Re: [Qemu-devel] Question about qemu firmware configuration (fw_cfg) device

[Gleb Natapov <gleb@redhat.com>]

On Mon, Jul 19, 2010 at 10:00:04AM +0100, Richard W.M. Jones wrote:
> On Mon, Jul 19, 2010 at 11:40:41AM +0300, Gleb Natapov wrote:
> > On Mon, Jul 19, 2010 at 09:34:11AM +0100, Richard W.M. Jones wrote:
> > > On Mon, Jul 19, 2010 at 10:55:33AM +0300, Gleb Natapov wrote:
> > > > Why not put then on cdrom or disk?
> > > 
> > > It simplifies device and mountpoint enumeration not to have a separate
> > > disk.  It would also mean we couldn't use standard Fedora paths, or
> > > we'd have to have bind-mount /bin etc on to the disk mount point,
> > > which again complicates things.
> > > 
> > Can't help you here, but if it's doable you can speedup your startup
> > time much more then by a second.
> 
> This isn't true.
> 
> The most we could save is 0.8 seconds [time taken to load the 100MB
> initrd by the kernel] less the time taken to probe and mount a CD ISO
But you do not need all 100MB of application, so with disk approach
you load things you need on demand.

> [0.2 seconds - measured using guestfish] less the time taken to load
> programs from this CD.  So the most we could save would be 0.6
> seconds, and in reality it'd be less than this if we actually loaded
> and ran any programs from the CD at all.
> 
> My patch saves 1 second, and all the programs are in RAM.
> 
And it will take 100M of a host ram.

> > Most users load initrd from a disk not by -initrd option.
> 
> It's unusual, but on my production webserver I use -kernel and -initrd
> options explicitly.  That's because I want all my VMs to share a
> single kernel.
> 
How often you restart them?

> virt-install is another program that uses explicit -initrd.
> 
Installation takes a lot of time. Saving 1 second there will not be
noticeable. And during lifetime of installed VM initrd will be loaded
from its disk.

--
			Gleb.