[GLLUG] Link two RAIDs in one LVM?

[Dr. Axel Stammler]

Thank you.

On Sun 2020-05-10 07.39.31, Greater London Linux User Group wrote:
>On 09/05/2020 20:32, Dr. Axel Stammler via GLLUG wrote:
>>On Sat 2020-05-09 11.24.15, Greater London Linux User Group wrote:
>>
>>>How do you intend to combine them? You won't be able to put your
>>>existing array into the LVM without destroying its contents.
>>
>>https://www.debian.org/doc/manuals/debian-handbook/advanced-administration.en.html#sect.raid-and-lvm
>>
>>
>>says you can use lvresize and resize2fs to do just that. I am still 
>>hoping this will work.
>
>I think you may be mis-reading the manual.  What you can do is 
>increase the size of an existing Logical Volume, but you can't do an 
>in-situ conversion of an existing non-LV filing system.

You are quite right, I was mis-reading the manual. So I cannot incorporate an existing filesystem (no matter whether regular or on a RAID-1 system) into a new Logical Volume system; I see.

It really sounded to good to be true: ”RAID-1+0: This isn't strictly speaking, a RAID level, but a stacking of two RAID groupings. Starting from 2×N disks, one first sets them up by pairs into N RAID-1 volumes; these N volumes are then aggregated into one, either by “linear RAID” or (increasingly) by LVM. This last case goes farther than pure RAID, but there's no problem with that.“

>One thing which might work for you is to make your two new drives into 
>a RAID1 set and then use resulting device as your first Physical 
>Volume. Create a large logical volume within it, copy all your 
>existing files over (boring), delete the partition on the old RAID1 
>set, create a second PV, add it to your Volume Group and then expand 
>the LV.
>
>It does mean moving all your data from old drives to new, but at least 
>you'd then end up with one much larger filing system.

Hmm. How long would it take to copy (nearly) 8 TB? Obviously I would have to prevent any write access for that period of time.
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