Category Archives: Linux

Command to re-read new VM disk CentOS/RHEL

System: CentOS/RHEL 6 and CentOS/RHEl 7

Added new VM disk in CentOS not showing the new VM disk when run fdisk -l.

Login as root, run this command to re-read the new VM disk without reboot the server.

ls /sys/class/scsi_host/ | while read host ; do echo "- - -" > /sys/class/scsi_host/$host/scan ; done

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Setup Banner on Console and Remote login CentOS/RHEL 6

Example add below banner.

* *
* This system is for the use of authorized users only. Usage of *
* this system may be monitored and recorded by system personnel. *
* *
* Unauthorized access to this machine is prohibited *
* Press Ctrl+D if you are not an authorized user *
* *
* Anyone using this system expressly consents to such monitoring *
* and is advised that if such monitoring reveals possible *
* evidence of criminal activity, system personnel may provide the *
* evidence from such monitoring to law enforcement officials. *
* *

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Install Oracle Java JDK CentOS command line

Scenario: How to install Oracle Java JDK rpm on CentOS/RHEL using command line.

1) Go to Oracle Java download page here -> select Java rpm package then right click copy link address.
2) Putty to server then use curl command below

curl -v -j -k -L -H "Cookie: oraclelicense=accept-securebackup-cookie" #paste here the url# > /tmp/jdk-file.rpm

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Decrease Logical Volume size in CentOS 6

Scenario: Decrease Logical Volume /home to 1GB size in CentOS/RHEL 6
System: CentOS/RHEL 6

– Check the drive and LV Path:

[root@centos6 ~]# df -h
Filesystem Size Used Avail Use% Mounted on
/dev/mapper/VolumeGroup-lv_root 9.9G 1.8G 7.6G 20% /
tmpfs 931M 0 931M 0% /dev/shm
/dev/sda1 485M 39M 421M 9% /boot
/dev/mapper/VolumeGroup-lv_home 2.5G 68M 2.3G 3% /home
/dev/mapper/VolumeGroup-lv_opt 2.5G 68M 2.3G 3% /opt
/dev/mapper/VolumeGroup-lv_var 9.9G 199M 9.2G 3% /var
[root@centos6 ~]# lvdisplay | grep Path
 LV Path /dev/VolumeGroup/lv_root
 LV Path /dev/VolumeGroup/lv_swap
 LV Path /dev/VolumeGroup/lv_var
 LV Path /dev/VolumeGroup/lv_opt
 LV Path /dev/VolumeGroup/lv_home
[root@centos6 ~]# vgs
 VG #PV #LV #SN Attr VSize VFree
 VolumeGroup 1 5 0 wz--n- 39.51g 10.51g
[root@centos6 ~]#

got the logical volume home path is /dev/VolumeGroup/lv_home with 2.5GB size.
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Command to re-read added new VM disk that not showing real size CentOS 6

System: CentOS 6.9

– User request in ESXi to add new 30 GB VM disk(2nd disk it mean sdb) but accidentally wrong added VM disk only 10 GB,after remove the 10 Gb and added 30 GB when run fdisk -l still showing 10 GB VM disk size.

Login as root, run this command to re-read the new disk size without reboot the server.

# echo 1 > /sys/block/sdb/device/rescan

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Increase or Decrease LV Swap in CentOS 6/RHEL 6

Scenario: How to increase or decrease swap space(logical volume swap) in CentOS 6/RHEL 6

System: CentOS 6/ RHEL 6

Steps increase Swap space
Example increase 1 GB swap space.

1) You must check the path of swap logical volume and the free space in the swap volume group before continue to step 2.
# free -m
# lvdisplay | grep swap
# vgs

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Remove Logical Volume on CentOS/RHEL

Scenario: Example remove Logical Volume that mount in /cache
System: CentOS/RHEL 6 or 7

– Check partition df -h
– Unmount /cache partition.
– Check df -h again to make sure /cache already unmounted
– Check logical volume path that we will remove.
– Edit /etc/fstab and remove or comment the logical volume path that we will remove.
– Type vgs to check Volume Group size before remove logical volume.
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