[Cialug] Replacing failed RAID 1 drive

[Matthew Nuzum]

This would make a great topic for a LUG meeting sometime. Bring a box in,
set it up with RAID then fail and swap a drive.

It's one of those things we all know we should do, and many of us do it,
but the actual hands-on experience of dealing with a failed drive is not
nearly as common. (I've never done in with software RAID, only done a
cold-swap with a hardware RAID controller)

On Tue, Sep 30, 2014 at 5:35 PM, Rob Cook <rdjcook at gmail.com> wrote:

> "When you say “LVM on top of RAID” I assume you mean something like this:
>         /dev/sd* (physical block devices) => md0 (mdadm array) => pv1 (LVM
> “physical” volume) => vg1 (LVM volume group) => lv1 (LVM logical volume) =>
> /mnt/foo (filesystem)"
>
> Yes, like that exactly.
>
> Ok, off to buy a new drive.
>
> On Tue, Sep 30, 2014 at 5:25 PM, Zachary Kotlarek <zach at kotlarek.com>
> wrote:
>
> >
> > On Sep 30, 2014, at 3:07 PM, Rob Cook <rdjcook at gmail.com> wrote:
> >
> > > I have a CentOS 6.5 box with 2 1.5Tb drives in a RAID 1 with LVM
> > partitions
> > > on top of that. One of the drives /dev/sdb has failed.
> > >
> > > I've been googling quite a bit and I think that I should be ok
> following
> > > this guide:
> > >
> > > http://www.howtoforge.com/replacing_hard_disks_in_a_raid1_array
> > >
> > > Fail then remove the drive from the array, replace with similar or
> larger
> > > then recreate. The one question I have is what to do with the LVM
> > > partitons? Naively they should recreate given this is a RAID 1 so it's
> > the
> > > same data on both drives so I shouldn't have to worry. Or is that to
> > > simplistic of a view?
> >
> >
> > When you say “LVM on top of RAID” I assume you mean something like this:
> >         /dev/sd* (physical block devices) => md0 (mdadm array) => pv1
> (LVM
> > “physical” volume) => vg1 (LVM volume group) => lv1 (LVM logical volume)
> =>
> > /mnt/foo (filesystem)
> >
> > If that’s the case then the LVM physical volume and everything higher in
> > the stack has no idea that you’re swapping disks and doesn’t need to be
> > told anything.
> >
> > —
> >
> > On a related note, sometimes mdadm commands that reference physical
> > devices, like this:
> >         mdadm --manage /dev/md1 --fail /dev/sdb2
> > will fail with an error like:
> >         No such device: /dev/sdb2
> > because the file /dev/sdb2 no longer exists (because the disk is dead or
> > pulled).
> >
> > But you still need to tell mdadm about it so it can update the array.
> > Instead you should use the short name:
> >         mdadm --manage /dev/md1 --fail sdb2
> > or whatever other device name shows up when you ask mdadm about the array
> > or look at /proc/mdstat. That bypasses any device-file lookup and uses
> the
> > references that mdadm tracks internally.
> >
> >         Zach
> >
> >
> > _______________________________________________
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> > http://cialug.org/mailman/listinfo/cialug
> >
> >
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-- 
Matthew Nuzum
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