[Libguestfs] [Qemu-block] v2v: -o rhv-upload: Long time spent zeroing the disk

[Nir Soffer]

On Tue, Apr 10, 2018 at 6:53 PM Richard W.M. Jones <rjones at redhat.com>
wrote:

> On Tue, Apr 10, 2018 at 03:25:47PM +0000, Nir Soffer wrote:
> > On Tue, Apr 10, 2018 at 5:50 PM Richard W.M. Jones <rjones at redhat.com>
> > wrote:
> >
> > > On Tue, Apr 10, 2018 at 02:07:33PM +0000, Nir Soffer wrote:
> > > > This makes sense if the device is backed by a block device on oVirt
> side,
> > > > and the NBD support efficient zeroing. But in this case the device is
> > > backed
> > > > by an empty sparse file on NFS, and oVirt does not support yet
> efficient
> > > > zeroing, we just write zeros manually.
> > > >
> > > > I think should be handled on virt-v2v plugin side. When zeroing a
> file
> > > raw
> > > > image,
> > > > you can ignore zero requests after the highest write offset, since
> the
> > > > plugin
> > > > created a new image, and we know that the image is empty.
> > > >
> > > > When the destination is a block device we cannot avoid zeroing since
> a
> > > block
> > > > device may contain junk data (we usually get dirty empty images from
> our
> > > > local
> > > > xtremio server).
> > >
> > > (Off topic for qemu-block but ...)  We don't have enough information
> > > at our end to know about any of this.
> > >
> >
> > Can't use use this logic in the oVirt plugin?
> >
> > file based storage -> skip initial zeroing
> > block based storage -> use initial zeroing
> >
> > Do you think that publishing disk capabilities in the sdk will solve
> this?
>
> The plugin would have to do some complicated gymnastics.  It would
> have to keep track of which areas of the disk have been written and
> ignore NBD_CMD_WRITE_ZEROES for other areas, except if block-based
> storage is being used.  And so yes we'd also need the imageio API to
> publish that information to the plugin.
>
> So it's possible but not trivial.
>

I think this should be fixed in qemu-img. The current zero optimization may
have insignificant improvement when the backend supports fast zeroing
(e.g fallocate, blkzeroout) by minimizing number of syscalls, but it has
severe
performance issue when the desntiation does not support fast zero
(e.g. NFS < 4.2).

We plan to improve zero performance in 4.2.z, but this will not solve the
issue
on NFS < 4.2.

By the way I think we're slowly reimplementing NBD in the imageio API.
>

You can also look at it as - slowly showing that HTTP can replace
NBD :-)


> Dan Berrange pointed out earlier on that it might be easier if imageio
> just exposed NBD, or if we found a way to tunnel NBD requests over web
> sockets (in the format case nbdkit would not be needed, in the latter
> case nbdkit could act as a bridge).
>

I'm not the more complex web sockets are needed for moving images
around. web sockets are good when the clients is a browser, and you need
to pass lot of small messages. If you have a real socket you don't need
web sockets. And the overhead of http headers is insignificant when you
move lot of data around.

Exposing NBD using the same authentication mechanism can be interesting,
but only if clients have an easy way to use this.

Will this allow client (e.g. backup vender) to download and upload images
using qemu-img?

    qemu-img <- nbd-> ovirt-imageio <- ndb -> qemu/ndb-server

We also need streaming support - with http you can stream the data from
anywhere, while qemu-img needs a file. I don't think that backup vendors
or users will be happy to implement nbdkit plugins.

If qemu-img would support input and ouptut from stdin/stdout instead of
files, this model sounds much more interesting.

Nir
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