Re: [Libguestfs] [PATCH nbdkit filters-v2 2/5] Introduce filters.

> Right, but this patch is against the version 1 API. We will need to > add flags at some point. So, do we ever want filters to be usable via version 1 instead of version 2 backend API, or should we be aiming that all filters are automatically version 2 from the point where we release this?

>> Would it ever make sense to allow a filter to intentionally request a >> lower concurrency level than the underlying plugin? At connection time, >> you'd compute the threading level as the lowest level requested by any >> element of the chain; it might make it easier to write plugins that >> aren't fully parallel (such filters may penalize the speed that can >> otherwise be achieved by using the plugin in isolation - but again, it >> may be useful for testing). That would imply adding a >> backend.thread_model() callback, which needs to be exposed to filters >> but not to plugins (plugins continue to use the #define at compile >> time); if the filter does not define the .thread_model callback, it >> defaults to fully-parallel; but if it does define the callback, it can >> result in something less concurrent. > > Yes, this is a good idea actually. Are you planning on writing that, or do you want me to take a shot at it?

> I couldn't think of a case where we'd need the same filter multiple > times. It could be made to work but we'd have to ban global > variables and modify the load() function. I can (sort of) see it if we support composition; but again, if composition is done right, you still have just a single use of each plugin per nbdkit process. Let's say I want to access [0-100M) and [200M-300M) of a given file via composition. Here's a possible way to write it: nbdkit --filter=compose compose='--filter=offset offset=0 range=100M file file=FILE' --filter=offset offset=200M range=100M file file=FILE Is there a way to rewrite it so I don't have to nest quotes when doing more than one composition? If there is, then --filter=offset appears twice, as does 'file file=FILE'. But either way, under the hood, we'd implement it as two separate nbdkit processes if composition is a filter: nbdkit --> compose --> offset 0/100 --> file \-> nbd ==> nbdkit --> offset 200/100 --> file