Re: Linux mdadm superblock question.

[Bill Davidsen <davidsen@tmr.com>]

Mr. James W. Laferriere wrote:
>  Hello Robin ,
>
>  On Fri, 12 Feb 2010, Robin Hill wrote:
> > On Thu Feb 11, 2010 at 05:52:41PM -0800, Michael Evans wrote:
> >
> >> On Thu, Feb 11, 2010 at 3:00 PM, Justin Piszcz
> >> <jpiszcz@lucidpixels.com> wrote:
> >>> Hi,
> >>>
> >>> I may be converting a host to ext4 and was curious, is 0.90
> >>> still the only superblock version for mdadm/raid-1 that you can
> >>> boot from without having to create an initrd/etc?
> >>>
> >>> Are there any benefits to using a superblock > 0.90 for a
> >>> raid-1 boot volume < 2TB?
> >>>
> >>> Justin.
> >>
> >> You need the superblock at the end of the partition:  If you read
> >> the manual that is clearly either version 0.90 OR 1.0 (NOT 1.1
> >> and also NOT 1.2; those use the same superblock layout but
> >> different locations).
> >>
> > You also need it to be auto-assembled by the kernel, which is only
> > version 0.90.
> >
> > As for benefits, there's a number of benefits of 1.x over 0.90 - I
> > don't think any of them are terribly important for a small boot
> > partition though.  There's also benefits to having an initrd
> > anyway.
> >
> > Cheers, Robin
>
>  One can use the 'append=""' functionality of lilo & I am user that
>  Grub2(and family) has some method of doing the same . ie: append="
>  md=1,/dev/sda1,/dev/sdb1,/dev/sdc1,/dev/sdd1
>  md=2,/dev/sda2,/dev/sdb2,/dev/sdc2,/dev/sdd2 md=3,/dev/sda3,/dev/sdb3
>  md=4,/dev/sda4,/dev/sdb4 vt.default_utf8=0 sysrq_always_enabled=1"
>
You *really* don't want to do that, you want to let mdadm find the 
arrays and use UUID to associate array with mount point. Any minor issue 
with the hardware will cause this whole structure to fail.

>  Not sure if this would be usable with a 1.1+ version of the MD
>  headers .

I'm not sure it's usable at all. Not that it doesn't work in a perfect 
world, we just don't live there. This method is fragile.

-- 
Bill Davidsen <davidsen@tmr.com>
  "We can't solve today's problems by using the same thinking we
   used in creating them." - Einstein