Improve disk performance

[dennis95]

Disk access is currently slow. There are a few changes that could improve performance significantly.

• [x] Disk caching: We should cache blocks that have been accessed on disk in memory. This will improve read access to blocks that have been accessed before. Since the ext2 driver is reading important data structures repeatedly, this will improve both read and write performance of ext2 filesystems.
• [x] Do not flush the ATA device cache on every write. Instead flush the ATA cache when unmounting a filesystem and provide a utility to manually flush caches. This will improve write performance.
• [x] Use DMA transfers instead of PIO. This should also improve performance.

[dennis95]

The ATA cache is now no longer flushed after every write. This has already resulted in a significant performance improvement.

[dennis95]

Blocks that are read or written on a ATA driver are now being cached in RAM.

[dennis95]

DMA transfers have now been implemented. We now have good disk performance.

Another performance improvement would be to perform writes to the block cache only and asynchronously write modified blocks to the disk. But that is something to be considered later.