Re: [Libguestfs] [nbdkit PATCH v3 15/16] multi-conn: New filter

On Fri, Mar 05, 2021 at 05:31:19PM -0600, Eric Blake wrote: > Implement a TODO item of emulating multi-connection consistency via > multiple plugin flush calls to allow a client to assume that a flush > on a single connection is good enough. This also gives us some > fine-tuning over whether to advertise the bit, including some setups > that are unsafe but may be useful in timing tests. > > Testing is interesting: I used the sh plugin to implement a server > that intentionally keeps a per-connection cache. > > Note that this filter assumes that multiple connections will still > share the same data (other than caching effects); effects are not > guaranteed when trying to mix it with more exotic plugins like info > that violate that premise. > ---

> +++ b/filters/cache/nbdkit-cache-filter.pod > @@ -41,7 +41,9 @@ an explicit flush is done by the client. > > This is the default caching mode, and is safe if your client issues > flush requests correctly (which is true for modern Linux and other > -well-written NBD clients). > +well-written NBD clients). Note that this mode is able to advertise > +multi-connection consistency even without the use of > +L<nbdkit-multi-conn-filter(1)>. I would say this hunk is confusing and unnecessary. I mean if you shouldn't be using --filter=multi-conn, then just don't say anything :-) If it really must be mentioned somewhere then probably better to say it in the multi-conn man page instead IMHO.

> +++ b/filters/fua/nbdkit-fua-filter.pod > @@ -16,6 +16,12 @@ This filter can be used to disable FUA and flush requests for speed > server fallbacks, and for evaluating timing differences between proper > use of FUA compared to a full flush. > > +Note that by default, the NBD protocol does not guarantee that the use > +of FUA from one connection will be visible from another connection > +unless the server advertised NBD_FLAG_MULTI_CONN. You may wish to > +combine this filter with L<nbdkit-multi-conn-filter(1)> if you plan on > +making multiple connections to the plugin. This is a case where mentioning the other filter _is_ a good idea.

> +++ b/filters/multi-conn/nbdkit-multi-conn-filter.pod [...] About the rest of the patch, ACK. It has to be said I did find the documentation a bit hard to follow. If you push this I may follow up with my own documentation fixes (I'll run it by you first though).