Re: [PATCH v2 1/4] block: nbd: Fix convert qcow2 compressed to nbd

[Eric Blake]

On 7/28/20 8:15 AM, Vladimir Sementsov-Ogievskiy wrote:
> 28.07.2020 00:58, Nir Soffer wrote:
> > When converting to qcow2 compressed format, the last step is a special
> > zero length compressed write, ending in call to bdrv_co_truncate(). This
> > call always fails for the nbd driver since it does not implement
> > bdrv_co_truncate().

> > +static int coroutine_fn nbd_co_truncate(BlockDriverState *bs, int64_t 
> > offset,
> > +                                        bool exact, PreallocMode 
> > prealloc,
> > +                                        BdrvRequestFlags flags, Error 
> > **errp)
> > +{
> > +    BDRVNBDState *s = bs->opaque;
> > +
> > +    if (offset != s->info.size && exact) {
> > +        error_setg(errp, "Cannot resize NBD nodes");
> > +        return -ENOTSUP;
> > +    }
> > +
> > +    if (offset > s->info.size) {
> > +        error_setg(errp, "Cannot grow NBD nodes");
> > +        return -EINVAL;
> > +    }
>
> I think that ENOTSUP actually is valid error code for both cases.. NBD 
> protocol has experimental extension NBD_CMD_RESIZE, so one day we'll 
> implement this. So, I think, it's not invalid, but just not supported 
> yet. Still, not a big deal, so with ENOTSUP or EINVAL:
> Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

Indeed, if we ever get around to fully specifying how NBD_CMD_RESIZE 
should even work, we'll be revisiting this code to implement that.

> Also, may be better to manage it in generic layer:
> If driver doesn't implement bdrv_co_truncate handler (or return 
> ENOTSUP), and we are truncating to the same size, or to smaller size 
> with exact=false, we just report success? Then we'll drop same code from 
> file-posix.c

That was also my question on v1; but given the closeness to the release, 
this is a minimal change appropriate for -rc2, while changing the 
generic layer may have unintended consequences.

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org