1. How to scan a new disk added to Redhat linux server. Manual method with out any script or softwares.

echo "- - -" > /sys/class/scsi_host/hosth/  scan

or
echo "c t l" >  /sys/class/scsi_host/hosth/  scan

c   is the channel on the HBA,
t    is the SCSI target ID  ,
l    is the LUN and
h   is  the HBA number

2. What is difference between ping and ping6 Commands?

Both ping and ping6 commands are same. Regular ping command works with IPV4 and ping6 works with ipv6 IP address.

3.  SAN connectivity to a server comes through which port.

 SAN storage connected through HBA/FC ports.

4. How to check if the RHEL server is physical or virtual after login to server using command line.

dmidecode command will help us to determind physical or virtual server,

For example : Pysical


dmidecode | grep "Product Name"

        Product Name: 300E4C/300E5C/300E7C

        Product Name: NP300E5X-A08IN

For example : Virtualization


dmidecode | grep "Product Name"

        Product Name: PowerEdge R420

        Product Name: 072XWF

5. What is DHCP and what is its use.

Dynamic Host Configuration Protocol, a protocol for assigning dynamic IP addressesand configuration information to clients.

Minimum basic information like IP Address, Default Gateway and Subnet Mask. DHCP also supports a mix of static and dynamic IP addresses.

This means that a new computer can be added to a network without the hassle of manually assigning it a unique IP address .

6.  What is RAID and what is its advantage. Name 3 basic RAID types widely used.

RAID expands to "“Redundant Array of Inexpensive Disks” or "“Redundant Array of Independent Disks”

RAID is basically used to achieve redundancy and/or with faster I/O.

RAID 0:
Need minumum 2 disks
blocks are striped
no mirror, no parity
We do not use this level to any critical system

RAID 1:
Need minumum 2 disks
data mirrored
no striping and no parity

RAID 5:
parity protected
Need minimum 3 disks.
extra data written to identify errors
blocks are striped

7. How to check the environment variables for a login session

We can check using  env or do need to check in the respective open the  .profile file on users home directory.

8. What is multipathing I/O and how can we check if native multipathing configured in RedhatLinux server or not.

Mutipathing is a fault tolerant technique, more than one physical path between the CPU in the computer systems and its main storage devices through the buses, controllers, Switches and other bridge devices connecting them Fibre Channel (FC) or iSCSI SAN environments.

displays the multipathed disks
 
multipath -ll

9. What is the difference between snapshot and clone in vmware.

A clone is a copy of a virtual machine that can be used to create many clone. Once completed the colning operation it will be a separate virtual machine.

We can take snapshot of a virtual machine backup at a specific point in time.

It's includes virtual machine settings and contents of VM Memory.
The state of all the virtual machine's disks.

10. Unfortunately you must restart or downtime server  how you will inform to all connected User's ?

wall command sends a message to all connected Users and message length is limited to 20 lines.

More interview questions :

Interview Questions
Interview Questions L2 - I
Interview Questions L2 - II