Fedora 15, new and exciting plans

[Lennart Poettering]

On Sat, 13.11.10 23:41, Richard W.M. Jones (rjones at redhat.com) wrote:

> 
> On Fri, Nov 12, 2010 at 06:26:48PM +0000, "Jóhann B. Guðmundsson" wrote:
> > *DE could consider switching the default to use EXT4 directly without 
> > LVM. [1]
> > 1. http://fedoraproject.org/wiki/Features/NoDefaultLVM

> Anyway, I think LVM is jolly useful:
> 
> - You can expand the root filesystem (eg. into spare space or
> across block devices).
> 
> - You can live pvmove filesystems from one device to another.
> 
> It may be that the tooling is not there to make these features
> available for non-experts, but that's a problem with lack of tools,
> not with LVM.  Partition tables are horrible and inflexible in
> comparison to LVM.

Well, there's no doubt that LVM has its uses, but that doesn't mean we
should install it by default on every Fedora installation.

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out-of-date on modern systems.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scsi_wait_scan" are used to work around this out-of-date design on the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.

We definitely should stop setting up LVM by default on Fedora, because
it allows us to disable these unnecessary enumeration delays that are
broken by design anyway.

If we don't have LVM on default installs, we also don't need
scsi_wait_scan anymore, and that would be great.

Lennart

-- 
Lennart Poettering - Red Hat, Inc.