[GLLUG] Link two RAIDs in one LVM?

James Roberts j.roberts at stabilys.com
Mon May 11 16:39:58 UTC 2020

On 11/05/2020 11:58, Andy Smith via GLLUG wrote:
>> Yes
> I actually think it is possible and is a reasonable plan, though
> backups will still be advised. I didn't suggest this at first
> because initially we thought there were unequal-sized devices (4T
> and 8T).

Same here.

> I believe modern mdadm can reshape a RAID-1 into a RAID-0 then a
> RAID-0 into a RAID-10 and then add extra devices.
>      https://www.berthon.eu/2017/converting-raid1-to-raid10-online/

I have done it myself long ago... see below

> There will be a scary time when it is RAID-0 and therefore no
> redundancy.

Yes depending on how it's done.

> My main uncertainty about this is that I'm fairly sure converting
> from RAID-1 to RAID-0 leaves you with a RAID-0 of one device and one
> marked as spare, then I'm not sure if it does support going to
> RAID-10 from that. Should be easy to prove with a test on small
> loopback files as block devices though.
> Another way it can be done now that we know all the devices are the
> same size is to:
> 1. create a new RAID-10 array that is missing two members.
> 2. Bring it online, put a filesystem (or LVM) on it,
> 3. copy data over to it,
> 4. boot from it making sure that everything works,
> 5. nuke the old array, add its members to the new RAID-10 thus
> making it fully redundant again.

And I seem to recall that's how I did it.

> again, for the time period where the second RAID-10 has two members
> missing it has no redundancy at all.

Indeed. But the new disks are then the non-redundant RAID-10 which may 
be safer.

> I think it can be done only with mdadm though.

I believe so.

On further consideration if it was my machine I'd either follow Andy's 
plan or do this:

1. Buy a Seagate 8/10TB USB backup device. They are generally cheaper 
than a raw disk (or were, pre covid-19, I am certain of this as I just 
then bought two to backup client data.

2. Replicate the data to the backup disk

3. Verify backup

4. Destroy existing raid and wipe disks (if paranoid, keep just one 
until later)

5. Test existing disks (and if cautious, the new ones)

6. Build new 4-unit RAID10 (if paranoid, with one existing disk missing 
as per above)

7. Copy data back

8. If paranoid once happy wipe test add the other old disk.

Really I would not be happy having half my data array on 5 year old 
disks even in RAID 10 - it can stand 2 disk loss but you need to feel 
lucky. Disks DO fail together. I do have systems (well one backup 
server) with older (2TB 7+ year old!) disks (but only as a small 
minority in RAID 6 or 60 arrays). But each to their own... and I did 
lose two at once in that system.

I'm very fond of LVM and have used it on large filesystems without an 
underlying partition in the days when Red Hat did not support >2TB, as a 
workaround, now not needed. It was 100% solid over the 5 year life of 
the system. This approach risked confusing people though.

But the only times I have lost data (twice) on mdadm-backed RAID is with 
LVM over large RAID5 and multiple disk failures, making recovery 
impossible, so I tend to avoid LVM on RAID (data restored from backup). 
But then I don't use RAID 5 any more on >2TB disks. Or RAID6, indeed. 
It's all RAID 10 now for me, and maybe ZFS in the future if it ever gets 
more performant on Linux...


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