by a single space, represent an item of information relating to the glock immediately before them in the file

Lines in the debugfs file starting with H: (holders) represent lock requests either granted or waiting to be granted

The flags field on the holders line f: shows which: The 'W' flag refers to a waiting request, the 'H' flag refers to a granted request

The glocks which have large numbers of waiting requests are likely to be those which are experiencing particular contention.

Having identified a glock which is causing a problem, the next step is to find out which inode it relates to. The glock number (n: on the G: line) indicates this. It is of the form type/number and if type is 2, then the glock is an inode glock and the number is an inode number. To track down the inode, you can then run find -inum number where number is the inode number converted from the hex format in the glocks file into decimal.

If the glock that was identified was of a different type, then it is most likely to be of type 3: (resource group). If you see significant numbers of processes waiting for other types of glock under normal loads, then please report this to Red Hat support.

f you do see a number of waiting requests queued on a resource group lock there may be a number of reason for this. One is that there are a large number of nodes compared to the number of resource groups in the filesystem. Another is that the filesystem may be very nearly full (requiring, on average, longer searches for free blocks). The situation in both cases can be improved by adding more storage and using the gfs2_grow command to expand the filesystem. 




The W flag indicates that this is waiting for a glock
UN (unlocked) state
SH (shared) lock

du command usage

1. du -hs *  (shows linux directory size)
2. du * | sort -n (finding large files with sort option)

bash: fork: Resource temporarily unavailable

The error is generated because of recursive defunct processes going on in the memory and is unable to allocate specific amount of memory for the newly created process. Basically the general structure of process generation is as followed.

This is the normal life of a program.
fork()INIT->exec()->RUN->exit()->ZOMBIE->done

INIT(fork)
The program is started by a parent process, an action called fork()
The fork makes a copy(the child) of the calling process(the parent).
exec() The child then issues an exec() system call which replaces the new
process with the intended executable file.

SRUN/URUN (system/user run space) the new child program now runs. Now the parent is either waiting(in a SLEEP) for the child to finish or checks for the childs completion or the system notifys the parent on exit
of the child process.

exit()
The child exits and returns the resources(memory) to the system.
ZOMBIE At this point the child has terminated and is in ZOMBIE. THIS IS NORMAL!! It will stay in this state until the parent process acknowledges receipt of the appropriate signal or terminates.

If the parent process has died unexpectedly or something else has prevented acknowledgment from the parent then process ID #1 (init) takes over and becomes the childs parent until reboot.

So...
A zombie does not tie up memory but it still has a slot in the process table. I/O devices can get locked out.

You can't kill a ZOMBIE process because......IT'S ALREADY DEAD!!!

So finally try to check these defunct processes going on server You can do this by giving the ps -dfa command and killing manually. If this work you can start by simply relogin into the shell and if not you will need a reboot to the server in order to refresh the memory states.

packet drops in Linux

Packet drops can happen at two layers. one at the NIC level or at the Network stack level.

Check 'ifconfig' output. 

RX packets:297126179 errors:0 dropped:3981 overruns:0 frame:0
         TX packets:233589229 errors:0 dropped:0 overruns:0 carrier:0

That means packets drops at the NIC level. These are most likely caused by exhaustion of the RX ring buffer. Increase the size of the ethernet device ring buffer.

First inspect the output of "ethtool -g eth0".  If the "Pre-set maximums" are more than the what's listed in the current hardware settings it's recommend to increase this number.  As an example:

    # ethtool -g eth0
    Ring parameters for eth0:
    Pre-set maximums:
    RX:             1020
    RX Mini:        0
    RX Jumbo:       16320
    TX:             255
    Current hardware settings:
    RX:             255
    RX Mini:        0
    RX Jumbo:       0
    TX:             255


To increase the RX ring buffer to 4080 you would run "ethtool -G eth0 rx 1020". 

when we bring the node back , failback is not happening

when poweroff  command is given through terminal , fail over is working  but when we bring the node back , failback is not happening?

kernel panic due to openafs filesystem

By the looks of the logs kernel panic occurred due to 'openafs' file system corruption.