[GLLUG] Link two RAIDs in one LVM?

John Winters john at sinodun.org.uk
Sun May 10 13:23:54 UTC 2020

On 10/05/2020 14:12, Dr. Axel Stammler via GLLUG wrote:
>> 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.

Hard to say how long it would take - it depends very much on your 
system.  However, depending on what you use your disk storage for you 
might not need to disable write access for that long.

(It's an annoying bootstrap problem - if only you were already using LVM 
there are all sorts of clever things which it can do to make it look 
like you have a copy of your data whilst the data are still copying. 
Not an option in this case though.)

If the rate of change to your data is not high, you could potentially 
copy it all over using rsync (rsync -at) whilst leaving your source 
disks write enabled and having fresh data arrive whilst the copy 
happens.  As soon as the initial copy completes (probably several hours) 
issue the same command again and it will complete in a very few minutes. 
  Do it again and it will be faster still.

Once the time gets down to a period acceptable for a "no writing" 
interval, disable writes, issue the command one last time.  Unmount the 
source, mount the destination in its place and Robert's your 
progenitor's male sibling.


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