[Libguestfs] IO rate of tar-in, what can we expect on a qcow2 image?

[Richard W.M. Jones]

On Tue, Feb 26, 2019 at 11:00:41AM +0100, Martin Blapp wrote:
> Hi all,
> 
> We have done several speed tests on a qcow2 Linux Image to test how fast tar-in with a big
> tarball can be. Virtio seems to be active, and we get transfers in a range from 100-160MB/sec,
> independent of the disk speed on the host. 
> 
> For example we had a 20 core host system with 900MB for serial writing and 350MB for mixed
> read/write on the native filesystem. We've expected a faster tar-in for qcow2 there than on a
> small test system with some slow disks and only two cores. But the rates where nearly the same.
> We tried really hard to do some optimizing and using big files inside the tar (for emulating serial
> writing), but we managed only to get those 160MB/sec, but not more. 
> 
> The IO-rate for serial writing inside a running VM-Image on the same qcow2 image is much more
> higher, dependent of course on the underlying disks of the host. Here we get clear benefits of fast
> storage.
> 
> Has anybody of you got more than those 160MB/sec with tar-in, and if so, how did you do it?
> Are there maybe some limitations in the XDR/RPC mechanisms of tar-in ?

I doubt that XDR (note: RPC is not used) is a problem.  There is a
special streaming mode used for tar-in and similar "upload" commands,
and apart from probably an extra copy or two there's very little
overhead.

Did you try changing the cache mode?

https://rwmj.wordpress.com/2013/09/02/new-in-libguestfs-allow-cache-mode-to-be-selected/

You could also try running virt-rescue to get direct access to an
appliance and run some tests from within that environment.  This gives
you an idea of how qemu+virtio is performing directly, without any
possible libguestfs overhead.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
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