Re: qemu-img convert vs writing another copy tool

[Richard W.M. Jones]

On Thu, Jan 23, 2020 at 07:53:57PM +0100, Max Reitz wrote:
> On 23.01.20 19:35, Richard W.M. Jones wrote:
> >  - NBD multi-conn.  In my tests this makes a really massive
> >    performance difference in certain situations.  Again, virt-v2v has
> >    a lot of information that we cannot pass to qemu: we know, for
> >    example, exactly if the server supports the feature, how many
> >    threads are available, in some situations even have information
> >    about the network and backing disks that the data will travel over
> >    / be stored on.
> 
> As far as I understand it, you use qemu-img convert with an NBD source
> or target, too?

Virt-v2v has many modes, but yes generally there will be either an NBD
source & target, or an NBD source to a local file target.

> I suppose it’s always easier to let a specialized and freshly written
> tool handle such information.  But it sounds like if such information is
> useful and makes that big of a difference, then it would be good to be
> able to specify it to qemu’s NBD block driver, too.

qemu-img convert has worked really well for us, and I'm actually _not_
confident that I could do better with a specialized tool.  But there's
definitely more info we could pass, such as the amount of parallelism
we believe is available in the NBD server / processors / disks.

> >  - Machine-parsable progress bars.  You can, sort of, parse the
> >    progress bar from qemu-img convert, but it's not as easy as it
> >    could be.  In particular it would be nice if the format was treated
> >    as ABI, and if there was a way to have the tool write the progress
> >    bar info to a precreated file descriptor.
> 
> It doesn’t seem impossible to add this feature to qemu-img, although I
> wonder about the interface.  I suppose we could make it an alternative
> progress output mode (with some command-line flag), and then the
> information would be emitted to stdout (just like the existing progress
> report).  You can of course redirect stdout to whatever fd you’d like,
> so I don’t know whether qemu-img itself needs that specific capability.
> 
> OTOH, if you need this feature, why not just use qemu itself?  That is,
> a mirror or a backup block job in an otherwise empty VM.

I don't think we've really thought before about this approach.  Maybe
the launching of a VM (even an empty / stopped one) could be a
problem.  I guess this is what the new tool that was recently proposed
upstream might help with?  (Was it called qemu-block-storage?  I can't
find it right this minute)

> >  - External block lists.  This is a rather obscure requirement, but
> >    it's necessary in the case where we can get the allocated block map
> >    from another source (eg. pyvmomi) and then want to use that with an
> >    NBD source that does not support extents (eg. nbdkit-ssh-plugin /
> >    libssh / sftp).  [Having said that, it may be possible to implement
> >    this as an nbdkit filter, so maybe this is not a blocking feature.]
> 
> That too seems like a feature that’s easily implementable in a
> specialized tool, but hard to implement in qemu-img.
> 
> I suppose we’d want a dirty bitmap copy mode which copies only the
> regions that the bitmap reports as dirty – but at that point you’re
> probably again better off not using qemu-img, but qemu itself.  Then
> we’d need some way to import bitmaps, and I actually don’t think we have
> that yet.
> 
> But again, if this is a generally useful feature, I think we want it in
> qemu anyway.

I think this is actually one we can more easily implement as an nbdkit
filter.  I'm going to try this and see.

> > One thing which qemu-img convert can do which nbdcp could not:
> > 
> >  - Read or write from qcow2 files.
> > 
> > So instead of splitting the ecosystem and writing a new tool that
> > doesn't do as much as qemu-img convert, I wonder what qemu developers
> > think about the above missing features?  For example, are they in
> > scope for qemu-img convert?
> 
> What I think is that there may be features that we don’t want in
> qemu-img, because they are more appropriate for the mirror or backup
> block job.  For example, I don’t think we want to let qemu-img convert
> mess around with dirty bitmaps.
> 
> But apart from that, the features you propose all seem useful to have in
> qemu itself.  Maybe some of them are too hard to implement (specifically
> importing bitmaps from external sources), then it might be pragmatic to
> write a new tool where such features can be easily implemented because
> they don’t need to be integrated into an existing API.
> 
> As for performance, well, if qemu’s NBD driver is slow, then naively I’d
> think that’s a bug, isn’t it?  And if improving performance requires
> knobs, then that’s how it is.

Thanks,

Rich.

-- 
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