[Libguestfs] [v2v PATCH 2/2] Consolidate handling of temporary files/dirs

[Richard W.M. Jones]

On Mon, Apr 06, 2020 at 10:48:35AM -0500, Eric Blake wrote:
> Side note:
...
> >+    (* Although glance can slurp in a stream from stdin, unfortunately
> >+     * 'qemu-img convert' cannot write to a stream (although I guess
> >+     * it could be implemented at least for raw).  Therefore we have
> >+     * to write to a temporary file.  XXX
> >+     *)
> >+    List.map (fun (_, ov) -> TargetFile (cachedir // ov.ov_sd)) overlays
> 
> I know this was just code motion, but does the nbdkit streaming
> plugin help here?

As Pino said in the other answer, -o glance is obsolete.

In more direct answer to your question, the streaming plugin doesn't
help.  There are two problems:

(1) It only works if ‘qemu-img convert’ guarantees to write from start
to end without ever going backwards.  I don't think this is true even
for raw.

(2) Because Glance’s sparseness support is broken, it's generally
recommended that you use virt-v2v to convert the format to qcow2 when
writing to Glance.  (Virt-v2v gives you a "free" format conversion
because under the hood we use ‘qemu-img convert’ to do the copy.)
However qcow2 certainly cannot be written linearly unless Kevin has
performed a miracle.

Rich.

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