[Fedora-xen] Kickstart slowness with virt-install

[Daniel P. Berrange]

On Wed, Mar 07, 2007 at 02:20:11PM -0800, Hanks, Dan wrote:
> > On Wed, Mar 07, 2007 at 01:50:43PM -0800, Hanks, Dan wrote:
> > > I've been able to kickstart a number of VMs using the virt-install
> tool,
> > > and so far have had pretty good success. One aspect of these
> installs
> > > has me a bit concerned, though. In most cases, anytime the kickstart
> > > needs to do intensive disk activity (such as formatting partitions,
> or
> > > installing all the rpms) there are noticible hangs, which I'm
> guessing
> > > come from some kind of IO wait. The result is that a kickstart which
> > > should take < 10 minutes ends up taking a half-hour or so.
> > 
> > What kind of virtual disk image are you using for the guest ? A
> partition
> > or a file - if the latter is it sparse, or non-sparse. Basically
> sparse
> > files will be horribly slow because every time the host OS has to
> extend
> > the sparse file to allocate real blocks it needs to do a journal sync
> on
> > the host FS. This destroys performance of I/O from the guest until the
> > sparse file is fully-allocated.
> 
> I'm using files. I've been using a command-line such as this for the
> install:
> 
> virt-install -m "00:16:3e:00:00:01" -n hostname -r 500 --vcpus=2 -f
> /var/lib/xen/images/myhost.xen.img -s 4 --nographics -p -l
> ftp://host/fc6/distro/i386/os -x "ks=http://kshost/ks.cfg"
> 
> Adding --nonsparse into the args looks like it fixes the speed issues
> (albeit adds a bit of extra time right up front to allocate the entire
> disk image). I imagine using partitions instead of files would be faster
> altogether. I'll have to explore that option (files are just so nice to
> ship around when needed).

If pre-allocated, files should be within a few % of real partitions.

> What does the "slower" here refer to? Slower up-front time to create the
> image file? Yes, but the actual install will be faster since it wont
> have to keep allocating more space for the disk image. Thoughts? 

Yes, upfront time will be longer due to the need to pre-allocate the
disk, this should be more than offset by the faster install time. So
I'd use --nonsparse as a general rule.

Dan.
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