Linux Filesystems 101 - Block Devices


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Discovering block devices

The block devices on a system can be discovered with the lsblk (list block devices) command. Try it in the VM below. Type lsblk at the command prompt and then press Enter.

Start the VM

The lsblk command should give you the following output:

fd0      2:0    1    4K  0 disk
sda      8:0    0    2G  0 disk
└─sda1   8:1    0  1.4G  0 part /
sdb      8:16   0    3M  0 disk
└─sdb1   8:17   0    3M  0 part /scripts
sr0     11:0    1 1024M  0 rom

There are three different kinds of devices represented here, indicated by the first 2 characters in the device name:

  • fd - A floppy drive (Yeah, yeah... Comes with the VM. Just ignore it.)
  • sd - A hard disk (Originally SCSI, but now includes SATA, SAS, and others.)
  • sr - An optical drive

We will be focusing on hard disk devices, in this case thesda and sdb devices, for this course.

Do you see the sda1 and sdb1 devices in the list? They are partitions of the sda and sdb disks. A disk partition is a portion of the drive that is set aside for a specific use. In the case of sda, the sda1 partition uses 1.4 GiB of the 2.0 GiB drive. So there is approximately 600 MiB of unused space on sda. There is no free space on sdb. The sdb1 partition uses all 3 MiB of that drive.

Notice that sda1 is mounted on / (the filesystem root). Unless you have changed directories, the command prompt should be in the /home/techio directory. You can verify this with the pwd (print working directory) command. Try typing the following df (file system disk space usage) command:

df -h .

NOTE: The -h flag tells the df command to print byte counts in "human readable" format, i.e. 1.2G vs. 1220172. The . (period) at the end of the command indicates that we're only interested in disk usage for the device that contains the current directory.

You should see the following:

Filesystem      Size  Used Avail Use% Mounted on
/dev/sda1       1.4G  1.2G  103M  93% /

This confirms some information that we already knew. The partition sda1 is mounted on /, and is of size 1.4 GiB. There's some extra info in here as well. First, we see that the partition is labeled as /dev/sda1. The /dev directory is a special place in Linux. It is the location of files that can be used to represent the devices in your system. Try executing the command ls /dev. Do you see all the block devices in this list?

We can also see that the filesystem in sda1 is about 93% full; 1.2 GiB are used and 103 MiB are still available. Try the following command:

dd if=/dev/zero of=junk bs=1M count=50

This will create a 50 MiB file called junk in the current directory. What do you think this will do to the available space in the filesystem? Try running the df command again to check your hypothesis. Try removing the file with rm junk, then run the df command again.

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