[Libguestfs] Possible to speed up guestmount?

[Richard W.M. Jones]

On Thu, Feb 06, 2014 at 02:53:05AM +0000, Patrick Schleizer wrote:
> Hi,
> 
> Apparently,
> 
>     guestmount -o allow_other -a "/path/to/raw_file" -m /dev/sda1
> "/path/to/mountfolder"
> 
> is much slower than
> 
>     kpartx -av "/path/to/raw_file"
>     mount /dev/mapper/loop0p1 /path/to/mountfolder
> 
> (Doing lots of read/write inside the image.)

For general performance tips, see this page (I think you've seen it
already):

http://libguestfs.org/guestfs-performance.1.html

> I thought guestmount "only" scripts the above. Seems I was wrong on that.

Guestmount provides a FUSE interface to the libguestfs API.

> I am currently using libguestfs 1.18.1-1 (because it comes with Debian
> wheezy/stable) and read the FAQ [1] [2], but still have questions.
> 
> Seems my version is higher than 1.13.16, so far so good. I am using
> guestmount inside a virtual machine (to prevent damaging my hosts due to
> own stuff).

However yes the real problem here as you've diagnosed is that you're
using TCG (software emulation) instead of baremetal hardware
virtualization.

Likely there are two (or three) things you can do:

(1) Use the libguestfs API directly instead of FUSE.  (eg guestfish or
a language binding like Sys::Guestfs).  This cuts out all the FUSE
layers, and should be quite a lot faster.

(2) Use UML instead of qemu.  This requires you to update your version
of libguestfs to something more recent (1.24 ideally), and follow the
instructions here:

  http://libguestfs.org/guestfs.3.html#user-mode-linux-backend

UML might be "uncool", but the UML backend is fully tested for each
release and supported by us.  UML has the advantage that its
performance is reasonably consistent between baremetal and under
virtualization.

(3) Run libguestfs on baremetal (!)

I'd love to say that you could use nested virtualization to get
baremetal-like virt performance in a guest, but unfortunately it
doesn't work well -- see recent discussion on this list.

[...]

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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