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

[Richard W.M. Jones]

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.)

Rich.

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