Tips and Tricks
- Screen I am not the first one to mention that. I just didn’t use it for such a long time even after knowing it. It a great tool and must for all the SysAdmins and Developers.
- Resetting the master on github
Reset your local master to the point you want, then `git push –force origin’
Some powerful commands on Linux
Adding ISO as repository on Fedora/RHEL/CentOS
Do a loop-back mount of the ISO and then add following in the yum.conf file.
Any file in /etc directory is treated as configuration files.They are not removed after un-installing the package. To remove those
$dpkg –purge <package name>
Make sure you don’t remove them manually. If you did then run:
$ dpkg –force-confnew <package>.deb
This will re-install the configuration files.
In my case, in the post section I was removing them manually, which was causing the problem next time while installing package.
I had to install a init script and dpkg was giving following error:-
update-rc.d: /etc/init.d/blah: file does not exist
dpkg was assuming that the file “/etc/init.d” is already and the not copying it again.
dd command status
?dd command doesn’t give us the status, So if you put very large copy using then you just have to wait till it finishes.
Run following command
kill -USR1 <pid of dd process>
and you get some status on the terminal you were running the command. You can run above command as many times you want to get the status.
Finding the SCSI id for a disk and its controller
If we look at “/proc/scsi/scsi” file then we see something like following:-
Host: scsi1 Channel: 00 Id: 00 Lun: 22
Vendor: HP Model: P2000 G3 FC Rev: T201
Type: Direct-Access ANSI SCSI revision
And we look at dmesg output then we see :-
sd 1:0:0:22: [sde] 683593728 512-byte logical blocks: (349 GB/325 GiB)
sd 1:0:0:22: [sde] Write Protect is off
sd 1:0:0:22: [sde] Mode Sense: 93 00 00 08
sd 1:0:0:22: [sde] Attached SCSI disk
From above we can conclude that “sde” is the disk which is attached on host 1 and have Lun Id as 22. We can also see that the disk is coming from HP P2000 G3 FC storage.
A typical SCSI nameing convention is
<host> <channel> <target> <lun>
Scanning for newly attached disk on the FC HBA
To scan the given “host (h)” , “channel(c)””, “target(t)” and “lun(l)” , run following command :-
$echo “<c> <t> <l>” > /sys/class/scsi_host/host<h>/scan
And to scan all channels, target and luns for a give host run following:-
$ echo “- – -” > /sys/class/scsi_host/host<h>/scan
Using SAR (system activity report)
It is used to collect, report, or save system activity information. It can report CPU, Disk, Memory, Network, paging and other information. If configured with cron then it collects information everyday and store the statistics in “/var/log/sa” folder. The “saX” files are binary files and “sarX” file are human readable data, where is day of the month. We can collect all those stats manually as well.
Following command will collect Disk, CPU, Memory, paging, load average stats in every 5 mins and save it in “logfile” file in binary format:-
$ sar -dp -u -r -B -q 5 -o <logfile>
And with following the binary file can be coverted into CSV file:-
$ sadf -d — -dp -u -r -B -q 5 <logfile> > <csvfile>
Make sure “sar” and “sadf” is run on the same system, else the name of the disks in CSV file would change.