"Yes" - Writes to the server are done asynchronously.
When your NFS client sends a write request to an NFS
server, the server returns a positive response to your client
before writing any data to its disk. Your client can then send
another write request without waiting for the server to
complete the previous write.
Write performance is better with asynchronous writes than
with synchronous writes. However, if the server crashes
after sending the positive response to the client but before
writing the data to disk, the data is lost, and the client is not
notified.
"No" - Writes to the server are done synchronously. When
your NFS client sends a write request to an NFS server, the
server writes the data to its disk before returning a positive
response to your client. Your client must wait until the server
writes the data to disk before it can send another write
request.
Write performance is not as good with synchronous writes
as with asynchronous writes.
The default is "No".