Linux Interview Scenarios

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How to set up and use iSCSI target on Linux

What is iSCSI?
iSCSI is an Internet Protocol based storage networking standard for linking data storage facilities. By carrying SCSI commands over IP networks, iSCSI can facilitate data transfers over local area networks (LANs), wide area networks (WANs), or the Internet. Through iSCSI, the space on storage server will be regarded as local disks by client's operation system. But in reality, all data transferred to the disk are actually transferred over network to the storage server.


What is iSCSI Target and LUN?
In SCSI terminology, A LUN (logical unit number) represents an individually addressable (logical) SCSI device that is part of a physical SCSI device (called "Target"). An iSCSI environment emulates the connection to a SCSI hard disk. An iSCSI Target is like a connection interface, and LUNs are essentially numbered disk drives. When an iSCSI Target is connected by an iSCSI initiator (a client), all the LUNs mapped to the iSCSI Target are virtually attached to the client's operation system. Therefore, Initiators can establish and manage file systems on iSCSI LUNs, as they would a raw SCSI or IDE hard drive.

https://www.synology.com/en-us/knowledgebase/DSM/tutorial/Virtualization/How_to_set_up_and_use_iSCSI_target_on_Linux 

Detailed iSCSI targets from 1st stage onwards will be found in above URL.

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You Noticed A Kernel Error Message During The Boot Process, But It Scrolled Before You Could Read This? What Log File Could You Check To Find Out That Message?

The log file that contains kernel boot messages is /var/log/dmesg

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high memory utilization in Linux troubleshooting process

top (check the load avge and memory)
free -h (check pagecache and clear it if required.)
vmstat -s
check for memory ballooning.
sar -r (check at what time it is utilizing more memory)



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 On my RHEL 7 setup the rsyslog service fails to start but the problem is once the rsyslog server fails I do not get any messages in /var/log/messages hence I am unable to debug or find the problem why the rsyslog service is failing. Where should I check my system messages in such scenarios?

 A. On RHEL 7 we have "journal" which is a component of systemd that is responsible for viewing and management of log files. Logging data is collected, stored, and processed by the Journal's journald service. It creates and maintains binary files called journals based on logging information that is received from the kernel, from user processes, from standard output, and standard error output of system services or via its native API. These journals are structured and indexed, which provides relatively fast seek times. Journal entries can carry a unique identifier. The journald service collects numerous meta data fields for each log message. The actual journal files are secured, and therefore cannot be manually edited.

To view the log files you can use


# journalctl
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How to implement logrotate on audit log files ?


To implemet logrotate on audit log files, we need to modify minimum below listed three parameters :

1. num_logs = 90 (Number of logs to keep on the server based on the disk space)
2. max_log_file_action = rotate (This parameter tells the system what action to take when the system has detected that the max file size limit has been reached)
   if we keep the second parameter as "keep_logs" for second one,  it does not use the num_logs setting.
3. max_log_file = 6 (Maximum each log file size in MB's)

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Disk space not getting freed even after deleting files ?
https://access.redhat.com/solutions/2316
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 Showmount command not working getting hang what could be done in this scenario ?

https://access.redhat.com/solutions/64542

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What happens when you type an URL www.example.com in the browser ?

 Browser ---> OS --->  OS memory --> OS ---> Resolver(ISP) --> DNS

In DNS we have three different units..

Root servers
Domain extension servers
Authoritative servers.

Root server will store the location of all extension servers like .org.com .in...
In our case it is .com server.

Domain extension server or .com server stores the location of authorative servers of all domains ending with .com
Authorative servers stores the IP address for some domains.

In simple the route will be like this

 Browser ---> OS --->  OS memory --> OS ---> Resolver(ISP) -->Root server (it redirects to .com servers) --> Authorative servers (IP address) --> Resolver (ISP) --> OS --> Browser.

Browser establishes the connection between the OS and Authorative server.
This complete process will completes in fraction of seconds.

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After upgrading kernel the machine fails to boot, what will you do?

 A. The very first thing to be done here is to edit the grub menu at boot stage and make the system boot with alternative kernel (assuming the last kernel is still installed) or else try booting the system with using the rescue option from the grub menu.

Once the node is UP then you can analyse the issue of why the node is failing to boot from new kernel. Many times the kernel is not properly installed and all the libraries are not available which leads to this problem. or the GRUB can be corrupted so you can regerate the initramfs using grub2-mkconfig

# grub2-mkconfig -o /boot/grub2/grub.cfg

If there is a kernel panic observed then boot the system with alternate kernel or rescue and then enable kdump. Share the kdump with the support engineers as they can then further try to debug the source of the problem

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Moving a volume group to another System


You can move an entire LVM volume group to another system. 
It is recommended that you use the vgexport and vgimport commands when you do this. 
Note
As of Red Hat Enterprise Linux 6.5, you can use the --force argument of the vgimport command. 
This allows you to import volume groups that are missing physical volumes and subsequently run the  
vgreduce --removemissing command.
The vgexport command makes an inactive volume group inaccessible to the system, which allows you to detach its physical volumes. The vgimport command makes a volume group accessible to a machine again after the vgexport command has made it inactive.
To move a volume group form one system to another, perform the following steps:
  1. Make sure that no users are accessing files on the active volumes in the volume group, then unmount the logical volumes. 
  2. Use the -a n argument of the vgchange command to mark the volume group as inactive, which prevents any further activity on the volume group. 
  3. Use the vgexport command to export the volume group. This prevents it from being accessed by the system from which you are removing it. 
    After you export the volume group, the physical volume will show up as being in an exported volume group when you execute the pvscan command, as in the following example. 
    # pvscan
  PV /dev/sda1    is in exported VG myvg [17.15 GB / 7.15 GB free]
  PV /dev/sdc1    is in exported VG myvg [17.15 GB / 15.15 GB free]
  PV /dev/sdd1   is in exported VG myvg [17.15 GB / 15.15 GB free]
  ...
    When the system is next shut down, you can unplug the disks that constitute the volume group and connect them to the new system. 
  4. When the disks are plugged into the new system, use the vgimport command to import the volume group, making it accessible to the new system. 
  5. Activate the volume group with the -a y argument of the vgchange command. 
  6. Mount the file system to make it available for use.
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