Re: [nfsv4] review of "draft-ietf-nfsv4-layrec-00"

[Jeff Layton <[e-mail]>]

On Fri, 2024-03-01 at 09:17 -0500, David Noveck wrote:
> 
> 
> On Thursday, February 29, 2024, Jeff Layton <[e-mail]> wrote:
> > Most of this document looks fine to me, but it has this statement in
> > section 2:
> > 
> > "The metadata server MUST NOT have been resilvering the file such
> > that it has a different layout (set of mirror instances) than the
> > client before the restart of the metadata server."
> > 
> 
> I also had problems with this statement but my reaction is different.  See below.
>  
> > 
> > The rationale for this seems clear,
> > 
> 
> The rationale is clear, but not how to enforce this constraint.  Given the use of the perfect aspect here, it seems you'd need a time machine to go back to prevent a resilvering that started before the MDS restart or a way to absolutely prevent MDS restart, which seems impossible.
> 
> Perhaps something else is intended here?
>  
> > 
> > 
> >  but it seems like resilvering the
> > file while the clients hold any sort of RW layout would be a bad idea,
> > no?
> > 
> 
> Yes, but rfc2119 does not define "A BAD IDEA".
> 
> It appears resilvering is not valid unless it can be known that there are no outstanding RW layouts.
>  

That's my take on the matter too, but then again, we occasionally find
ways to safely do things like this.  Perhaps there is some way to
temporarily fence off the clients while you resilver so that it doesn't
involve recalling layouts? That's why I'd go for something more weasel-
wordy:

"Resilvering must only be done when there is no risk of corruption by
clients holding RW layouts."


> > 
> > In any case, adding another sentence or two explaining this requirement
> > (even if it seems obvious) wouldn't go amiss.
> > 
> 
> I'm not sure how much is needed but it needs to be clearer if this is really a requirement, and, if it is, how that requirement is to be enforced
>  
> 

Yeah, some more explanation here would certainly be a good thing.
-- 
Jeff Layton <[e-mail]>