Block filesystems

  • Storage devices are classified in two main types: block devices and flash devices
    • They are handled by different subsystems and different filesystems
  • Block devices can be read and written to on a per-block basis, in random order, without erasing.
    • Hard disks, floppy disks, RAM disks
    • USB keys, SSD, Compact Flash, SD card, eMMC: these are based on flash storage, but have an integrated controller that emulates a block device, managing the flash in a transparent way.
  • Raw flash devices are driven by a controller on the SoC. They can be read, but writing requires erasing, and often occurs on a larger size than the “block” size.
    • NOR flash, NAND flash

Block device list

  • The list of all block devices available in the system can be found in /proc/partitions $ cat /proc/partitions major minor #blocks name
  • And also in /sys/block/


  • Block devices can be partitioned to store different parts of a system
  • The partition table is stored inside the device itself, and is read and analyzed automatically by the Linux kernel
    • mmcblk0 is the entire device
    • mmcblk0p2 is the second partition of mmcblk0
  • Two partition table formats:
    • MBR, the legacy format
    • GPT, the new format, not yet used everywhere, but becoming more and more common
  • Numerous tools to create and modify the partitions on a block device: fdisk, gdisk, cfdisk, sfdisk, parted, etc.

Transfering data to a block device

  • It is often necessary to transfer data to or from a block device in a raw way
    • Especially to write a filesystem image to a block device
  • This directly writes to the block device itself, bypassing any filesystem layer.
  • The block devices in /dev/ allow such raw access
  • dd is the tool of choice for such transfers:
  • dd if=/dev/mmcblk0p1 of=testfile bs=1M count=16 Transfers 16 blocks of 1 MB from /dev/mmcblk0p1 to testfile
  • dd if=testfile of=/dev/sda2 bs=1M seek=4 Transfers the complete contents of testfile to /dev/sda2, by blocks of 1 MB, but starting at offset 4 MB in /dev/sda2

Available filesystems

Standard Linux filesystem format: ext2, ext3, ext4

  • The standard filesystem used on Linux systems is the series of ext{2,3,4} filesystems
    • ext2
    • ext3, brought journaling compared to ext2
    • ext4, mainly brought performance improvements and support for even larger filesystems
  • ext4 is now the default filesystem used on most Linux distributions
  • It supports all features Linux needs from a filesystem: permissions, ownership, device files, symbolic links, etc.

Journaled filesystems

Designed to stay in a coherent state even after system crashes or a sudden poweroff

  • Writes are first described in the journal before being committed to files (can be all writes, or only metadata writes depending on the configuration)
  • Allows to skip a full disk check at boot time after an unclean shutdown

Filesystem recovery after crashes

  • Thanks to the journal, the recovery at boot time is quick, since the operations in progress at the moment of the unclean shutdown are clearly identified
  • Does not mean that the latest writes made it to the storage: this depends on syncing the changes to the filesystem.

Other Linux/Unix filesystems

  • btrfs, intended to become the next standard filesystem for Linux. Integrates numerous features: data checksuming, integrated volume management, snapshots, etc.
  • XFS, high-performance filesystem inherited from SGI IRIX, still actively developed.
  • JFS, inherited from IBM AIX. No longer actively developed, provided mainly for compatibility.
  • reiserFS, used to be a popular filesystem, but its latest version Reiser4 was never merged upstream. All those filesystems provide the necessary functionalities for Linux systems: symbolic links, permissions, ownership, device files, etc.

F2FS: filesystem for flash-based storage

  • Filesystem that takes into account the characteristics of flash-based storage: eMMC, SD cards, SSD, etc.
  • Developed and contributed by Samsung
  • Available in the mainline Linux kernel
  • For optimal results, need a number of details about the storage internal behavior which may not easy to get
  • Benchmarks: best performer on flash devices most of the time: See
  • Technical details:
  • Not as widely used as ext3,4, even on flash-based storage.

Squashfs: read-only filesystem

  • Read-only, compressed filesystem for block devices. Fine for parts of a filesystem which can be read-only (kernel, binaries...)
  • Great compression rate, which generally brings improved read performance
  • Used in most live CDs and live USB distributions
  • Supports several compression algorithm (LZO, XZ, etc.)
  • Benchmarks: roughly 3 times smaller than ext3, and 2-4 times faster (
  • Details:

Compatibility filesystems

Linux also supports several other filesystem formats, mainly to be interopable with other operating systems:

  • vfat for compatibility with the FAT filesystem used in the Windows world and on numerous removable devices
    • This filesystem does not support features like permissions, ownership, symbolic links, etc. Cannot be used for a Linux root filesystem.
  • ntfs for compatibility with the NTFS filesystem used on Windows
  • hfs for compatibility with the HFS filesystem used on Mac OS
  • iso9660, the filesystem format used on CD-ROMs, obviously a read-only filesystem

tmpfs: filesystem in RAM

  • Not a block filesystem of course!
  • Perfect to store temporary data in RAM: system log files, connection data, temporary files...
  • More space-efficient than ramdisks: files are directly in the file cache, grows and shrinks to accommodate stored files
  • How to use: choose a name to distinguish the various tmpfs instances you could have. Examples: mount -t tmpfs varrun /var/run mount -t tmpfs udev /dev
  • See Documentation/filesystems/tmpfs.txt in kernel sources.

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