svn commit: r230516 - in head/sys: fs/nfsclient nfsclient

[Bruce Evans]

On Tue, 24 Jan 2012, Rick Macklem wrote:

> Bruce Evans wrote:
>> On Wed, 25 Jan 2012, Rick Macklem wrote:
>>
>>> Log:
>>>  If a mount -u is done to either NFS client that switches it
>>>  from TCP to UDP and the rsize/wsize/readdirsize is greater
>>>  than NFS_MAXDGRAMDATA, it is possible for a thread doing an
>>>  I/O RPC to get stuck repeatedly doing retries. This happens
>>>  ...

>> Could it wait for the old i/o to complete (and not start any new
>> i/o?). This is little different from having to wait when changing
>> from rw to ro. The latter is not easy, and at least the old nfs
>> client seems to not even dream of it. ffs has always called a
>> ...

> As you said above "not easy ... uses complicated suspension of i/o".
> I have not tried to code this, but I think it would be non-trivial.
> The code would need to block new I/O before RPCs are issued and wait
> for all in-progress I/Os to complete. At this time, the kernel RPC
> handles the in-progress RPCs and NFS doesn't "know" what is
> outstanding. Of course, code could be added to keep track of in-progress
> I/O RPCs, but that would have to be written, as well.

Hmm, this means that even when the i/o sizes are small, the mode switch
from tcp to udp may be unsafe since there may still be i/o's with higher
sizes outstanding.  So to switch from tcp to udp, the user should first
reduce the sizes, when wait a while before switching to udp.  And what
happens with retries after changing sizes up or down?  Does it retry
with the old sizes?

Bruce