Linux kernel continued

 

Linux supports several file systems. The Virtual File System Interface allows Linux to support many file systems via a common interface. It is designed to allow access to files as fast and efficiently as possible.

Ex2fs was the original file system, and it became widely popular allowing typical file operations such as to create, update, and delete files, directories, hard links, soft links, device special files, sockets, and pipes. It suffered from one limitation that if the system crashed, the entire file system would be validated and corrected for inconsistencies before it is remounted. This was improved with journaling where every file system operation is logged before the operation is executed and the log is replayed to bring the file system to consistency.

Linux Volume Managers and Redundant Array of Inexpensive Disks (RAID) provide a logical abstraction of a computer’s physical storage devices and can combine several disks into a single logical unit to provide increased total storage space as well as data redundancy. Even on a single disk, they can divide the space into multiple logical units, each for a different purpose.

Linux provides four different RAID levels. RAID-Linear which is a simple concatenation of disks that comprise the volume. Raid-0 is a simple striping where the data that is written is interleaved in equal-sized “chunks” across all disks in the volume. RAID-1 is mirroring where  all data is replicated on all disks in the volume. A RAID-1 volume created from n disks can survive the failure of n-1 of those disks. RAID-5 is striping with parity which is similar to RAID-0 but with one chunk in each stripe containing parity information instead of data. RAID-5 can survive the failure of any single disk in the volume.

A Volume-Group could be used to form a collection of disks also called Physical-Volumes. The storage space provided by these disks is then used to create Logical-Volumes. It is also resizable.  New volumes are easy to add as extents and the Logical Volumes can be expanded or shrinked and the data on the LVs can be moved around within the same Volume-Group.

Beyond the hard disk, keyboard and console that a Linux system supports by default, a user-level application can create device special files to access other hardware devices. They are mounted as device nodes in the /dev directory. Usually, these are of two types: a block device and a character device. Block devices allow block-level access to the data residing on a device and the character devices allow character-level access to the devices. The ls -l command will show a ‘b’ for block device and a ‘c’ for character device in the permission string. The virtual file system devfs is an alternative to these special devices. It reduces the system administrative task of creating device node for each device.  A system administrator can mount the devfs file system many times at different mount points but changes to a device node is reflected on all the mount points. The devfs namespace exists in the kernel even before it is mounted which makes the device node, to become available independently of the root file system.