Tuesday, 5 August 2014

Linux-Filesystem

A filesystem is the methods and data structures that an operating system uses to keep track of files on a disk or partition; that is, the way the files are organized on the disk. The word is also used to refer to a partition or disk that is used to store the files or the type of the filesystem. Thus, one might say I have two filesystems meaning one has two partitions on which one stores files, or that one is using the extended filesystem, meaning the type of the filesystem.

Before a partition or disk can be used as a filesystem, it needs to be initialized, and the bookkeeping data structures need to be written to the disk. This process is called making a filesystem.

Most UNIX filesystem types have a similar general structure, although the exact details vary quite a bit. The central concepts are superblock, inode, data block, directory block, and indirection block. The superblock contains information about the filesystem as a whole, such as its size (the exact information here depends on the filesystem). An inode contains all information about a file, except its name. The name is stored in the directory, together with the number of the inode. A directory entry consists of a filename and the number of the inode which represents the file. The inode contains the numbers of several data blocks, which are used to store the data in the file. There is space only for a few data block numbers in the inode, however, and if more are needed, more space for pointers to the data blocks is allocated dynamically. These dynamically allocated blocks are indirect blocks; the name indicates that in order to find the data block, one has to find its number in the indirect block first.

 Like UNIX, Linux chooses to have a single hierarchical directory structure. Everything starts from the root directory, represented by /, and then expands into sub-directories instead of having so-called 'drives'.

 In general, 'block devices' are devices that store or hold data, 'character devices' can be thought of as devices that transmit or transfer data. For example, diskette drives, hard drives and CD-ROM drives are all block devices while serial ports, mice and parallel printer ports are all character devices.

A few terms to understand:
tty: Video console terminal (abbreviation for “Teletype”)
ttyS: Serial console terminal
pts: Virtual console terminal (pseudo-tty or pty but stands for Pseudo-Terminal Slave (PTS))
[root@host ~]# w
16:58:50 up 19:22,  3 users,  load average: 0.81, 0.90, 0.71
USER     TTY      FROM              LOGIN@   IDLE   JCPU   PCPU WHAT
root     pts/0    client.domain. 16:57    0.00s  0.01s  0.00s w
root     ttyS0    –                16:58    3.00s  0.00s  0.00s -bash
root     tty1     –                16:56    1:24   0.02s  0.02s -bash
As you can see from this, we have 1 user logged in via a virtual console (such as SSH), defined as pts/0. Another user logged in via serial console, defined as ttyS0. And another user logged in via the video console port, defined as tty1.

/lost+found directory:
 ---------------------------------

 Linux should always go through a proper shutdown. Sometimes your system might crash or a power failure might take the machine down. Either way, at the next boot, a lengthy filesystem check (the speed of this check is dependent on the type of filesystem that you actually use. ie. ext3 is faster than ext2 because it is a journalled filesystem) using fsck will be done. Fsck will go through the system and try to recover any corrupt files that it finds. The result of this recovery operation will be placed in this directory. The files recovered are not likely to be complete or make much sense but there always is a chance that something worthwhile is recovered. Each partition has its own lost+found directory. If you find files in there, try to move them back to their original location.

Mounting:
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 Before one can use a filesystem, it has to be mounted. The operating system then does various bookkeeping things to make sure that everything works. Since all files in UNIX are in a single directory tree, the mount operation will make it look like the contents of the new filesystem are the contents of an existing subdirectory in some already mounted filesystem.

/Proc:
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/proc is very special in that it is also a virtual filesystem. It's sometimes referred to as a process information pseudo-file system. It doesn't contain 'real' files but runtime system information (e.g. system memory, devices mounted, hardware configuration, etc).


/sbin should contain only binaries essential for booting, restoring,
  recovering, and/or repairing the system in addition to the binaries
  in /bin. 
 
 

Reference:

 http://www.tldp.org/LDP/Linux-Filesystem-Hierarchy/html/glossary.html

 


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