RISC OS Filing System Update

This page describes plans for changes to update the RISC OS filing system stack.

Goals

• Support large files (64 bit offsets).
  
• Support foreign format discs (e.g. FAT32 USB drives) larger than 2GB.
  
• Support a modern disc format to use as a native filesystem, that will handle large discs more efficiently.
  
• Provide a standard method of partitioning discs.
  
• Provide support for inserting additional block driver layers to implement things like RAID.
  
• Implement file and directory cacheing.
  
• Provide an interface for storing arbitrary metadata with files on filing systems that support it.
  
• Add support for symlinks.
  
• Simplify interfaces (for new code) wherever possible.
  
• Retain backwards compatibility for userspace applications.
  
• Retain backwards compatibility for old filing systems.
  
• Provide a unified formatter which can write any supported formats to any hardware.
  
• Provide a standard way of creating filer iconbar icons.
  
• Some support for nonblocking/async IO?

Plan

The following is an initial plan on how the changes can be split into smaller steps that can be implemented and tested individually.

Some of the phases could be done in a different order, or in parallel.

Phase 1

• Design new userspace APIs
  
• Trivially implement them, mapping to the old ones.
  
• Provide a patch to do the same for older OSes.
  
• Ideally work with ROL so the same can be added to RO6.

Phase 2

• Design new interface between fileswitch and filesystems.
  
• Implement fileswitch changes (possibly as a separate module, so it can be in C).
  
• Non-disc filing systems (e.g. Sunfish) could then be ported to the new interface.

Phase 3

• Design new block driver interface (common to discs and image filing systems).
  
• Port some filing systems (e.g. Filecore, DOSFS) to the new interface, and test as an imagefs.

Phase 4

• Port or rewrite some hardware drivers to implement the new block interface.
  
• Can now test with a normal ADFS/Filecore hard disc using the new stack.

Phase 5

• Implement disc formatter.
  
• Implement disc partitioning.
  
• Implement file cacheing.

Phase 6

• Port a new filing system (e.g. ext3, ZFS) to RISC OS, using the new interfaces.