Libguestfs - Mar 2019 - Re: [PATCH nbdkit v5 FINAL 00/19] Implement extents.

On Thu, Mar 28, 2019 at 09:15:17PM -0500, Eric Blake
wrote:
> On 3/28/19 11:18 AM, Richard W.M. Jones wrote:
> > This has already been pushed upstream.  I am simply posting these here
> > so we have a reference in the mailing list in case we find bugs later
> > (as I'm sure we will - it's a complex patch series).
> > 
> > Great thanks to Eric Blake for tireless review on this one.  It also
> > seems to have identified a few minor bugs in qemu along the way.
> > 
> 
> Should nbdkit-rate-filter.pod mention that extents is considered
> effectively free (same as zeroing and trimming)?
I guess it's covered by the existing docs.  Extents aren't quite as
"free" as zeroing and trimming but the structured reply message is
still quite small.
> I know that I want the nbd plugin to do extents passthrough, but I have
> some work to do to teach it do to NBD_OPT_GO/structured replies first.
> 
> The full and null plugins should probably have a trivial .extents that
> reports completely sparse, all of the time :)
I think it would be wrong for full wouldn't it?  It could mean that
the client would skip the read, thus not getting the intended ENOSPC
error.  For null I agree.
> Various plugins like partitioning and linuxdisk might be interesting to
> support extents on (particularly since we know we pad in between files);
> but it may be trickier to write.
> 
> Can curl or ssh access extents information?
Not sure about curl, but I checked ssh (sftp) and it cannot.  However
at some point I intend to submit an extension to the sftp protocol
(similar to my "fsync@openssh.com" extension) to support it.  We will
need it for efficient ssh access in virt-v2v one day.
> Language bindings will also be an interesting exercise ;)
Yes, the language bindings lag behind what's available because they
aren't generated ...

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
virt-p2v converts physical machines to virtual machines.  Boot with a
live CD or over the network (PXE) and turn machines into KVM guests.
http://libguestfs.org/virt-v2v

On 3/29/19 3:52 AM, Richard W.M. Jones wrote:
>> The full and null plugins should probably have a trivial .extents that
>> reports completely sparse, all of the time :)
> 
> I think it would be wrong for full wouldn't it?  It could mean that
> the client would skip the read, thus not getting the intended ENOSPC
> error.  For null I agree.
> 
The full plugin always reads as 0 successfully. It is only writes that
differ (successfully ignored vs ENOSPC) between the two plugins.

>> Language bindings will also be an interesting exercise ;)
> 
> Yes, the language bindings lag behind what's available because they
> aren't generated ...
I still need to revive my patches for adding fua support to python
bindings...

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

On Fri, Mar 29, 2019 at 07:13:18AM -0500, Eric Blake
wrote:
> On 3/29/19 3:52 AM, Richard W.M. Jones wrote:
> 
> >> The full and null plugins should probably have a trivial .extents
that
> >> reports completely sparse, all of the time :)
> > 
> > I think it would be wrong for full wouldn't it?  It could mean
that
> > the client would skip the read, thus not getting the intended ENOSPC
> > error.  For null I agree.
> > 
> 
> The full plugin always reads as 0 successfully. It is only writes that
> differ (successfully ignored vs ENOSPC) between the two plugins.
Ah yes, my mistake.  In that case yes we can return extents.
I'll add the simple patch.

I already did this for the null plugin:

https://github.com/libguestfs/nbdkit/commit/8b5adddbf06a6ba785e7d0409910281922cbea42

So the full plugin would be the same.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
virt-builder quickly builds VMs from scratch
http://libguestfs.org/virt-builder.1.html