The free-list structure, while efficient for allocations, had one big
issue: complexity. Storing free blocks as a simple fifo made sense
when dealing with a single file, but as soon as you have two files
open for writing, updating the free list atomicly when the two files
can not necessarily even be written atomicly proved problematic. It's a
solvable problem, but requires many writes to keep track of everything.
Now changing direction to pursue a more "drop it on the floor" strategy.
Since allocated blocks are tracked by the filesystem, we can simply
subtract from all available blocks the blocks we know of to allocate new
blocks. This is very expensive (O(blocks in use * blocks on device)),
but greatly simplifies any interactions that result in deallocated
blocks.
Additionally, it's impossible to corrupt the free list structure
during a power failure. Anything blocks that aren't tracked are simply
"dropped on the floor", and can be allocated later.
There's still a bit of work around the actually allocator to make it
run in a somewhat reasonable frame of time while still avoiding
dynamic allocations. Currently looking at a bit-vector of free
blocks so at least strides of blocks can be skipped in a single
filesystem iteration.
Missing seek, but these are the core filesystem operations
provided by this filesystem:
- Read a file
- Append to a file
Additional work is needed around freeing the previous file, so
right now it's limited to appending to existing files, a real
append only filesystem. Unfortunately the overhead of the free
list with multiple open files is becoming tricky.
This comes with a lot of scafolding put into place around the core
of the filesystem.
Added operations:
- append an entry to a directory
- find an entry in a directory
- iterate over entries in a directory
Some to do:
- Chaining multiple directory blocks
- Recursion on directory operations
The core algorithim that backs this filesystem's goal of fault
tolerance is the alternating of "metadata pairs". Backed by a
simple core function for reading and writing, makes heavy use
of c99 designated initializers for passing info about multiple
chunks in an erase block.
Really started working out how the internal structure of the driver
will be organized. There are a few hazy lines between the intended
data structures with the goal of code reuse, so the function boundaries
may end up a bit weird.
The primary data structure backing the little fs was planned
to be a little ctz based skip-list for O(logn) lookup and
O(1) append.
Was initially planning to start with a simple linked list of
index blocks, but was having trouble implementing the free-list
on top of the structure. Went ahead and adopted the skip-list
structure since it may have actually been easier.
