Re: [PATCH] nfs: add 'noextend' option for lock-less 'lost writes' prevention

[Christoph Hellwig <hch@infradead.org>]

On Wed, Jun 19, 2024 at 02:15:16PM +0000, Trond Myklebust wrote:
> BTW: We've been asked about the same issue from some of our customers,
> and are planning on solving the problem by adding a new per-file
> attribute to the NFSv4.2 protocol.
> 
> The detection of that NOCACHE attribute would cause the client to
> automatically choose O_DIRECT on file open, overriding the default
> buffered I/O model. So this would allow the user or sysadmin to specify
> at file creation time that this file will be used for purposes that are
> incompatible with caching.

Can we please come up with coherent semantics for this on the fsdevel
list?  We've had quite a few requests for something similar for local
file systems as well.  The important fine points are things like keeping
the cache coherent if there already is one and only bypassing it
for new data (or alternatively writing it back / invalidating it
beforehand), and the lack of alignment requirements that O_DIRECT
usually has (although that doesn't apply to NFS).