[haiku-development] Re: Writing a filesystem driver
- From: "Axel Dörfler" <axeld@xxxxxxxxxxxxxxxx>
- To: haiku-development@xxxxxxxxxxxxx
- Date: Mon, 26 Oct 2009 14:12:56 +0100
Alexander Botero-Lowry <alex.boterolowry@xxxxxxxxx> wrote:
> > BFS in this way is time well spent - it would probably be a better
> > idea
> > to implement a new file system based on ideas found in more
> > recently
> > developed file systems (but, of course, still with support for
> > queries).
> What are the issues with BFS, and what new features are compelling
> enough to warrant a new file system?
BFS has several issues that can only be solved with breaking binary
compatibility. To solve all of them, I would think it's a better idea
to start a new file system (based on the BFS sources) than to try to
remain compatible.
To mention the ones I currently remember without further investigation:
* The space of files that were deleted, but still in use is lost after
a system crash (checkfs fixes that, so it's a good idea to run it from
time to time after crashes).
* There is a global block bitmap that could solved better using some
tree.
* The index is not written for user queries (looking up wildcards can
be pretty expensive).
* There is a single fixed size log which theoretically prevents some
operations from happening (those that doesn't fit into the log
anymore).
* Writing meta data is essentially single threaded (as there is only
one log).
* The double indirect block range is implemented in a stupid way which
makes it almost useless (it could cover a lot more space easily).
* No support for hard links.
* One inode covers a whole block, and it only embeds attribute data
into that block (could do the same with file data).
* Potentially scatters inodes over the whole disk (somewhat balanced by
its allocation policies) that aren't optimal for directory reading
speed.
* The B+trees fragment over time.
* No data correctness facilities (ie. data cannot be placed in the log,
and there are no checksums either).
* The indexes hurt performance as they are updated synchronously.
Nothing too bad, but still a lot of room for improvements.
Bye,
Axel.
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