[GLLUG] Link two RAIDs in one LVM?

James Courtier-Dutton james.dutton at gmail.com
Tue Apr 28 12:19:50 UTC 2020

On Tue, 28 Apr 2020 at 12:18, Dr. Axel Stammler via GLLUG
<gllug at mailman.lug.org.uk> wrote:
> Hi,
> I have a 4 TB RAID system (two identical hard disks combined in a RAID-1, created using mdadm). Now, after a few years, this has reached 90% capacity, and I am thinking about first adding another similar 8 TB RAID system and then combining them into one 12 GB RAID 1+0 filesystem. I should be grateful for any tips, especially about buying two 8 TB harddisks.
> Which hardware parameters should I look at?
> Which method should I use to combine both RAID systems into one?
> - linear RAID
> - RAID-0
> - Large Volume Management (using pvcreate, vgcreate, lvcreate)


First for RAID, avoid SMR HDDs. (Shingled magnetic recording)
I would probably RAID 5 them.
4+4 = 8 for the first disk, against the two other 8 disks.
So, say disks are A(4TB), B(4TB), C(8TB), D(8TB)
Partitions the 8TB in half.
A(4TB), B(4TB), C1(4TB), C2(4TB), D1(4TB), D2(4TB)
RAID 5: A,C1,D1
RAID 5: B,C2,D2
Then put the two RAID arrays in the same LVM VG, so that they look
like one big disk for the OS.

Another alternative is using XFS or BTRFS and configure them with replicas.
That is where the filesystem does the replication, thus not needing RAID at all.

Or, you could take the approach I take. I remove the old 4TB disks and
only copy the few files I need on to the 8TB disks going forward.
I can always plug the old 4TB disks in if I need an old file.
I have written my own indexer for this. It scans the whole disk,
creating an index and thumbnails and then only store the index and
thumbnails on the 8TB disks.
The index is stored in Elastic Search, so makes it easy to find the
files again, and also which disk they are on.
So, files I hardly ever need are stored on powered off disks.

Kind Regards


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