For the pNFS case, when I implemented a pNFS server for FreeBSD, I
that the Metadata server only needed to return a correct size for a file
to a client doing a Getattr with a read/write layout after it did a LayoutCommit.
This implementation worked ok for the FreeBSD client, but did not work
correctly for the Linux-4.17-rc2 kernel.
To fix the server implementation to interoperate with the above Linux kernel,
I had to add a tunable that makes the server get an up to date size for the
Getattr operation for a client when a read/write layout is issued to the client.
Just to be clear, I am referring to the case where the Getattr was done by the
client that holds the read/write layout and not another client.
(This does result in additional overheads whenever a client holds a read/write
layout for the file.)
I can't remember exactly how the Linux client failed, but I suspect it would
see a premature EOF when the size returned by the MDS was smaller than
the actual size of the file on the DS.
I honestly don't know if this is a bug in the Linux client or a misinterpretation
A little more info on this.
If I recall correctly, the Linux client only did LayoutCommits after Commits.
As such, no LayoutCommit were done after writes to a DS that were FILE_SYNC
(or stable, if you prefer). Without the LayoutCommit, the pNFS server didn't
know when it needed to get an up to date size.
The FreeBSD client does a LayoutCommit after writing to a DS whenever it is done
writing, such as an fsync() or close() syscall on the file or an unlock of a write lock
on the file.