Files opened with O_DIRECT will now bypass the disk cache in read/write
operations (though metadata operations will still hit the disk cache.)
This will allow us to test actual disk performance instead of testing
disk *cache* performance, if that's what we want. :^)
There's room for improvment here, we're very aggressively flushing any
dirty cache entries for the specific block before reading/writing that
block. This is done by walking the entire cache, which may be slow.
We can't be calling the virtual FS::flush_writes() in order to flush
the disk cache from within the disk cache, since an FS subclass may
try to do cache stuff in its flush_writes() implementation.
Instead, separate out the implementation of DiskBackedFS's flushing
logic into a flush_writes_impl() and call that from the cache code.
This way clients are not required to have instantiated ByteBuffers
and can choose whatever memory scheme works best for them.
Also converted some of the Ext2FS code to use stack buffers instead.
Instead of having DiskBackedFS allocate a ByteBuffer, leave it to each
client to provide buffer space.
This is significantly faster in many cases where we can use a stack
buffer and avoid heap allocation entirely.
The hashmap cache was ridiculously slow and hurt us more than it helped
us. This patch replaces it with a flat memory cache that keeps up to
10'000 blocks in cache with a simple dirty bit.
The syncd task will wake up periodically and call flush_writes() on all
file systems, which now causes us to traverse the cache and write all
dirty blocks to disk.
There's a ton of room for improvement here, but this itself is already
drastically better when doing repeated GCC invocations.
This way you can spam small write()s on a file without the kernel writing
to disk every single time. Flushes are included in the FS::sync() operation
and will get triggered regularly by syncd. :^)
