Logical Volumes vs. Partitions - the way of things

[Ted Potter]

speaking for myself you have clarified far more than a bit !
:-)
Go Rick Go !



On 2/12/08, Rick Stevens <rstevens at internap.com> wrote:
>
>
> On Mon, 2008-02-11 at 22:48 -0500, Dennis D. Calhoun wrote:
> > Hello Mr. Stevens;
> >
> > Firstly, I've admired and respected your insights, obviously deep
> > knowledge, and more than substantial contributions to this list for many
> > years. I've learned much in the process and still have a great deal more
> > to learn.
> >
> > My comment and question: Logical Volumes offer much more capability, in
> a
> > number of ways, than old fashioned partitions do. However, many current
> > operating systems cannot boot from a logical volume. What can you share
> > with us about this paradox?
> >
> > Afterword: I don't know enough about the current Linux offerings to know
> > whether or not any Flavor of Linux can boot from a Logical Volume, but I
> > do know that Windows, of any flavor, cannot.
>
> You're correct, no OS can boot from a logical volume.  Booting requires
> the BIOS to be able to access the data directly.  I don't know of any
> PC-type BIOS that groks logical volumes.  Even in the Linux world there
> are two types of logical volume systems, LVM1 and LVM2 (kernel 2.6 and
> later) and they're not compatible (LVM1 systems can't talk to LVM2
> volumes and vice-versa).  You could convert LVM1 to LVM2, but not back.
>
> The BIOS must be able to decipher how the LVMs are structured to access
> the data and as I said, I know of no BIOS that can do it.  If you toss
> Windows LVMs into the mix (or BSD LVMs or Solaris LVMs or whatever), a
> BIOS would have a really hard time.
>
> So, to boot, the BIOS must see a regular partition, pull in the
> appropriate level 1 boot loader and run it.  Level 1 code can only
> occupy a maximum of 512 bytes (one disk block), so it's operation is
> limited.  Typically, the level 1 code then brings in the level 2 code,
> which in turn loads and starts the kernel (in Linux, it really uses a
> level 1, level 1.5 and a level 2...you really don't need to know why
> there's three, just trust me on that).
>
> That all being said, this does NOT mean that the root file system cannot
> be on a logical volume.  Once the kernel starts up (remember, it was
> loaded by the level 2 boot loader and comes from the standard /boot
> partition), it can load the LVM management modules from the ramdisk
> image (also on the standard /boot partition) and access them.  In these
> types of operations, the ramdisk image would normally contain the LVM
> modules, SCSI driver (if needed) and any special filesystem modules (for
> example, the ext3 filesystem).
>
> I hope that clarifies it a bit for you.
>
> ----------------------------------------------------------------------
> - Rick Stevens, Principal Engineer             rstevens at internap.com -
> - CDN Systems, Internap, Inc.                http://www.internap.com -
> -                                                                    -
> -  Animal testing is futile.  They always get nervous and give the   -
> -                             wrong answers                          -
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-- 
Ted Potter
tpotter at techmarin.com
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