Linux devices overview

• Devices are special files stored in /dev that provide interfaces to the physical hardware

• There are two main types of devices under all Unix systems, character and block devices

• character devices

  • Provide unbuffered direct access to the corresponding hardware

  • Reads and writes are always performed synchronously

  • Examples of character devices are :

    path	description
    /dev/mem	system RAM
    /dev/fb0	graphics card frame buffer
    /dev/sg0	generic SCSI layer
    /dev/input/*	input peripherals (keyboard, mouse, etc)
    /dev/tty*	native terminals
    /dev/pts*	emulated terminals (ssh, etc)

• block devices

  • Provide buffered/cached access to the corresponding hardware

  • As a result reads and writes can be asynchronous

  • Examples of block devices are :

    path	description
    /dev/sda	hard disk (SCSI driver)
    /dev/sda1	system boot partition (fat32)
    /dev/sda2	system and data partition (ext4)
    /dev/sda3	swap partition
    /dev/sdb	second hard disk (unpartitioned)

Note : in Unix-like systems, devices can exist with no corresponding physical hardware (pseudo-devices)

• The kernel provides an abstract interface to the system hardware through device drivers which implement low-level interactions
• Device drivers are part of the linux kernel, and make up for more than half of its source code
• They allow hardware discovery and hardware addition/removal event detection among other things
• Once a device driver is loaded, the kernel sends hardware events to the userspace daemon udevd which maintains the virtual filesystem mounted at /dev
• Running programs can then access the hardware through system calls or by reading/writing to the device files

Note : many shell commands are in fact wrappers around system calls

• Linux always creates block devices for storage hardware (file systems can only be mounted if they are on a block device)
• It is important to understand the concept of blocks and sectors and how they relate to block devices :

1. sector size :

   • it is the minimal number of contiguous bytes that can be read/written on a physical storage disk

     sector size	media
     512 bytes	HDD
     2048 bytes	CD-ROM/DVD-ROM
     4096 bytes	SSD/newer HDD (AF - advanced format)

   • it depends on storage media manufacturers

   • each sector stores metadata (address, error correction information, flaw indicator ...) as well as user data

   • AF uses 4096 bytes sectors because of the increase in disks size (less sector metadata to store)

   • for backwards compatibility, AF disks emulate 512 bytes sectors to the operating system (logical vs physical sectors size)

   • if a file does not fill a whole sector, the remainder of the sector will be filled with zeroes

2. block size :

   • it is the minimal number of contiguous disk sectors that can be read/written when accessing a file system
   • it depends on file system options (for instance in ext4 it is a group of sectors between 1KiB and 64KiB)
   • block size cannot be smaller than the corresponding disk sector size and is often a multiple of it
   • if a file size is not a multiple of the file system block size, its last block will only be partially filled (fragmentation)

3. related shell commands :

# storage media sector size
sudo fdisk -l /dev/sda | grep "Sector size"

# block device block size
sudo blockdev --getbsz /dev/sda2

# print the number of blocks used by a file or a directory (recursive)
bash -c 'du -a -B "$(stat -f --format="%S" "$0")" "$0"' "$HOME/test.file" | sort -gr

# hex + ASCII dump blocks 65536 to 131072 from /dev/sda2 (4096 equals 0x1000)
# it's funny to scroll down and see your files contents appear on the file system blocks
sudo xxd -c 32 -seek 0x10000000 -l 0x10000000 /dev/sda2 | less

# you can also use the same command to see which blocks contain specific text values
# (remember that utf8 and utf16 are supersets of ASCII ...)
sudo xxd -c 32 -seek 0x10000000 -l 0x10000000 /dev/sda2 | grep confidential

Notes :

• linux always assume 512 bytes (logical) block size by default regardless of the filesystem block size
• for example, if the disk uses AF, any non empty file is reported by stat as using at least 8 blocks
• filesystems with a small block size use space more efficiently, but are slower