Re: [PATCH 000 of 5] md: Introduction

[Kyle Moffett]

On Jan 22, 2006, at 20:25, Neil Brown wrote:
> Changing the size of the devices is a separate operation that has  
> been supported for a while. For each device in turn, you fail it  
> and replace it with a larger device. (This means the array runs  
> degraded for a while, which isn't ideal and might be fixed one day).
>
> Once all the devices in the array are of the desired size, you run
>   mdadm --grow /dev/mdX --size=max
> and the array (raid1, raid5, raid6) will use up all available space  
> on the devices, and a resync will start to make sure that extra  
> space is in-sync.

One option I can think of that would make it much safer would be to  
originally set up your RAID like this:

               md3 (RAID-5)
       __________/   |   \__________
      /              |              \
md0 (RAID-1)   md1 (RAID-1)   md2 (RAID-1)

Each of md0-2 would only have a single drive, and therefore provide  
no redundancy.  When you wanted to grow the RAID-5, you would first  
add a new larger disk to each of md0-md2 and trigger each resync.   
Once that is complete, remove the old drives from md0-2 and run:
  mdadm --grow /dev/md0 --size=max
  mdadm --grow /dev/md1 --size=max
  mdadm --grow /dev/md2 --size=max

Then once all that has completed, run:
  mdadm --grow /dev/md3 --size=max

This will enlarge the top-level array.  If you have LVM on the top- 
level, you can allocate new LVs, resize existing ones, etc.

With the newly added code, you could also add new drives dynamically  
by creating a /dev/md4 out of the single drive, and adding that as a  
new member of /dev/md3.

Cheers,
Kyle Moffett

--
I lost interest in "blade servers" when I found they didn't throw  
knives at people who weren't supposed to be in your machine room.
  -- Anthony de Boer


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