Filesystems

When you are finished partitioning the hard disk, you can put a file systemfile system on each partition.

This chapter builds on the partitions from the previous chapter, and prepares you for the next one where we will mount the filesystems. about file systems ==================

A file system is a way of organizing files on your partition. Besides file-based storage, file systems usually include directoriesdirectory and access control, and contain meta information about files like access times, modification times and file ownership.

The properties (length, character set, ...) of filenames are determined by the file system you choose. Directories are usually implemented as files, you will have to learn how this is implemented! Access control in file systems is tracked by user ownership (and group owner- and membership) in combination with one or more access control lists.

man fs

The manual page about filesystems is accessed by typing man fs.

[root@rhel65 ~]# man fs

/proc/filesystems

The Linux kernel will inform you about currently loaded file system drivers in /proc/filesystems.

root@rhel53 ~# cat /proc/filesystems  | grep -v nodev
    ext2
    iso9660
    ext3

/etc/filesystems

The /etc/filesystems file contains a list of autodetected filesystems (in case the mount command is used without the -t option.

Help for this file is provided by man mount.

[root@rhel65 ~]# man mount

common file systems

ext2 and ext3

Once the most common Linux file systems is the ext2ext2 (the second extended) file system. A disadvantage is that file system checks on ext2 can take a long time.

ext2 was being replaced by ext3ext3 on most Linux machines. They are essentially the same, except for the journalingjournaling which is only present in ext3.

Journaling means that changes are first written to a journal on the disk. The journal is flushed regularly, writing the changes in the file system. Journaling keeps the file system in a consistent state, so you don\'t need a file system check after an unclean shutdown or power failure.

creating ext2 and ext3

You can create these file systems with the /sbin/mkfsmkfs(1) or /sbin/mke2fsmke2fs(1) commands. Use mke2fs -j to create an ext3 file system.

You can convert an ext2 to ext3 with tune2fs -jtune2fs(1). You can mount an ext3 file system as ext2, but then you lose the journaling. Do not forget to run mkinitrdmkinitrd(1) if you are booting from this device.

ext4

The newest incarnation of the ext file system is named ext4 and is available in the Linux kernel since 2008. ext4 supports larger files (up to 16 terabyte) and larger file systems than ext3 (and many more features).

Development started by making ext3 fully capable for 64-bit. When it turned out the changes were significant, the developers decided to name it ext4.

xfs

Redhat Enterprise Linux 7 will have XFS as the default file system. This is a highly scalable high-performance file system.

xfs was created for Irix and for a couple of years it was also used in FreeBSD. It is supported by the Linux kernel, but rarely used in dsitributions outside of the Redhat/CentOS realm.

vfat

The vfatvfat file system exists in a couple of forms : fat12 for floppy disks, fat16fat16 on ms-dos, and fat32fat32 for larger disks. The Linux vfat implementation supports all of these, but vfat lacks a lot of features like security and links. fat disks can be read by every operating system, and are used a lot for digital cameras, usb sticks and to exchange data between different OS\'ses on a home user\'s computer.

iso 9660

iso 9660iso9660 is the standard format for cdroms. Chances are you will encounter this file system also on your hard disk in the form of images of cdroms (often with the .iso extension). The iso 9660 standard limits filenames to the 8.3 format. The Unix world didn\'t like this, and thus added the rock ridgerock ridge extensions, which allows for filenames up to 255 characters and Unix-style file-modes, ownership and symbolic links. Another extensions to iso 9660 is jolietjoliet, which adds 64 unicode characters to the filename. The el toritoel torito standard extends iso 9660 to be able to boot from CD-ROM\'s.

udf

Most optical media today (including cd\'s and dvd\'s) use udfudf, the Universal Disk Format.

swap

All things considered, swap is not a file system. But to use a partition as a swap partitionswap partition it must be formatted and mounted as swap space.

gfs

Linux clusters often use a dedicated cluster filesystem like GFS, GFS2, ClusterFS, ...

and more...

You may encounter reiserfsreiserfs on older Linux systems. Maybe you will see Sun\'s zfszfs or the open source btrfsbtrfs. This last one requires a chapter on itself.

/proc/filesystems

The /proc/filesystems/proc/filesystems file displays a list of supported file systems. When you mount a file system without explicitly defining one, then mount will first try to probe /etc/filesystems/etc/filesystems and then probe /proc/filesystems for all the filesystems without the nodevnodev label. If /etc/filesystems ends with a line containing only an asterisk (*) then both files are probed.

paul@RHELv4u4:~$ cat /proc/filesystems 
nodev   sysfs
nodev   rootfs
nodev   bdev
nodev   proc
nodev   sockfs
nodev   binfmt_misc
nodev   usbfs
nodev   usbdevfs
nodev   futexfs
nodev   tmpfs
nodev   pipefs
nodev   eventpollfs
nodev   devpts
        ext2
nodev   ramfs
nodev   hugetlbfs
        iso9660
nodev   relayfs
nodev   mqueue
nodev   selinuxfs
        ext3
nodev   rpc_pipefs
nodev   vmware-hgfs
nodev   autofs
paul@RHELv4u4:~$

putting a file system on a partition

We now have a fresh partition. The system binaries to make file systems can be found with ls.

[root@RHEL4b ~]# ls -lS /sbin/mk*
-rwxr-xr-x  3 root root 34832 Apr 24  2006 /sbin/mke2fs
-rwxr-xr-x  3 root root 34832 Apr 24  2006 /sbin/mkfs.ext2
-rwxr-xr-x  3 root root 34832 Apr 24  2006 /sbin/mkfs.ext3
-rwxr-xr-x  3 root root 28484 Oct 13  2004 /sbin/mkdosfs
-rwxr-xr-x  3 root root 28484 Oct 13  2004 /sbin/mkfs.msdos
-rwxr-xr-x  3 root root 28484 Oct 13  2004 /sbin/mkfs.vfat
-rwxr-xr-x  1 root root 20313 Apr 10  2006 /sbin/mkinitrd
-rwxr-x---  1 root root 15444 Oct  5  2004 /sbin/mkzonedb
-rwxr-xr-x  1 root root 15300 May 24  2006 /sbin/mkfs.cramfs
-rwxr-xr-x  1 root root 13036 May 24  2006 /sbin/mkswap
-rwxr-xr-x  1 root root  6912 May 24  2006 /sbin/mkfs
-rwxr-xr-x  1 root root  5905 Aug  3  2004 /sbin/mkbootdisk
[root@RHEL4b ~]#

It is time for you to read the manual pages of mkfsmkfs(1) and mke2fsmke2fs(1). In the example below, you see the creation of an ext2 file systemext2 on /dev/sdb1. In real life, you might want to use options like -m0 and -j.

root@RHELv4u2:~# mke2fs /dev/sdb1
mke2fs 1.35 (28-Feb-2004)
Filesystem label=
OS type: Linux
Block size=1024 (log=0)
Fragment size=1024 (log=0)
28112 inodes, 112420 blocks
5621 blocks (5.00%) reserved for the super user
First data block=1
Maximum filesystem blocks=67371008
14 block groups
8192 blocks per group, 8192 fragments per group
2008 inodes per group
Superblock backups stored on blocks: 
8193, 24577, 40961, 57345, 73729

Writing inode tables: done                            
Writing superblocks and filesystem accounting information: done

This filesystem will be automatically checked every 37 mounts or
180 days, whichever comes first.  Use tune2fs -c or -i to override.

tuning a file system

You can use tune2fstune2fs(1) to list and set file system settings. The first screenshot lists the reserved space for root (which is set at five percent).

[root@rhel4 ~]# tune2fs -l /dev/sda1 | grep -i "block count"
Block count:              104388
Reserved block count:     5219
[root@rhel4 ~]#

This example changes this value to ten percent. You can use tune2fs while the file system is active, even if it is the root file system (as in this example).

[root@rhel4 ~]# tune2fs -m10 /dev/sda1 
tune2fs 1.35 (28-Feb-2004)
Setting reserved blocks percentage to 10 (10430 blocks)
[root@rhel4 ~]# tune2fs -l /dev/sda1 | grep -i "block count"
Block count:              104388
Reserved block count:     10430
[root@rhel4 ~]#

checking a file system

The fsckfsck(1) command is a front end tool used to check a file system for errors.

[root@RHEL4b ~]# ls /sbin/*fsck*
/sbin/dosfsck  /sbin/fsck         /sbin/fsck.ext2  /sbin/fsck.msdos
/sbin/e2fsck   /sbin/fsck.cramfs  /sbin/fsck.ext3  /sbin/fsck.vfat
[root@RHEL4b ~]#

The last column in /etc/fstab/etc/fstab is used to determine whether a file system should be checked at boot-up.

[paul@RHEL4b ~]$ grep ext /etc/fstab 
/dev/VolGroup00/LogVol00   /             ext3    defaults        1 1
LABEL=/boot                /boot         ext3    defaults        1 2
[paul@RHEL4b ~]$

Manually checking a mounted file system results in a warning from fsck.

[root@RHEL4b ~]# fsck /boot
fsck 1.35 (28-Feb-2004)
e2fsck 1.35 (28-Feb-2004)
/dev/sda1 is mounted.

WARNING!!!  Running e2fsck on a mounted filesystem may cause
SEVERE filesystem damage.

Do you really want to continue (y/n)? no

check aborted.

But after unmounting fsck and e2fscke2fsck(1) can be used to check an ext2 file system.

[root@RHEL4b ~]# fsck  /boot
fsck 1.35 (28-Feb-2004)
e2fsck 1.35 (28-Feb-2004)
/boot: clean, 44/26104 files, 17598/104388 blocks
[root@RHEL4b ~]# fsck -p /boot
fsck 1.35 (28-Feb-2004)
/boot: clean, 44/26104 files, 17598/104388 blocks
[root@RHEL4b ~]# e2fsck -p /dev/sda1
/boot: clean, 44/26104 files, 17598/104388 blocks

practice: file systems

1. List the filesystems that are known by your system.

2. Create an ext2 filesystem on the 200MB partition.

3. Create an ext3 filesystem on one of the 300MB logical drives.

4. Create an ext4 on the 400MB partition.

5. Set the reserved space for root on the ext3 filesystem to 0 percent.

6. Verify your work with fdisk and df.

7. Perform a file system check on all the new file systems. solution: file systems ======================