RAID and root

[Martin Maney]

On Mon, Sep 27, 2004 at 09:59:48AM -0500, Jimmie Houchin wrote:
> The install is going fine but I want to use RAID1 on my 4 250gb SATA drives.
> 
> The installer says that Ubuntu doesn't do RAID on root.

Inherited from Debian, yes.

> What is the smallest root that I can do and symlink to everything else
> on the RAID partitions. In other words I would like to make the root
> partition as small as possible but as large as necessary.
> 
> The reason for this is I want all the drives identical. Since I cant
> RAID boot or root, I will dd boot and root to the other drives when my
> install is finalized. That way the system can reboot as necessary off
> any drive.
> 
> I have never done this before and all RAID setup is new to me.
> 
> Any help or wisdom greatly appreciated.

The best RAID is hardware RAID, of course - all these problems just go
away if you can see your way to picking up a nice 3ware card, plus the
entire boot process gets protected against single drive failures.

Linux itself has no problem doing root on RAID these days; it's only
the install step that balks.  There are plenty of writeups on ways of
getting from here to there going back to the early days (pre-version
0.9 of the RAID code, IIRC) when getting root onto software RAID was
hairy.  Now it's just a nuisance, though you may still need to build
your own kernel with RAID compiled in to make it easy (that's how I've
done it on several machines with Woody).  One trick is to do the
install not on RAID, then setup a broken mirror (only one of the two drives
online), copy the system over to the half mirror, and finally add the
original install drive to the RAID set and let it rebuild the mirror.

The simplest way is to give up on RAID for the root.  I generally use
1/4G for root in multi-partition setups, with separate partitions (all
LVM these days) for /usr, /var, /home, and usually /tmp - you can
probably get away without the last on a single-user machine if you want
to.  I don't know if it would work with ubuntu, but with Woody I
wouldn't expect any problem doing a limited install to, say, hda1
(root) and hdb1 (temporary /usr) before setting up the software mirror
(hda2 and hdb2, say).  Then carve it up with LVM and copy things around
so you can boot into the real setup.

BTW, if you want RAID to keep the system up despite a drive failure,
you really want to put the swap on a RAID parititon.  Yes, you can do
that - even on an LVM over RAID1, I happen to know.   :-)

There, that's my sketchy brain dump on Linux (mostly Debian) versus
RAID.  Feel free to ask for more details, but be warned that since I've
fallen in geek lust with 3ware controllers I haven't been setting up a
lot of software RAID boxes.

-- 
These are people who panic when the power goes off
and they're trapped on an escalator.  -- John Walker