In writing the initial allocator, I ran into the rather
difficult problem of trying to iterate through the entire
filesystem cheaply and with only constant memory consumption
(which prohibits recursive functions).
The solution was to simply thread all directory blocks onto a
massive linked-list that spans the entire filesystem.
With the linked-list it was easy to create a traverse function
for all blocks in use on the filesystem (which has potential
for other utility), and add the rudimentary block allocator
using a bit-vector.
While the linked-list may add complexity (especially where
needing to maintain atomic operations), the linked-list helps
simplify what is currently the most expensive operation in
the filesystem, with no cost to space (the linked-list can
reuse the pointers used for chained directory blocks).
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.
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.
