Bootstrapping Raid 5 w/GEOM

[Stijn Hoop]

On Thu, Jul 21, 2005 at 10:56:39PM +1000, Norberto Meijome wrote:
> Stijn Hoop wrote:
> > On Thu, Jul 21, 2005 at 05:11:18PM +1000, Norberto Meijome wrote:
> > > I want to setup software Raid 5 , and I want it to affect ALL 
> > > partitions, including / (so if any one of the drives fails, it will 
> > > still boot up properly). I am planning on using GEOM, but open to 
> > > suggestions.
> >
> > I don't think it's possible -- the loader doesn't know about the mirror
> > even in a mirrored / setup; it justs treats one of the disks of the mirror
> > as 'the boot disk'. It's not until the kernel is loaded that gvinum RAID-5
> > can do something.
> 
> Would the same be true (not possible to boot) for a RAID 1 + 0 with GEOM?

Hmm, tricky. I guess if you can get the disk layout to mirror that of
a non-RAID one, the loader would be able to cope. Striping would be
impossible (at least not unless you can guarantee that the whole of
the kernel + / and /boot directory entries are available on one
stripe, and even then I'm not sure).

> > So, in your case, the loader would read random RAID-5 data instead of
> > a kernel and refuse to boot.
> >
> > IE, it's not possible until someone writes a RAID-5 capable loader.
> 
> right - so that's why I couldnt find any reference to this anywhere :)

Probably.

> > I would advise you to either use gmirror for booting, or define a few
> > gvinum mirror plexes (it is possible to boot from a mirrored gvinum
> > setup although it's tricky to get right).
> 
> I guess I could create a boot slice in 2 of the drives, mirror that with 
> gmirror and use that to boot. Then RAID-5 the rest of the drives (minus 
> the size of the boot partition in the other 2 drives of course).

Yes. While on the subject, you could use 2 drives to mirror the boot
disk and have swap, and then use the same amount of space on the other
two drives to mirror /usr and /var. Then use RAID-5 for your data
partitions.  This way you'll have speed & reliability.

Be aware that RAID-5 is not ideal for many-write situations (most home
directories), although certainly tolerable with modern drives.

--Stijn

-- 
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