Bug 155929

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Description of problem:
Certain workloads that remove lots of files run slowly on Linux clients using
NFSv4 and with delegation enabled on the server.  By examining a network trace,
we saw that the Linux NFSv4 client does not send a DELEG_RETURN before deleting
a file when there is an outstanding delegation on that file.  Instead, the
server delays the remove operation, does a call back, and the client then does
the DELEG_RETURN.  The client redrives the REMOVE request a second later.

Customers using NFSv4 against an NFSv4 server with delegation enabled will see
terrible performance on workloads that remove lots of files after reading or
writing to them.

Version-Release number of selected component (if applicable):


How reproducible:
Always

Steps to Reproduce:
1. Enable delegation on an NFSv4 server
2. Mount one of the server's exports from Linux client using NFSv4
3. Try a kernel build on that share
  

Actual Results:  Every on-the-wire remove operation results in an NFS4ERR_DELAY and a server call
back.  The client takes a second to recover and redrive the remove operation.

Expected Results:  The client should do a DELEG_RETURN before removing a file that has been
granted a delegation.

Additional info:

There is a policy question about just when the client should return a
delegation -- just after closing the file, or should it wait as long as
possible (until the file's cached pages are reclaimed or the file
itself is deleted by the client).

In addition we don't want to add a second round trip by doing the
DELEG_RETURN in a separate compound, so some cleverness will be needed to
make nfs4_proc_remove check if it needs to do the return and build the
DELEG_RETURN op into the same compound as the REMOVE.

Trond knows about this issue and is working on a solution for 2.6 kernels.