File Systems In AIX-A Primer

Generally talking, the File Systems are collection of files, directories etc. within a single partition or Logical Volume in a Volume Group. Collecting files and directories from different Logical Volumes or Partitions does not qualify to be named as File System. Aix supports mainly three types of File Systems: JFS, NFS and CDRFS. All these are being discussed here in this article. For more information you can see the previous article: Aix Logical Volume Manager A Brief Introduction.

What Are The Components of A File System
The File System normally has the below mentioned components, depending upon the partition/logical volume and Operating System type:

1. Boot Block.
2. Super Block.
3. Bitmaps.
4. Allocation Groups.

 
Aix Supported File Systems
The Aix OS supports mainly three types of File Systems:

1. Journaled File System(JFS/JFS2).
2. Network File System(NFS).
3. CD-ROM File System(CDRFS).

 
Journaled File System
The Journaled File System is native to AIX. This File System retain a journal for every I/O transaction on a Physical Volumes. One Logical Volume can contain only one Journaled File System. In order to use the File System, the Operating System has to mount it. The File Systems which are critical to the booting and operation of the Operating System, are mounted during boot time operation.

The Journaled File Systems are of two types: JFS and JFS2. AIX 5.3 onwards JFS2 is the default File System. Earlier it used to be JFS. JFS2 has some enhancements over JFS.

Network File System
The NFS is used to allow the users to access the files and directories over the network. NFS has been in use since the very beginning for almost all kind of *nix Operating Systems. Almost every *nix OS supports NFS. NFS is a distributed File System and provides seamless access to the files from the server side.

CD-ROM File System
The CDRFS is the one typical to CDs.

Journaled File System Structure
The entire JFS is nothing but a set of logical blocks.

Boot Record: This is the first logical block in the File System and is reserved for bootstrap program. File System does not use it.

Superblock: The super block keeps and maintains information about total File System Its size is 4096Bytes. It has several types of information:

1. File System size.
2. Total Number of data blocks in File System.
3. Flag to indicate the state of File System.
4. Allocation Group Sizes.

 
Allocation Groups: The total File System is a collection of logical blocks. After leaving initial few logical blocks, the logical blocks in rest of the File System are grouped together. Then additional space is allocated in group of these logical blocks. That’s why these are called allocation groups. The i-nodes refer to these data blocks. So, allocating the i-nodes means allocating these groups of data blocks.