Logical Volumes vs. Partitions - the way of things

[Rick Stevens]

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.

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- Rick Stevens, Principal Engineer             rstevens at internap.com -
- CDN Systems, Internap, Inc.                http://www.internap.com -
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-  Animal testing is futile.  They always get nervous and give the   -
-                             wrong answers                          -
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