Hi
As my PhD thesis, I am designing and writing a filesystem, and it's now in
a state that it can be released. You can download it from
http://artax.karlin.mff.cuni.cz/~mikulas/spadfs/
It has some new features, such as keeping inode information directly in
directory (until you create hardlink) so that ls -la doesn't seek much,
new method to keep data consistent in case of crashes (instead of
journaling), free space is organized in lists of free runs and converted
to bitmap only in case of extreme fragmentation.
It is not very widely tested, so if you want, test it.
I have these questions:
* There is a rw semaphore that is locked for read for nearly all
operations and locked for write only rarely. However locking for read
causes cache line pingpong on SMP systems. Do you have an idea how to make
it better?
It could be improved by making a semaphore for each CPU and locking for
read only the CPU's semaphore and for write all semaphores. Or is there a
better method?
* This leads to another observation --- on i386 locking a semaphore is 2
instructions, on x86_64 it is a call to two nested functions. Has it some
reason or was it just implementator's laziness? Given the fact that locked
instruction takes 16 ticks on Opteron (and can overlap about 2 ticks with
other instructions), it would make sense to have optimized semaphores too.
* How to implement ordered-data consistency? That would mean that on
internal sync event, I'd have to flush all pages of a files that were
extended. I could scan all dirty inodes and find pages to flush --- what
kernel function would you recommend for doing it? Currently I call only
sync_blockdev which doesn't touch buffers attached to pages.
Mikulas
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