We've discovered that delivering a CB_OFFLOAD operation can be
unreliable in some pretty unremarkable situations. Examples
include:
- The server dropped the connection because it lost a forechannel
NFSv4 request and wishes to force the client to retransmit
- The GSS sequence number window under-flowed
- A network partition occurred
When that happens, all pending callback operations, including
CB_OFFLOAD, are lost. NFSD does not retransmit them.
Moreover, the Linux NFS client does not yet support sending an
OFFLOAD_STATUS operation to probe whether an asynchronous COPY
operation has finished. Thus, on Linux NFS clients, when a
CB_OFFLOAD is lost, asynchronous COPY can hang until manually
interrupted.
I've tried a couple of remedies, but so far the side-effects are
worse than the disease and they have had to be reverted. So
temporarily force COPY operations to be synchronous so that the use
of CB_OFFLOAD is avoided entirely. This is a fix that can easily be
backported to LTS kernels. I am working on client patches that
introduce an implementation of OFFLOAD_STATUS.
Note that NFSD arbitrarily limits the size of a copy_file_range
to 4MB to avoid indefinitely blocking an nfsd thread. A short
COPY result is returned in that case, and the client can present
a fresh COPY request for the remainder.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
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