Re: RAID superblock confusion

[Mike Fedyk <mfedyk@matchmail.com>]

On Sat, Apr 13, 2002 at 06:00:13PM -0600, Richard Gooch wrote:
> Mike Fedyk writes:
> > On Sat, Apr 13, 2002 at 01:29:05PM -0600, Richard Gooch wrote:
> > > Luigi Genoni writes:
> > > > 
> > > > > > >
> > > > > > > Ehh, I ran into this a while ago.  When you compile raid as modules
> > > > > > > it doesn't use the raid superblocks for anything except for
> > > > > > > verification.  I took a quick glance at the source and the
> > > > > > > auto-detect code is ifdefed out if you compiled as a module.
> > > > > >
> > > > > > Exactly where is this? A scan with find and grep don't reveal this.
> > > > > >
> > > > >
> > > > > drivers/md/md.c
> > > > >
> > > > > in the ifndef MODULE sectioin.
> > > > >
> > > > > > > Ever since I have had raid compiled into my kernels.
> > > > > >
> > > > > > This is my relevant .config:
> > > > > > CONFIG_MD=y
> > > > > > CONFIG_BLK_DEV_MD=y
> > > > > > CONFIG_MD_LINEAR=m
> > > > > > CONFIG_MD_RAID0=m
> > > > > > CONFIG_MD_RAID1=m
> > > > > > CONFIG_MD_RAID5=m
> > > > > > CONFIG_MD_MULTIPATH=m
> > > > > >
> > > > >
> > > > > Set this to =y and you're set.
> > > > >
> > > > > I'd like to see this working from modules though.
> > > > 
> > > > NO, please. There are hundreds of scenarios where that could be
> > > > dangerous.  Suppose you load the RAID module when all partitions are
> > > > mounted, and two partiton in mirror are mount on different mount
> > > > point (you can do this, raid module is not loaded, and so...). And
> > > > now you load the module and md device is registered. That would not
> > > > be really nice, also if it is ulikely that you could damnage your
> > > > system
> > > 
> > > The RAID code checks to see if there are busy inodes for each device
> > > in a RAID set. So your hundreds of scenarios are not a problem.
> > > 
> > 
> > I had a machine that had raid1 setup correctly but was accidentally
> > configured to root=/dev/hda1 (one member of the md0 raid1 set).
> > 
> > All was well until I noticed I wasn't rooting from md0, so reboot with new
> > root=/dev/md0 and now my filesystem is b0rked (maybe because hdc1 was the
> > primary mirror?).
> > 
> > Luckily I was still setting up that machine so I just reinstalled it.
> > 
> > This was with raid compiled into the kernel, so it's not a module checking
> > issue, and I consider it a user error.  But maybe someone else thinks
> > different...
> 
> Yep, user error. Just like if you dd if=/dev/zero of=/dev/hda2 but
> meant to write to /dev/hda1 instead, and /dev/hda2 has your OS while
> /dev/hda1 had M$ which you wanted to erase and re-install. Not much to
> be done about that. One learns best by fucking up :-)
> 

One can also argue that raid (sh|c)ould lock the devices that it has been
started on also...

But it is the unix way to give the root user lots of rope to hang
him/herself, and what keeps the root user from trying to erase the entire
drive (hda instead of hda1) instead of just the locked partition?