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

[Sagi Grimberg <sagi@grimberg.me>]

On 18/06/2024 21:59, Trond Myklebust wrote:
> Hi Dan,
>
> On Tue, 2024-06-18 at 18:33 +0300, Dan Aloni wrote:
>> There are some applications that write to predefined non-overlapping
>> file offsets from multiple clients and therefore don't need to rely
>> on
>> file locking. However, NFS file system behavior of extending writes
>> to
>> to deal with write fragmentation, causes those clients to corrupt
>> each
>> other's data.
>>
>> To help these applications, this change adds the `noextend` parameter
>> to
>> the mount options, and handles this case in `nfs_can_extend_write`.
>>
>> Clients can additionally add the 'noac' option to ensure page cache
>> flush on read for modified files.
> I'm not overly enamoured of the name "noextend". To me that sounds like
> it might have something to do with preventing appends. Can we find
> something that is a bit more descriptive?

nopbw (No page boundary writes) ?

>
> That said, and given your last comment about reads. Wouldn't it be
> better to have the application use O_DIRECT for these workloads?
> Turning off attribute caching is both racy and an inefficient way to
> manage page cache consistency. It forces the client to bombard the
> server with GETATTR requests in order to check that the page cache is
> in synch, whereas your description of the workload appears to suggest
> that the correct assumption should be that it is not in synch.
>
> IOW: I'm asking if the better solution might not be to rather implement
> something akin to Solaris' "forcedirectio"?

This access pattern represents a common case in HPC where different workers
write records to a shared output file which do not necessarily align to 
a page boundary.

This is not everything that the app is doing nor the only file it is 
accessing, so IMO forcing
directio universally is may penalize the application.