Scenario : Setting up a physical standby from Exadata to a non-Exadata single instance. tnsping from standby to primary works fine but tnsping from primary to standby fails with:
TNS-12543: TNS:destination host unreachable
I am able to ssh standby from primary, can ping as well but tnsping doesn’t work. From the error description we can figure out that something is blocking the access. In this case it was iptables that was enabled on the standby server.
Stopping the service resolved the issue.
service iptables stop
chkconfig iptables off
The error is an obvious one but sometimes it just doesn’t strike you that it could be something simple like that.
One of my friend today asked me about removing Linux partitions & GRUB (from a dual boot system) and return back to windows alone. Removing Linux involves just formatting/removing the partitions. Now to remove GRUB either do fdisk /mbr from a Windows 98 bootable CD or do fixmbr after booting into repair mode with Windows XP CD. But if you have none then to remove GRUB you will need some utility like this one and if you reboot before doing that it might make GRUB unable to boot into Windows. It will get stuck at GRUB> prompt only. So there is an option: to manually boot the OS you want (ie Windows). A quick search gave link to this thread. It involves few commands on the GRUB prompt:
grub> rootnoverify (hd0,0)
grub> chainloader +1
It will load the NTLDR where your Windows is installed in Partition 1 on HDD 1.
Last week I attended a training on AIX system administration from IBM (organized by company, obviously 😉 It was a 7 days course covering all of the system administration stuff. There was a lot of new stuff to learn, LVM being the most number of times uttered word, once we did the chapter on LVM. It was a nice experience as a whole as for the first time I attended any training on Unix.
The sessions (specially after lunch) were sleepy also. I find this ppt method of training pretty boring. The trainer (most of) strictly, stupidly follows the slides and slide, I feel is a dumb sort of thing, makes you feel sleepy except at the moments when there are few eye opening bullet points.
There should be bare minimum number of slides and for rest of the things, trainer should use white board so that everybody follows that and doesn’t sleep 🙂
Anyways it was really enjoying to be familiar with so many things in Unix.
I was searching for some good tutorials on awk. Found a very nice (brilliant indeed) article on Oracle website by Emmett Dulaney. A very good introduction for beginners. I searched for some other links as well. Have a read:
1. AWK: The Linux Administrators’ Wisdom Kit
2. A Guided Tour Of Awk
3. AWK Programming
4. UNIX Utilities – awk
Happy awk’ing 🙂