Inode is a data structure that keeps track of all the information about a file.
You store your information in a file, and the operating system stores the information about a file in an inode(sometimes called as an inode number).
Information about files(data) are sometimes called metadata. So you can even say it in another way, “An inode is metadata of the data.”
Whenever a user or a program needs access to a file, the operating system first searches for the exact and unique inode (inode number), in a table called as an inode table. In fact the program or the user who needs access to a file, reaches the file with the help of the inode number found from the inode table.
To reach a particular file with its “name” needs an inode number corresponding to that file. But to reach an inode number you dont require the file name. Infact with the inode number you can get the data.How does the structure of an inode look like?

This is the most important part to understand in terms of an inode. Here we will be discussing the contents of an inode.

Inode Structure of a Directory:

Inode structure of a directory just consists of Name to Inode mapping of files and directories in that directory.

you can see the first two entries of (.) and (..) dot dot. You might have seen them whenever you list the contents of a directory.(most of the times they are hidden. You will have to use -a option with “ls” command to see them).

And people who are more into Linux or any NIX system, knows that the command “cd .” will change the directory to the current directory itself(which means it does nothing..because you are already in that directory.).

And the command “cd ..” will take you to the previous directory or call it the parent directory of the current directory. Now why that happens?

Lets understand why this happens with an example.

[root@rafi log]# pwd
/var/log

[root@rafi log]# ls -ai
393456 .                     392471 boot.log       393598 gdm                392887 pm-powersave.log   404347 spooler
392449 ..                    392943 btmp           393624 httpd              393490 ppp                393609 spooler-20160330
392779 anaconda.ifcfg.log    393568 btmp-20160401  393561 lastlog            523827 prelink            393484 tallylog
392465 anaconda.log          393489 ConsoleKit     404086 mail               393000 puppet             392865 vmware-tools-upgrader.log
392476 anaconda.program.log  404343 cron           404344 maillog            393903 rhsm               392806 wpa_supplicant.log
392478 anaconda.storage.log  392789 cron-20160330  393608 maillog-20160330   394649 sa                 393566 wtmp
392466 anaconda.syslog       393571 cups           404345 messages           393894 samba              392922 Xorg.0.log
392467 anaconda.xlog         392468 dmesg          393606 messages-20160330  404346 secure             392871 Xorg.0.log.old
392780 anaconda.yum.log      392469 dmesg.old      404342 mysqld.log         393607 secure-20160330    392938 yum.log
524731 audit                 393493 dracut.log     393610 ntpstats           392790 spice-vdagent.log

Now lets note down inode numbers of .(dot) and ..(dot dot).
393456 . 
392449 ..

Now lets do the directory listing of /var/ directory and see the inodes there.
[root@rafi log]# cd ..
[root@rafi var]# pwd
/var

[root@rafi var]# ls -ai
392449 .        392452 cache  393458 empty  392450 lib    393456 log    393466 nis       393469 run    393626 www
2 ..       523733 crash  393459 games  393462 local  393465 mail   393467 opt       393470 spool  393473 yp
524735 account  393457 db     393593 gdm    393463 lock   392990 named  393468 preserve  392474 tmp

So you can clearly note that inode of .(dot) inside /var/log directory is equal to inode of log directory. And inode of ..(dot dot ) inside /var/log/ is equal to inode of .(dot) inside /var/ directory.

.(dot) always means the current directory just because its inode is same as the directory’s inode. And ..(dot dot) means parent directory inode because its inode is same as the previous(parent) directory.

Inode Structure of a File


Mode:

This keeps information about two things, one is the permission information, the other is the type of inode, for example an inode can be of a file, directory or a block device etc.

Owner Info: Access details like owner of the file, group of the file etc.

Size:  This location store the size of the file in terms of bytes.

Time Stamps:  it stores the inode creation time, modification time, etc.

Now comes the important thing to understand about how a file is saved in a partition with the help of an inode.

Block Size: Whenever a partition is formatted with a file system.It normally gets formatted with a default block size. Now block size is the size of chunks in which data
will be spread. So if the block size is 4K, then for a file of 15K it will take 4 blocks(because 4K*4 16), and technically speaking you waste 1 K.

Direct Block Pointers:
In an ext2 file system an inode consists of only 15 block pointers. The first 12 block pointers are called as Direct Block pointers. Which means that these pointers point to the address of the blocks containing the data of the file. 12 Block pointers can point to 12 data blocks. So in total the Direct Block pointers can address only 48K(12 * 4K) of data. Which means if the file is only of 48K or below in size, then inode itself can address all the blocks
containing the data of the file.

Now What if the file size is above 48K?

Indirect Block Pointers:

whenever the size of the data goes above 48k(by considering the block size as 4k), the 13th pointer in the inode will point to the very next block after the data(adjacent block after 48k of data), which inturn will point to the next block address where data is to be copied.

Now as we have took our block size as 4K, the indirect block pointer, can point to 1024 blocks containing data(by taking the size of a block pointer as 4bytes, one 4K block can point to 1024 blocks because 4 bytes * 1024 = 4K).

which means an indirect block pointer can address, upto 4MB of data(4bytes of block pointer in 4K block, can point and address 1024 number of 4K blocks which makes the data size of 4M)

Double indirect Block Pointers:

Now if the size of the file is above 4MB + 48K then the inode will start using Double Indirect Block Pointers, to address data blocks. Double Indirect Block pointer in an inode will point to the block that comes just after 4M + 48K data, which intern will point to the blocks where the data is stored.

Double Indirect block pointer also is inside a 4K block as every blocks are 4K, Now block pointers are 4 bytes  in size, as mentioned previously, so Double indirect block pointer can address 1024 Indirect Block pointers(which means 1024 * 4M =4G). So with the help of a double indirect Block Pointer the size of the data can go upto 4G.

Triple Indirect Block Pointers:

Now this triple Indirect Block Pointers can address upto 4G * 1024 = 4TB, of file size. The fifteenth block pointer in the inode will point to the block just after the 4G of data, which intern will point to 1024 Double Indirect Block Pointers.

So after the 12 direct block pointers, 13th block pointer in inode is for Indirect block pointers, and 14th block pointer is for double indirect block pointers, and 15th block pointer is for triple indirect block pointers.

Now this is the main reason why there are limits to the full size of a single file that you can have in a file system.

Now an interesting fact to understand is that the total no of inodes are created at the time of creating a file system. Which means there is an upper limit in the number of inodes you can have in a file system. Now after that limit has reached you will not be able to create any more files on the file system, even if you have space left on the partitio