DTV Flash File System

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Contents

[edit] Standard DTV ROM file system

For a description of the normal DTV ROM file system (read only, device 1, directory at $010000, files at $014000) see http://galaxy22.dyndns.org/dtv/common/flash-files/ and DTV2 Kernal disassembly.

  • DTVFSEdit (GUI) can open, edit, and write DTV flash file systems, handling compression transparently
  • Spiff's dtvmkfs can create this type of filesystem (command line)
  • Spiff's dirdump can extract files from an image of this filesystem (command line)

[edit] DTVFS2

This is a description of a to-be implemented DTV flash filesystem that can be modified more easily and allows implementation of SAVE on the DTV.

Interesting links:

[edit] Requirements

  • fault tolerance - the DTV2/3's AT47 flash supposedly only allows about 100 erase cycles
    • erase only if absolutely necessary (= flash full, garbage collection needed)
    • wear-levelling - make sure no block gets stressed out of proportion
    • if individual blocks fail (= can't get erased properly) this shouldn't make the whole flash unusable
  • low overhead
    • on-medium: 2MB of flash are not much to begin with
    • RAM requirements should be generally low and only occur on medium access

Not required:

  • Truncating/changing files (delete and create is enough)
  • open/read/write/close support (would require persistent handles in memory)

[edit] Filesystem outline

[edit] Data structures

  • One file occupies one or more nodes à 1kB. One block is 64 nodes. Only complete blocks can be erased (= set to FF).
    • 64 nodes are reserved in order to keep garbage collection simple.
  • Node structure
    • File number: 16 bits (ff ff: free node, 00 00: bad/broken flash area, f0 00: obsoleted node, other: file number)
    • Node sequence number: 16 bits (00 00: bad/broken flash area, 00 01: start of file + data, other: data)
      • start of file: null-terminated filename, null-terminated feature list (each feature is a stream of bytes - MSB unset indicates end of feature), 3b data len, data

[edit] Algorithms

  • Read file: Search through seq==1 nodes for filename, get file number, walk through nodes with this file number ordered by sequence number.
  • Write file
    • Any block containing only obsoleted nodes? Erase block.
    • If (number of free nodes) > (nodes needed + 64), save nodes to random free nodes. Save done.
    • If (number of free+obsoleted nodes) < (nodes needed + 64), fail (not enough free space).
    • else: grab block (G) containing many obsoleted nodes randomly OR (small chance) any other block.
    • Number of non-obsoleted nodes (N) in G <= free nodes: Write N to free nodes.
    • else: fail (user has to delete files first - should happen only if flash is partially bad already)
    • Mark all nodes as obsoleted in G, erase G, check erase successful; no: mark bad nodes in G.
    • Repeat (at max n times, fail otherwise) until enough free nodes available.
    • TODO: A more sensible approach dealing with bad nodes would be good.
  • Erase file: Mark all file's nodes as obsoleted.
  • Filesystem verify (in case of crash/power outage): 1. Check for incompletely written files, mark nodes as obsolete (crash during file write). 2. Check for duplicate nodes (same seq/file num). Continue copy, mark old node as obsolete. (crash during node copy)

This does not handle unstable erases due to power outage completely.

[edit] Implementation plan

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