Block devices: blkid, fdisk -l, /dev

Everything in /dev is a file, but two kinds of file. Character devices hand bytes through one at a time: terminals, the serial port, /dev/null, /dev/zero. Block devices are addressable in fixed-size blocks you can seek around in: disks, partitions, CD-ROMs, and anything you can put a filesystem on. ls -l shows the difference in the very first column, c or b.

~% ls -l /dev/sr0 /dev/loop0 /dev/null /dev/ttyS0
brw-------    1 root     root        7,   0 Oct  4 18:51 /dev/loop0
crw-rw-rw-    1 root     root        1,   3 Oct  4 18:51 /dev/null
brw-------    1 root     root       11,   0 Oct  4 18:51 /dev/sr0
crw-------    1 root     root        4,  64 Oct  4 18:51 /dev/ttyS0

Where a regular file shows a size, a device shows two numbers: the major picks the driver (11 is SCSI CD-ROM, 7 is loop, 8 would be SCSI/SATA disks like sda) and the minor picks the instance. Names follow conventions: sda, sdb for whole disks, sda1, sda2 for their partitions, sr0 for the first optical drive, loop0–loop7 for loop devices.

Which disks do I have?

Three views, from rawest to friendliest. /proc/partitions is the kernel's list of block devices with their size in 1 KB blocks. fdisk -l prints every disk's geometry and its partition table; fdisk -l DEVICE limits it to one. blkid reads the first bytes of each device looking for a filesystem signature, and reports the UUID (a unique id assigned at format time), the LABEL if one was set, and the TYPE. UUIDs are how modern /etc/fstab files name disks, because /dev/sdb can become /dev/sdc after a reboot but the UUID never moves.

~% cat /proc/partitions
major minor  #blocks  name

  11        0       7550 sr0
~% fdisk -l /dev/sr0
Note: sector size is 2048 (not 512)
Disk /dev/sr0: 7 MB, 7731200 bytes, 15100 sectors
0 cylinders, 255 heads, 63 sectors/track
Units: cylinders of 16065 * 2048 = 32901120 bytes

Disk /dev/sr0 doesn't contain a valid partition table

The VM has a single block device, the boot CD, and no hard disk, so fdisk -l alone prints nothing and blkid alone is silent too. To have something to practise on, we will make a disk.

A disk made from a file: loop devices

A loop device makes a regular file behave as a block device. Create an empty 4 MB file with dd, attach it with losetup /dev/loop0 FILE, and /dev/loop0 now *is* that file as far as the kernel is concerned. mke2fs /dev/loop0 writes an ext2 filesystem onto it (-L NAME sets a label), after which blkid sees it, and mount /dev/loop0 DIR mounts it like any disk. losetup -a lists attachments, losetup -d /dev/loop0 detaches.

~% dd if=/dev/zero of=/root/lab/l66/disk.img bs=1M count=4
4+0 records in
4+0 records out
~% losetup /dev/loop0 /root/lab/l66/disk.img
~% mke2fs -L labdisk /dev/loop0 > /dev/null
~% blkid
/dev/loop0: LABEL="labdisk" UUID="487f006e-cba1-490a-9403-3d4513275e9f"
~% mount /dev/loop0 /root/lab/l66/mnt
~% df -h /root/lab/l66/mnt
Filesystem                Size      Used Available Use% Mounted on
/dev/loop0                3.9M     13.0K      3.7M   0% /root/lab/l66/mnt
~% ls /root/lab/l66/mnt
lost+found