Re: usb raid1 pendrive boot
On Friday 07 May 2010 08:22:00 deloptes wrote:
> Jozsi Vadkan wrote:
> > Did anyone managed to boot from RAID1?
> >
> > Lenny gives this error message:
> >
> > "mdadm: no devices found for /dev/md0"
> >
> You'll need a small i.e. 20-30MB partition out of raid to boot from with
> i.e. initrd which loads the md driver and your raid device becomes visible.
Just for the record, it actually *is* possible to boot off a partition
that's in RAID1, but you might have to use grub. The reason it works
is because grub can read past the md-raid1 metadata without getting
confused -- since the raid is not running when grub is looking at the
disks, it only sees one of the two, but since they're mirrored, and
since grub doesn't *write* to the partition, everything works.
I have several production systems that do this.
It's a bit complicated to set up, but generally, it goes
something like:
- Install a basic system onto one drive, with the correct
size of partition (call this /dev/sda1).
- Set up an "incomplete" RAID1 array, with one device and
one missing, with the second drive -- this is /dev/md0,
consisting of /dev/sdb1 and an empty slot.
- Configure /etc/mdadm/mdadm.conf, and re-build your initramfs
so that it has the md modules and knows about the array.
- Mount /dev/md0 somewhere and copy all the data off /dev/sda1 to it.
It's very important to get the *new* initramfs onto the array.
- Install grub in the bootloader of the 2nd drive. (The installer
already did this for the first one). This actually isn't
essential, but you may need it if/when /dev/sda fails.
- Boot on to the incomplete RAID array -- root=/dev/md0.
- Add /dev/sda1 to the array, and sync.
Now you have a bootable software raid-1 system.
I'm not saying this solves the OP's problem, necessarily,
but it's perfectly possible. I've never tried it with LILO
or grub2, but I would think it would work.
-- A.
--
Andrew Reid / reidac@bellatlantic.net
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