Re: moving LVM logical volumes to new disks
"Karl E. Jorgensen" <karl@jorgensen.org.uk> writes:
> Hi
>
> On Wed, Nov 12, 2014 at 10:09:43PM +0100, lee wrote:
>> Hi,
>>
>> what's the best way to move existing logical volumes or a whole volume
>> group to new disks?
>>
>> The target disks cannot be installed at the same time as the source
>> disks. I will have to make some sort of copy over the network to
>> another machine, remove the old disks, install the new disks and put the
>> copy in place.
>
> Having to do this over the network makes it slightly
> complicated.... But not impossible.
>
>> Using dd doesn't seem to be a good option because extend sizes in the
>> old VG can be different from the extend sizes used in the new VG.
>>
>> The LVs contain VMs. The VMs can be shut down during the migration.
>> It's not possible to make snapshots because the VG is full.
>
> Ok.
>
>> New disks will be 6x1TB RAID-5, old ones are 2x74GB RAID-1 on a
>> ServeRaid 8k. No more than 6 discs can be installed at the same time.
>
> Assuming that:
>
> * both machines can be online at the same time
>
> * there is a good network connection between them. The fatter the pipe
> the better
>
> * both run Debian. Obviously
One is running Fedora. I could install Debian on it if that helps.
> * The VMs are happy to (eventually) migrate to the new hardware box
The server remains the same, only the disks are being replaced. The
SeverRaid 8k doesn't like the WD20EARS (and they are slow), so I'm
replacing them.
> Then there is a sneaky way, which can help minimize the downtime: LVM
> and network block devices (or iSCSI. Either can work). Chunky,
> slightly hacky, but worth considering.
>
> The basic idea is:
>
> * On the receiving machine, prepare the disks. Export the *whole*
> disks (or rather: the RAID device(s)) using nbd, xnbd or iSCSI.
I'd have to attach the disks to a SmartArry P800, and the ServeRaid 8k
won't be able to read them. I'd have to use the 1TB spare disk instead
to move the volumes to. Once they're moved over, I could replace the
disks in the sending machine and move the LVs back the other way round.
> * On the sending machine: attach the disks over the network, using nbd
> client, xndb client or iSCSI.
Hm, I need to learn about that ...
Looking at things, aoe seems to be a good idea --- if that works with
hardware RAID and SATA.
> [...]
>
> Hope this helps
Yes, thank you :) It's a very interesting idea.
Fortunately, downtime isn't an issue. I also have a 32GB USB stick, and
all the LVs are smaller than 32GB.
Since there seems to be some agreement that it would be best to use
pvmove, I think I could, one after the other, move all the LVs to the
USB stick with pvmove, plug the USB stick into the other machine, move
the LVs onto a hard disk in the other machine, replace disks and move
the LVs back the same way.
I can keep the VMs shut down while doing this, which allows me to just
move the USB stick rather than moving over the network. However, over
the network might be more reliable, and I could move all VMs at once
with minimal downtime. Hmmm ...
--
Again we must be afraid of speaking of daemons for fear that daemons
might swallow us. Finally, this fear has become reasonable.
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