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

[J. Roeleveld]

On Thu, April 7, 2011 7:31 pm, BRM wrote:
> ----- Original Message ----
>
>> From: Joost Roeleveld <joost@××××××××.org>
>> 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
>>
>
> The attraction to LVM for me was that from what I could tell it supported
> and
> implemented a software-RAID
> so that I could help protect from disk-failure. I never got around to
> configuring that side of it, but that was my goal.
> Or are you saying I was misunderstanding and LVM _does not_ contain
> software-RAID support?

Unless I am mistaken, LVM does not provide redundancy. It provides
disk-spanning (JBOD) and basic striping (RAID-0).

For redundancy, I would use a proper RAID (either hardware or software).
On top of this, you can then decide to have a single filesystem, LVM or
even partition this.

I think the confusion might have come from the fact that both LVM and
Linux Software Raid use the "Device Mapper" interface in the kernel config
and they are in the same part.

Also, part of the problem is that striping is also called RAID-0. That, to
people who don't fully understand it yet, makes it sound like it is a
RAID.
It actually isn't as it doesn't provide any redundancy.

I do hope you didn't loose too much important data when you had this issue.

--
Joost