I tried your change but VirtualFull with multiple read devices is not
working. Restore is working but it was working even without your change.

I think I know why:
1. the code from reserve.c:637

 dcr = new_dcr(rctx.jcr, rctx.jcr->dcr, rctx.device->dev);

always changes the write device, never the read device. The function
new_dcr returns the rctx.jcr->dcr if it's not NULL. Your change of
removing the "rctx.jcr->dcr=" in front  has no effect when the dcr is
not NULL.
2. however, further down in reserve.c:716

  rctx.jcr->read_dcr = dcr;

this makes effectively the read_dcr the same with the write dcr. The old
read_dcr is lost.
3. why restoring from multiple devices works? Because after the first
pass, the read_dcr and the write dcr are the same, so in all the
subsequent passes, changing the write dcr will also change the read_dcr.
In restores the write dcr is not used so it all works.
4. why the virtual full backup works when there is only one read device?
Because the storage director selects first the read dcr (overwriting in
the same time the write dcr) but then it creates a brand new write dcr
in reserve.c:251. So the original write dcr is used for reading and a
new dcr is used for writing. This all happens before the backup starts
when the dcr pointers can still change (apparently).
5. why the virtual full backup doesn't work when there is more than one
device? Because the trick of transforming the write dcr into the
read_dcr doesn't work anymore first because the backup has started and
dcr pointers can't change anymore and second because the write dcr is
still needed.

nicolae


Kern Sibbald wrote:
> By the way, the new_dcr(), form me is OK as is.  It does indeed create a new 
> dcr, but if it is possible to reuse the existing one, it does rather than 
> mallocing a new one.  In any case, any change to new_dcr() is a bit more 
> disruptive than I would like to do just now.
>   
The name new_dcr is confusing (not anymore to me). The function returns
the dcr passed as argument unless it was null. It should be perhaps
called "renew_dcr". Maybe I'm spoiled by all these OOP languages where
new of something makes you a brand new object.



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