Re: [nfsv4] persistent sessions and compound atomicity

["J. Bruce Fields" <[e-mail]>]

On Wed, Dec 09, 2020 at 04:43:59PM +0000, Rick Macklem wrote:
> I would say that the ATOMIC refers to the individual operations of the
> compound. In other words, it must cache the reply to each operation
> when it is completed, in the persistent cache.
> 
> Then, if a retry of the compound is received after a server reboot,
> the reply for the first K operations that were completed prior to the
> reboot are generated from the cache and the remaining N-K are
> performed on the file system to complete the reply. (At least for the
> non-idempotent operations in the compound.)
> 
> For this to all work correctly, I think the server file system would
> need to perform each non-idempotent operation ATOMICALLY as well, such
> that after the reboot the file system is in either the pre-operation
> or post-operation state.

Great, sounds like everyone agrees, I'll calm down.

> rick, who has no intention of ever trying to implement a persistent
> session cache.

I can't decide.  It looks like kind of a pain.  It'll require some
cooperation with the filesystem folks--I think it needs to be integrated
with filesystem journaling somehow.

But I'm not looking forward to someone running into this, and having to
say, "oh yeah, we've known about that bug for decades; just give me
another few months or a year, and I'll fix it right up for you...."

--b.