Can someone elplain Synchronous Writes?

[Chris]

I did a search but was unable to find a good explanation of what 
synchronous writes are in filesystems.  A disk can only write one block 
of data at a time so I fail to see what is synchronous and why it is 
slow.  Is synchronous used in this case to mean serialized file operations?

Also modern hard drives have write caches so doesn't that defeat 
filesystem's consistency efforts?

Lastly "soft updates" I think caches file system meta data so it updates 
multiple records at once.  Isn't this similar to a hard drives write 
cache and SCSI and now IDE's "native command queing" which reoders and 
combines read and writes?

Chris