Linux Query or Interview question - 8

Query 1. Why only 4 primary partitions can be created on HDD?

Ans: The MBR (Master Boot Record) contains boot information & partition table information. MBR is located in 1^st sector of HDD or 0^th cylinder of HDD. The size of MBR is 512 bytes in that 446 bytes is used for boot sector and 46 bytes is allocated for partition table. The remaining 2 bytes are nothing but magic number which shows “YES” or “NO” value.

 

MBR is a small program created by using Assembly language. MBR gets stored in HDD at the time of installation.  In layman language the MBR will be present only on that disk on which OS is installed.

The partition table is divided into different programs which manages particular partition.

1 program = 16 bytes = /dev/sda1 = manages 1^st partition

2 program = 16 bytes = /dev/sda2 = manages 2^nd partition

3 program = 16 bytes = /dev/sda1 = manages 3^rd partition

4 program = 16 bytes = /dev/sda1 = manages 4^th partition

So the partition table of size 64 bytes is divided into 4 programs of size 16 bytes each to manage the 4 primary partitions & that is the reason we can create only 4 primary partitions.

Please refer post: http://kiranbjadhav.blogspot.in/2017/03/linux-query-or-interview-question-3.html

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Query 2: what is “SHUTDOWN PROCESS”?

Ans: Maximum interviewers ask you boot process to explain but sometime they may ask you to explain shutdown and reboot process/sequence.

Run below command for shutting down server.

[root@ testmachine ~]# poweroff

Or

[root@ testmachine ~]# shutdown –h now

Both commands calls #init 0 in background; the init0 run level is for halting the machine.

To get run level details refer post:

http://kiranbjadhav.blogspot.in/2017/03/linux-query-or-interview-question-5.html

The init0 will call to /etc/rc scripts.

Process tree:

/etc/rc = script responsible for stopping or starting the services.

| --  check run level

| -- /etc/rc$.d/k* = all k=kill files from specific run level

| -- /etc/rc$.d/s* = all s=start files from specific run level

| -- /etc/rc.local = use this file, if you want to run any command or script before login

So now the /etc/rc will check runlevel and it finds the run level 0 so it will call /etc/rc0.d/ and will run all the k* related scripts present under this directory.

And will start two processes i.e S00killall – this process will start and kill if any pending process which is still running on server. After this, S01halt process will start and this process will send signal to BIOS to halt/poweroff/shutdown the machine.

[root@testmachine rc0.d]# ls -lrt |egrep -i "killall|halt"

lrwxrwxrwx  1 root root 14 Aug 17  2015 S01halt -> ../init.d/halt

lrwxrwxrwx  1 root root 17 Aug 17  2015 S00killall -> ../init.d/killall

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Query 3: what is “REBOOT PROCESS”?

Ans: For rebooting the server run below command:

[root@ testmachine ~]# reboot

Or

[root@ testmachine ~]# shutdown –r now

Both commands calls #init 6 in background; the init6 run level is for rebooting the machine.

The init6 will call to /etc/rc scripts.

So now the /etc/rc will check runlevel and it finds the run level 6 so it will call /etc/rc6.d/ and will run all the k* related scripts present under this directory.

And will start two processes i.e S00killall – this process will start and kill if any pending process which is still running on server. After this, S01reboot process will start and it will send signal to BIOS to reboot/restart the machine.

[root@testmachine rc6.d]# ls -lrt |egrep -i "killall|reboot"

lrwxrwxrwx  1 root root 14 Aug 17  2015 S01reboot -> ../init.d/halt

lrwxrwxrwx  1 root root 17 Aug 17  2015 S00killall -> ../init.d/killall

Note: The only difference is that, in shutdown process OS send halt signal (S01halt) from /etc/rc0.d to BIOS and in reboot process we send reboot signal (S01reboot) from /etc/rc6.d to BIOS.

Thanks & Regards,

Kiran Jadhav