Fedora 15, new and exciting plans

Richard W.M. Jones rjones at redhat.com
Sun Nov 14 10:15:20 UTC 2010

On Sun, Nov 14, 2010 at 01:14:18AM +0100, Lennart Poettering wrote:
> LVM actually slows down boot considerably. Not primarily because its
> code was slow or anything, but simply because it isn't really written in
> the way that things are expected to work these days. The LVM assembly at
> boot is expected to be run at a time where all disks have been found by
> the kernel and identified. However, the idea that such a time exists is
> out-of-date on modern systems. There is simply no point in time where
> all disks have been enumerated, because they can always come and go and
> on many busses (for example USB), you never know whether you have
> enumerated all devices, because the bus doesn't support a notion like
> that. The right way how to implement a logic like this is to wait
> exactly until all disks actually *needed* have shown up and at that time
> assemble LVM. Currently, to make LVM work, we however try to wait until
> *everything* thinkable is enumerated, not only the disks that are
> actually needed. The fact that on many busses this point in time doesn't
> really exist is ignored, and awful hacks such as "modprobe
> scsi_wait_scan" are used to work around this out-of-date design on the
> other busses. To get to a fast system however, you should minimize the
> time you waste and continue withthe next step of booting the moment you
> have collected all devices you need for assembly.

Thanks for explaining what the "assembly" issue is all about.

I'd really like to hear from an LVM expert or two about this, because
I can't believe that it's impossible to make this work better for the
common single-disk-is-boot-disk single-PV case.  The LVM metadata
(which I've written code to read and decode in the past) contains the
information needed.


Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
virt-p2v converts physical machines to virtual machines.  Boot with a
live CD or over the network (PXE) and turn machines into Xen guests.

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