Re: [gentoo-user] LVM for data drives but not the OS

[Joost Roeleveld]

On Thursday 07 April 2011 06:52:26 BRM wrote:
> ----- Original Message ----
>
> > From: Joost Roeleveld <joost@××××××××.org>
> >
> > On Thursday 07 April 2011 06:20:55 BRM wrote:
> > > ----- Original Message  ----
> > >
> > > > From: Neil Bothwick <neil@××××××××××.uk>
> > > >
> > > > On Thu, 07 Apr 2011 05:22:41 -0500, Dale wrote:
> > > > >  I want to do it this  way because I don't trust LVM enough
> > > > >  to put  my
> > > > >
> > > > > OS
> > > > > on.  Just my  personal  opinion on LVM.
> > > >
> > > > This doesn't make sense. Your OS can  be  reinstalled in an hour
> > > > or two, your photos etc. are   irreplaceable.
> > >
> > > Makes perfect sense to me as well.
> > >
> > > Having installed LVM - and then removed it due to issues; namely,
> > > the  fact that one of the hard drives died taking out the whole LVM
> > > group,  leaving the OS unbootable, and not easily fixable. There
> > > was a thread on  that (started by me) a while back (over a year).
> > >
> > > So,  perhaps if I had a RAID to underly so I could mirror drives
> > > under LVM
> > >
> > >  for recovery I'd move to it again. But otherwise it is just a PITA
> > >   waiting
> > >
> > > to happen.
> > >
> > > Ben
> >
> > Unfortunately, any method  that spreads a filesystem over multiple disks
> > can be
> >
> > affected if one of  those disks dies unless there is some mechanism in
> > place that can handle the  loss of a disk.
> > For that, RAID (with the exception of striping, eg. RAID-0)  provides
> > that.
> >
> > Just out of curiousity, as I never had the need to look  into this, I
> > think that, in theory, it should be possible to recover data  from LVs
> > that were not
> >
> > using the failed drive. Is this assumption correct or  wrong?
>
> If you have the LV configuration information, then yes. Since I managed to
> find the configuration information, I was able to remove the affected PVs
> from the VG, and get it back up.
> I might still have it running, but I'll back it out on the next rebuild - or
> if I have a drive large enough to do so with in the future. I was wanting
> to use LVM as a bit of a software RAID, but never quite got
> that far in the configuration before it failed. It does do a good job at
> what it's designed for, but I would not trust the OS to it either since the
> LVM configuration is very important to keep around.
>
> If not, good luck as far as I can tell.
>
> Ben

LVM isn't actually RAID. Not in the sense that one gets redundancy. If you 
consider it to be a flexible partitioning method, that can span multiple disks, 
then yes.
But when spanning multiple disks, it will simply act like JBOD or RAID0. 
Neither protects someone from a single disk failure.

On critical systems, I tend to use:
DISK <-> RAID <-> LVM <-> Filesystem

The disks are as reliable as Google says they are. They fail or they don't.
RAID protects against single disk-failure
LVM makes the partitioning flexible
Filesystems are picked depending on what I use the partition for

--
Joost