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.)
Rich.
--
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
virt-top is 'top' for virtual machines. Tiny program with many
powerful monitoring features, net stats, disk stats, logging, etc.
http://people.redhat.com/~rjones/virt-top