Re: [nfsv4] file size and getattr

[Trond Myklebust <trondmy@hammerspace.com>]

On Wed, 2019-02-27 at 00:13 +0000, Rick Macklem wrote:
> Trond Myklebust wrote:
> [stuff snipped]
> > Please see the Errata ID 2751 
> > http://www.rfc-editor.org/errata/eid2751
> 
> I'll admit I hadn't seen this errata before. However, it seems to be
> specific to
> the File Layout. For the Flexible File Layout...
> 
> When I look in RFC-8435, I cannot find anything that states that a
> LayoutCommit
> is only required for case(s) where a Commit to the Storage Server is
> required.
> Sec. 2.1
>    Clearly states that a Commit to the Storage Server is required
> before the client
>    does a LayoutCommit when the write(s) were not done FILE_SYNC.
>    However, I do not see any indication that the LayoutCommit is not
> to be done
>    for the case where the write(s) are done FILE_SYNC.
> 
> FF_FLAGS_NO_LAYOUTCOMMIT can be used to indicate to a client that
> LayoutCommits are not required, but this does not be dependent on how
> the write(s) to the Storage Server were done.
> 
> The only way a Flexible File layout Metadata server can know what the
> current file size is (when a read/write layout is issued to a client)
> is to do a
> Getattr to the Storage Server.
> If a client is not required to do a LayoutCommit when the write(s) to
> the
> Storage Server are done FILE_SYNC, then the Metadata server must do
> Getattr RPCs to the Storage Server whenever it needs an up to date
> file size
> if a read/write layout is issued to a client.
> 
> This can result in a lot of overhead that can be avoided by requiring
> the
> LayoutCommit to be done by a client after writing to a Storage
> Server,
> irrespective of the need for a Commit to the Storage Server.
> As such, I would rather not have this errata applied to RFC-8435.
> 

Fair enough. I agree that the errata in question only applies to the
pNFS files layout, however you were talking about RFC5661 and whether
or not we were interpreting that correctly. Since RFC5661 only refers
to about the behaviour of the pNFS files layout, then I assumed that
was what you were referring to.

For flexfiles we may have a bug in the layoutcommit case. However note
that the counter argument to what you state above is that _if_ the
server requires a layoutcommit before it will acknowledge a file size
change, then pNFS is likely to underperform for applications such as
databases or VMs where each record is required to be written in stable
mode.
IOW: If all writes that need to be stable are also required to be
acknowledged with a layoutcommit (to the MDS), then your ability to
scale out your server will be in doubt.

-- 
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trond.myklebust@hammerspace.com