Disk Recovery Part III

[richb at pioneer.ci.net (Rich Braun)]

Matthew Gillen <me at mattgillen.net> wrote:
> Unless you're running raid-1 under LVM, with a 2-disk LVM you're
> doubling your chances of an unrecoverable error.  LVM lets the
> filesystem act like it's operating on a single device...

This I agree with and in fact I would add that splitting filesystems across
multiple physical devices by any other means (RAID0 etc) without mirroring is
just asking for trouble.

LVM per se is not the culprit, this warning does not affect the decision
whether to use LVM for your system.

I did a quick scan of the LVM Howto at tldp.org to see if your suggestion to
include warnings about multi-volume striping  has been implemented.  I
couldn't find any such warnings; the maintainer lists his email address as
aj(at)terrascale.com.

Would like to see actual details of any horror stories using LVM or RAID or
whatever posted here.  The details are useful not only for helping people
decide for themselves the risk/reward tradeoff, but also for people who are in
the middle of disaster recovery and looking for specific hints from those who
have traveled down the same path.  A statement like "Avoid XYZ, it's buggy and
I heard bad things" doesn't contain the same search value as "When I used XYZ
and suffered a drive failure, the error message was 'Foo Bar code 123' and the
tool I used to recover from the problem can be found in the rpm for 'asdf'".

Someone did ask for the commands to use for extending a volume.  When I
recommend LVM for maintaining spare storage, for myself what I am talking
about is setting aside say 10% of a 160GB mirrored physical volume to allocate
in the future--a year or three down the road--when I need space in a
filesystem and don't have time or inclination to shuffle filesystems across
drives.  It's not very likely I would use LVM extension in the case of a hard
drive upgrade, as the person who had asked, because in that case I'd be
creating new filesystems, not extending old ones.  But you *could* use these
commands after dd'ing the old drive's partition to the new drive.

  cfdisk (if you need to change the partition table)
  lvextend
  resize2fs (for ext3 or ...)
  resize_reiserfs (...for reiser)

These commands are noted with examples in section 11.9 of the
previously-mentioned Howto; the section can be accessed directly here: 
http://www.tldp.org/HOWTO/LVM-HOWTO/extendlv.html

Matthew, thanks for the heads-up about striping.  It's pretty much something I
wouldn't ever think to do but a lot of people are probably tempted by it
because you can get more storage quickly if you tack extra physical volumes
onto an existing setup.

Where would I actually contemplate striping?  In a data center application
where rotational latency and drive throughput of a single drive are too low to
meet the performance requirements.  In that case we're not talking about
two-drive setups on motherboard IDE controllers.

-rich