RH033 Unit16 The Linux Filesystem In-Depth

Ojectives Upon completion of this unit, you should be able to: Describe how filesystem information is organized
Describe the function of dentries and inodes
Describe how cp, mv and rm work at the inode level
Create symbolic links and hard links
Access removable media
Create archives using tar and gzip
Partitions and Filesystems 1) Disk drives are divided into partitions 2) Partitions are formatted with filesystems, allowing users to store data Default filesystems: ext3, the Third Extended Linux Filesystem
Other common filesystems:ext2 and msdos (typically used for floppies)
iso9660 (typically used for CDs)
GFS and GFS2 (typically used for SANs)

Inodes 1) An inode table contains a list of all files in an ext2 or ext3 filesystem 2) An inode (index node) is an entry in the table, containing information of a file (the metadata), including: file type, permissions, UID, GID
the link count (count of path names pointing to this file)
the file’s size and various stamps
pointers to the file’s data blocks on disk
other data about the file
Directories 1) The computer’s reference for a file is the inode number 2) The human way to reference a file is by file name 3) A directory is a mapping between the human name for the file and the computer’s inode number cp and inodes The cp command: Allocate a free inode number, placing a new entry in the inode table
Creates a dentry in the directory, associating a name with the inode number
Copies data into the new file
mv and inodes 1) If the destination of the mv command is on the same file system as the source, the mv command: Creates a new directory entry with the new file name
Deletes the old directory entry with the old file name
2) Has no imact on the inode table (except for a time stamp) or the location of data on the disk: no data is moved! 3) If the destination is a different filesystem, mv acts as a copy and remove rm and inodes 1) The rm command: Decrements the link count, thus freeing the inode number to be reused
Places data blocks on the free list
Removes the directory entry
2) Date is not actually removed, but will be overwritten when the data blocks are used by another file Hard Links 1) A hard link adds an additional pathname to reference a single file One physical file on the filesystem
Each directory references the same inode number
Increments the link countThe rm command decrements the link count
File exists as long as at least one link remains
When the link account is zero, the file is removed

Cannot span drives or partitions
2) Syntax: ln filename [linkname] Symbolic (or Soft) Links 1) A symbolic link points to another file ls –l displays the link name and the referenced file
File type: l for symbolic link
The content of a symbolic link is the name of the file that is references
2) Syntax: ln –s filename linkname The Seven Fundamental Filetypes

ls –l symbol	File Type
-	regular file
d	directory
l	symbolic link
b	block special file
c	character special file
p	named pipe
s	socket

Checking Free Space 1) df – Reports disk space usage Reports total kilobytes, kilobytes used, kilobytes free per file system
-h and –H display sizes in easier to read unit
2) du – Reports disk space usage Reports kilobytes used per directory
Includes subtotals for each subdirectory (-s option only reports single directory summary)
Also takes –h and –H options
3) Applications –> System Tools –> Disk Usage Analyzer or baobab – Reports disk space usage graphically Removable Media 1) Mounting means making a foreign filesystem look like part of the main tree 2) Before accessing, media must be mounted 3) Before removing, media must be unmouted 4) By default, non-root users may only mount certain devices (cd, dvd, floppy, usb, etc) 5) Mountpoints are usually under /media Mounting CDs and DVDs 1) Automatically mounted in Gnome/KDE 2) Otherwise, must be manually mouted CD/DVD Reader – mount /media/cdrom
CD/DVD Writer – mount /media/cdrecorder
3) eject command unmounts and ejects the disk Mounting USB Media 1) Detected by the kernel as SCSI devices /dev/sdaX or /dev/sdbX or similar
2) Automatically mounted in Gnome/KDE Icon created in computer window
Mounted under /media/Device ID – Device ID is built into device by vendor
Mounting Floppy Disks 1) Must be manually mounted and unmounted mount /media/floppy
umount /media/floppy
2) DOS floppies can be accessed with mtools Mounts and unmounts device transparently
uses DOS naming conventionsmdir a:
mcopy /home/file.txt a:

Archiving Files and Compressing Archives 1) Archiving places many files into on target file Easier to backup, store, and transfer
tar – standard Linux archiving command
2) Archives are commonly compressed Algorithm applied that compressed file
Umcompressing retores the original file
tar natively supports compression using gzip and gunzip, or bzip2 and bunzip2
Creating, Listing, and Extracting File Archives 1) Action arguments (one is required) -c create an archive
-t list an archive
-x extracts files from an archive
2) Typically requried: -f archviename name of file archive
3) Optional arguments: -z use gzip compression
-j use bzip2 compression
-v be verbose
Creating File Archives: Other Tools 1) zip and unzip Supports pkzip-compatible archives
2) file-roller Graphical, multi-format archiving tool
End of Unit16 1) Questions and Answers 2) Summary Linux filesystem structure
Using removable media
Using unformatted floppies
Archiving and compression