Replacing disk in Linux Software RAID 1

[Michael E. Webster]

On Tue, 2004-08-17 at 07:14, Nigel Wade wrote:
> Michael E. Webster wrote:
> > Michael,
> > 
> > Probably - I went with "ghost" because I had a boot floppy handy.
> 
> You should not need to ghost. Add the new drive as a hot spare and md should 
>   take care of building the new drive whilst the system is running (it does 
> need to be partitioned correctly first).
> 

You are absolutely correct.  I remember now why I ghosted the drive - it
was our main mail server and we wanted to have a "good" drive with all
of our data in case something went wrong during the rebuild.  That's why
I copied the drive and pulled it from the system. 

> > 
> > The main thing is to get the MBR from that first drive - without it,
> > you're out of luck.  
> > 
> > In software RAID, the MBR is only written to the first drive, which
> > really doesn't make sense.  I would think that if you had the first
> > drive fail, you should be able to boot off the second one - IMHO.
> > 
> > 
> > Mike.
> > 
> > 
> 
> I don't think that's right.
> 
> In the case of lilo it writes MBRs such that each drive will boot from 
> itself, and either drive can work with the other removed. OTOH, due to the 
> way grub works, only one drive can be used as the boot device (each will try 
> to boot from the same drive).
> 
> At least this was the case in RH9 which is the latest mirrored boot system 
> I've setup. It's entirely possible that lilo has since been broken so that 
> it no longer handles booting from mirrored disks.
> 

I honestly don't know how lilo handles the MBRs in software RAID - I was
talking about FC1 using grub.  With grub only one drive can be used as
the boot device.  Thus, if your main boot device crashes - and you don't
have the MBR copied onto the second drive you will have problems.

For more information, I found this helpful:
http://lists.us.dell.com/pipermail/linux-poweredge/2003-July/014331.html



> -- 
> Nigel Wade, System Administrator, Space Plasma Physics Group,
>              University of Leicester, Leicester, LE1 7RH, UK
> E-mail :    nmw at ion.le.ac.uk
> Phone :     +44 (0)116 2523548, Fax : +44 (0)116 2523555
>