Objectives

After completing this section, students should be able to identify a directory in the file system hierarchy and what storage device it is stored on.

Storage management concepts

A file system is an organized structure of data-holding files and directories residing on a storage device, such as a physical disk or partition. The file system hierarchy discussed earlier assembles all the file systems into one tree of directories with a single root, the / directory. The advantage here is that the existing hierarchy can be extended at any time by adding a new disk or partition containing a supported file system to add disk space anywhere in the file system tree. The process of adding a new file system to the existing directory tree is called mounting. The directory where the new file system is mounted is referred to as a mount point. This is a fundamentally different concept than that used on a Microsoft Windows system, where a new file system is represented by a separate drive letter.

Hard disks and storage devices are normally divided up into smaller chunks called partitions. A partition is a way to compartmentalize a disk. Different parts of it can be formatted with different file systems or used for different purposes. For example, one partition could contain user home directories while another could contain system data and logs. If a user fills up the home directory partition with data, the system partition may still have space available. Placing data in two separate file systems on two separate partitions helps in planning data storage.

Storage devices are represented by a special file type called block device. The block device is stored in the /dev directory. In Red Hat Enterprise Linux, the first SCSI, PATA/SATA, or USB hard drive detected is /dev/sda, the second is /dev/sdb, and so on. This name represents the whole drive. The first primary partition on /dev/sda is /dev/sda1, the second partition is /dev/sda2, and so on.

A long listing of the /dev/vda device file on serverX reveals its special file type as b, which stands for block device:

[student@serverX ~]$ ls -l /dev/vda
brw-rw----. 1 root disk 253, 0 Mar 13 08:00 /dev/vda

Another way of organizing disks and partitions is with logical volume management (LVM). With LVM, one or more block devices can be aggregated into a storage pool called a volume group. Disk space is made available with one or more logical volumes. A logical volume is the equivalent of a partition residing on a physical disk. Both the volume group and the logical volume have names assigned upon creation. For the volume group, a directory with the same name as the volume group exists in the /dev directory. Below that directory, a symbolic link with the same name as the logical volume has been created. For example, the device file representing the mylv logical volume in the myvg volume group is /dev/myvg/mylv.

It should be noted that LVM relies on the Device Mapper (DM) kernel driver. The above symbolic link /dev/myvg/mylv points to the /dev/dm-number block device node. The assignment of the number is sequential beginning with zero (0). There is a another symbolic link for every logical volume in the /dev/mapper directory with the name /dev/mapper/myvg-mylv. Access to the logical volume can generally use either of the consistent and reliable symbolic link names, since the /dev/dm-number name can vary with each boot.

Examining file systems

To get an overview about the file system mount points and the amount of free space available, run the df command. When the df command is run without arguments, it will report total disk space, used disk space, and free disk space on all mounted regular file systems. It will report on both local and remote systems and the percentage of the total disk space that is being used.

Display the file systems and mount points on the serverX machine.

[student@serverX ~]$ df
Filesystem     1K-blocks    Used Available Use% Mounted on
/dev/vda1        6240256 4003760   2236496  65% /
devtmpfs          950536       0    950536   0% /dev
tmpfs             959268      80    959188   1% /dev/shm
tmpfs             959268    2156    957112   1% /run
tmpfs             959268       0    959268   0% /sys/fs/cgroup

The partitioning on the serverX machine shows one real file system, which is mounted on /. This is common for virtual machines. The tmpfs and devtmpfs devices are file systems in system memory. All files written into tmpfs or devtmpfs disappear after system reboot.

To improve readability of the output sizes, there are two different human-readable options: -h or -H. The difference between these two options is that -h will report in KiB (2¹⁰), MiB (2²⁰), or GiB (2³⁰), while the -H option will report in SI units: KB (10³), MB (10⁶), GB (10⁹), etc. Hard drive manufacturers usually use SI units when advertising their products.

Show a report on the file systems on the serverX machine with all units converted to human-readable format:

[student@serverX ~]$ df -h
Filesystem      Size  Used Avail Use% Mounted on
/dev/vda1       6.0G  3.9G  2.2G  65% /
devtmpfs        929M     0  929M   0% /dev
tmpfs           937M   80K  937M   1% /dev/shm
tmpfs           937M  2.2M  935M   1% /run
tmpfs           937M     0  937M   0% /sys/fs/cgroup

For more detailed information about the space used by a certain directory tree, there is the du command. The du command has -h and -H options to convert the output to human-readable format. The du command shows the size of all files in the current directory tree recursively.

Show a disk usage report for the /root directory on serverX:

[root@serverX ~]# du /root
4	/root/.ssh
4	/root/.cache/dconf
4	/root/.cache
4	/root/.dbus/session-bus
4	/root/.dbus
0	/root/.config/ibus/bus
0	/root/.config/ibus
0	/root/.config
14024	/root

Show a disk usage report in human-readable format for the /var/log directory on serverX:

[root@serverX ~]# du -h /var/log
...
4.9M	/var/log/sa
68K	/var/log/prelink
0	/var/log/qemu-ga
14M	/var/log