Filesystem

Summary

The IGEL filesystem is used within IGEL OS, in order to manage various operating system components in a self-contained, independent manner; each partition is responsible for a specific OS component, such as the root filesystem, bootsplash, or a specific userspace application.

Each component can be updated individually, by replacing the partition. Whether an update is available is determined by the update_hash attribute of the partition header.

Most partitions are read-only SquashFS images, which are mounted to a particular mountpoint at boot, with a few exceptions for licensing or encrypted user data.

Disk

Once installed, there are usually three partitions on UEFI systems, in the following format:

• Partition #1
  • IGEL FS
• Partition #2
  • FAT32, ESP #1
• Partition #3
  • FAT32, ESP #2

For OS 12, it appears the IGEL FS partition is #4, and partition #1 is small (~9 MiB) and filled with null bytes.

igelfs can operate directly on these device files, or on an image of these physical partitions.

Filesystem

The filesystem is structured into fixed-length sections (262144 bytes). The first section (section #0) is reserved for the boot registry, which contains persistent configuration data used during boot, and the directory, which is used to efficiently look-up the location of partitions within the filesystem.

Please see the following snippet from igelfs.__init__ for a description of the filesystem structure:

• Section #0
  • Boot Registry
    • Boot Registry Entries
  • Directory
    • Partition Descriptors
    • Fragment Descriptors
• Section #1, Partition Minor #1
  • Section Header
  • Partition Block
    • Partition Header
    • Partition Extents * PartitionHeader.n_extents
  • Hash Block, optional
    • Hash Header
    • Hash Excludes * HashHeader.count_excludes
    • Hash Values => HashHeader.hash_block_size
  • Partition Data
    • Extents
    • Payload
• Section #2, Partition Minor #1
  • Section Header
  • Partition Data
• Section #3, Partition Minor #2...

In short, all partitions are stored in sections as a linked list. Each section has a section header, which contains the partition minor (ID) and the next section for the partition until 0xffffffff. The first section of a partition also contains a partition header and optionally a hash header.

Further Reading

For more information about these data structures, see Models.