Re: [Libguestfs] [PATCH nbdkit v4 05/15] cow: Disable extents information in this filter.

On Wed, Mar 27, 2019 at 09:45:58PM -0500, Eric Blake wrote: > On 3/26/19 4:17 PM, Richard W.M. Jones wrote: >> The cow filter doesn't support trimming and we should assume the >> underlying plugin is fully allocated too. Note that both of these >> limitations might be lifted with a more advanced filter >> implementation. >> >> However we ought to support this in future because xz files contain >> sparseness information, so add a note. > > Stale copy-and-paste comment? Yes that was a copy-paste bug (or rather commit -c). I've fixed it. > Also, since the series is mostly alphabetical, I suspect you're missing > 4.5/15 for filters/cache/cache.c (blindly passing through .extents to > the underlying plugin); easiest is probably forcing .can_extents false, > but nicer would be remembering status of portions that we have cached > locally to avoid repeated callouts to the plugin. I examined the cache filter and I don't think it needs any changes for correctness. It could - as you note - be improved so it caches extents information, but I think if it does nothing then it's still correct.

Note that nbdkit will automatically pass the extents call through - we don't need to add an explicit extents callback in filters which don't want to modify the result. Also .can_extents is properly taken into account. (I think the man page should be clearer about all this.)